Download Breakers, Relays, and Disconnects

Revision 3
Breakers, Relays,
and Disconnects
Student Guide
Approved by:
_
Chairperson, Industry OGF Working Group
Approved by:
5/17/2017
Date
5/17/2017
Manager, INPO Learning Development
Date
NOTE: Signature also satisfies approval of PowerPoint presentation and overview sheet.
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ii
Table of Contents
INTRODUCTION ..................................................................................................................... 2
TLO 1 CIRCUIT PROTECTION ................................................................................................ 2
Overview .......................................................................................................................... 2
ELO 1.1 Circuit Protection .............................................................................................. 3
ELO 1.2 Circuit Interrupting Devices .............................................................................. 6
ELO 1.3 Disconnect Switches ....................................................................................... 13
ELO 1.4 Safety and Equipment Protection .................................................................... 16
ELO 1.5 Electrical Drawings ......................................................................................... 19
TLO 1 Summary ............................................................................................................ 33
TLO 2 CIRCUIT BREAKERS ................................................................................................. 34
Overview ........................................................................................................................ 34
ELO 2.1 Circuit Breaker Construction and Function .................................................... 35
ELO 2.2 Racking Circuit Breakers ................................................................................ 48
ELO 2.3 Circuit Breaker Indications ............................................................................. 52
ELO 2.4 Circuit Breaker Control Power........................................................................ 58
TLO 2 Summary ............................................................................................................ 61
TLO 3 PARALLELING AC SOURCES .................................................................................... 62
Overview ........................................................................................................................ 62
ELO 3.1 Paralleling AC Sources ................................................................................... 62
ELO 3.2 Abnormal Conditions During Paralleling Operations ..................................... 65
TLO 3 Summary ............................................................................................................ 68
BREAKERS, RELAYS, AND DISCONNECTS SUMMARY .......................................................... 68
KNOWLEDGE CHECK ANSWER KEY ...................................................................................... 1
ELO 1.2 Circuit Interrupting Devices .............................................................................. 1
ELO 1.3 Transfer and Disconnect Switches .................................................................... 2
ELO 1.4 Safety and Equipment Protection ...................................................................... 4
ELO 1.5 Electrical Drawings ........................................................................................... 6
ELO 2.1 Circuit Breaker Construction and Function .................................................... 11
ELO 2.2 Racking Circuit Breakers ................................................................................ 13
ELO 2.3 Circuit Breaker Indications ............................................................................. 14
ELO 2.4 Circuit Breaker Control Power........................................................................ 17
ELO 3.1 Paralleling AC Sources ................................................................................... 19
ELO 3.2 Abnormal Conditions During Paralleling Operations ..................................... 20
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Breakers, Relays, and Disconnects
Revision History
Revision
Date
Version
Number
Purpose for Revision
Performed
By
11/6/2014
0
New Module
OGF Team
12/10/2014
1
Added signature of OGF
Working Group Chair
OGF Team
7/19/2016
2
Added OGF Working
Group Comments
OGF Team
10/27/2016
2.1
Incorporate user comments
OGF Team
5/17/2017
3
Incorporate additional user
comments
OGF Team
1
Introduction
In this module, you will learn about circuit interrupting devices and circuit
switching devices. The module discusses the types of devices, describes
their design and operation, and gives examples of the appropriate
application for each. This module also reviews safety measures for
operation of electrical circuits.
Understanding of how the different circuit switching and interrupting
devices operate is vital to the operator's ability to operate the plant safely.
Objectives
At the completion of this training session, the trainee will demonstrate
mastery of this topic by passing a written exam with a grade of 80 percent
or higher on the following Terminal Learning Objectives (TLOs):
1. Explain the purpose, safety precautions, and operation of electrical
circuit interrupting and circuit switching devices.
2. Explain the construction, operation, and indications for electrical
circuit breakers.
3. Describe the conditions that must be met prior to paralleling two
generators including effects of not meeting these conditions.
TLO 1 Circuit Protection
Overview
An operator needs to understand the principles of circuit protection and how
the circuit interrupting devices work, and have the ability to read and
interpret control drawings to respond to electrical system upsets, and
maintain plant safety.
When this section is complete, you will be able to read simple electrical
diagrams, identify various circuit interrupting and switching devices,
explain and follow safety precautions for using circuit these devices, and
describe the principles of circuit protection.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. Explain the principles and applications of circuit protection, including
selective tripping.
2. Describe the protection provided by each of the following:
a. Fuses
b. Protective relays
c. Circuit breakers
d. Overload devices
2
3. Describe the function and precautions associated with the operation of
disconnect switches.
4. Describe the personnel safety and equipment protection procedures
and precautions associated with circuit interrupting devices and relays.
5. Interpret symbols for breakers, relays, and disconnects in a simple
one-line diagram, and explain the operation of the control circuit.
ELO 1.1 Circuit Protection
Introduction
At the completion of this section, the student will be able to explain the
principles and applications of circuit protection.
Circuit Protection Guidelines
Circuit protection schemes must de-energize circuits when necessary to
protect the equipment and circuit from electrical system faults. They also
maximize the reliability of the system by avoiding unnecessary trips,
tripping only those portions of the circuit necessary to isolate the fault.
Circuit protection schemes incorporate the following key features in order
to accomplish the following goals:
Selective tripping — a system that includes tripping devices sized and
located to isolate faults at the lowest level in the distribution system.
2. Diverse circuit interrupting devices — a circuit includes different
types of devices to interrupt current flow in different places, so one
type of component failure does not prevent circuit protection.
3. Diverse fault sensors — a circuit with various sensor types to detect
different types of faults (overcurrent, undervoltage, underfrequency,
etc.) to ensure that faults are detected and isolated.
1.
Selective Tripping
Selective tripping is a term used to describe a method for protecting
electrical distribution systems by incorporating circuit breakers, fuses and
other protective devices at locations such that the protective device closest
to a fault will operate to remove the fault from the system and still maintain
the largest possible portion of the system energized. The following figure
depicts a simple example of selective tripping.
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Figure: Selective Tripping
In the figure above, the fuses, which are the protective devices closest to the
load, have 50 ampere (amp) ratings to load 1. If some sort of electrical fault
(short or ground) on the load causes current flow to the load to exceed 50
amps, the fuses will blow first, stopping current flow to the load and
isolating whatever faulted condition resulted in the excess current flow from
the rest of the system.
The output breaker for the generator is set to trip at 500 amps. In the fault
scenario described above, the output breaker for the generator would remain
shut and the generator would remain connected to the distribution system in
order to power other loads. By protecting the distribution system, power
remains available to load 2
Selective Tripping Example
A power plant internal electrical system must be set up to protect equipment
against the expected range of faults, and also to keep the equipment
available for operation so the power plant can stay in operation and reliably
generate power. In order to meet both goals, the power plant uses selective
tripping, diverse fault sensors, and diverse interrupting elements.
Selective tripping is a design that arranges protective devices so that the
device closest to a fault will trip to remove the fault from the circuit but not
interrupt the rest of the circuit. Consider an example power plant with four
condensate booster pumps. Full power operation requires all four pumps,
but the plant can operate indefinitely at 85 percent power with only three
condensate booster pumps available. The plant can continue operating at 60
percent power with two condensate booster pumps available.
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While these pumps and motors are reliable, there may be instances when a
failure occurs; rendering one or more pumps inoperable. A failure could
occur at the individual pump, or at the power supply level. During design
of the plant electrical systems, careful selection and placement of selective
tripping devices should minimize the impact of any of these possible faults.
If a fault occurs in an individual condensate booster pump or motor, we
need to de-energize the pump motor quickly to minimize damage to the
pump and motor. Depending on the severity of the fault, isolating it quickly
limits the damage and repair time required.
We also want to isolate this fault in such a manner that we only remove the
faulty condensate booster pump from service, which keeps the other three
condensate booster pumps and all other plant equipment available for use.
This requires the plant selective tripping design to have fault sensors and
circuit interrupting devices installed on the motor for each condensate
booster pump.
Induction motors have high initial currents when started. Therefore,
whatever overcurrent protection we choose for an induction motor, it must
satisfy the following requirements:


Allow the starting current surge for several seconds.
Interrupt the current flow quickly enough to prevent or minimize
damage to the component.
 Interrupt current flow quickly enough to contain any damage to that
component alone, as well as protect the rest of the electrical system.
Choices include fuses, overloads, or a protective relay to trip the motor
circuit breaker. Fuses are simple and reliable, but a fuse must be large
enough to allow high starting currents to pass. Fuses sized this large may
fail to interrupt a smaller fault. Other options would be a relay and circuit
breaker, or a magnetic overload with a time delay to allow starting currents.
For a large motor that is important to plant operation, an overcurrent relay
with a time delay to allow starting current to decay is frequently the
alternative chosen.
If the fault occurs at the power supply level at a switchgear that supplies
two of the four condensate booster pumps, and other important plant
equipment, the selective tripping scheme should isolate this switchgear to
de-energize the fault while maintaining power to the rest of the plant
electrical system. The selective trip design would include overcurrent
relays monitoring the switchgear and tripping the power supply breaker
coming into the switchgear.
Therefore, a fault at the switchgear could de-energize the switchgear,
causing the loss of two condensate booster pumps, but the plant could still
operate at 65 percent power while isolating the electrical fault to prevent or
minimize damage to important plant equipment.
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Such tripping schemes must consider all plant equipment in the same way
that we have considered condensate booster pumps here, and must consider
the impact of each to continued plant operation. However, a farsighted
tripping scheme design provides maximum plant equipment protection from
electrical faults while keeping the plant in operation.
ELO 1.2 Circuit Interrupting Devices
Introduction
Several different types of devices provide circuit protection. Fuses, relays,
circuit breakers, and overload protection devices all play a part in a
comprehensive circuit protection scheme.
Fuses
A fuse is a device that protects a circuit only from an overcurrent condition.
A fuse has a fusible link directly heated and destroyed by the current
passing through it. A fuse contains a current-carrying element sized so that
the heat generated by the flow of normal current through it does not cause
the element to melt; however, when an overcurrent or short-circuit current
flows through the fuse, the fusible link melts and opens the circuit. There
are several types of fuses in use; the figure on the next page shows common
fuse types.
Figure: Typical Types of Fuses
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The plug fuse is a fuse that consists of a zinc or alloy strip, a fusible element
enclosed in porcelain or borosilicate glass housing, and a screw base.
Circuits rated at 125 volts or less to ground normally use this type of fuse,
which has a maximum continuous current-carrying capacity of 30 amps.
The cartridge fuse consists of a zinc or alloy fusible element enclosed in a
cylindrical fiber tube with the element ends attached to a metallic contact
piece at the ends of the tube. Circuits rated from 250 volts to 600 volts
normally use this type of fuse, which has a maximum continuous currentcarrying capacity of 600 amps.
Fuses provide protection against circuit faults that lead to overcurrent
conditions only. They do not provide protection against undervoltage,
overvoltage, or underfrequency failures unless those failures lead to an
overcurrent condition.
Circuit Breaker Protective Relays
Various types of protective relays detect fault conditions then send signals
to trip one or more circuit breakers to isolate the fault, protecting equipment
from damage and personnel from injury. Sensors monitor different
parameters to provide prompt response to a fault condition while avoiding
unnecessary system interruptions.
Overload Relay Devices (also known as overcurrent)
These are the most common relay protection devices in use. Different
levels of overcurrent protection are frequently included in power systems to
provide the necessary protection without causing unnecessary system
interruptions.
Instantaneous Overcurrent
An instantaneous overcurrent relay (also known as a 50) can be set with a
high-current rating, without a time delay, as an instantaneous overcurrent
protection. If a large short or ground occurs, the current on the system
would be extremely high, and could potentially cause large-scale damage to
the electrical system.
The instantaneous overcurrent relay signals one or more circuit breakers to
open and isolate this fault. In this instance, opening with no time delay is
necessary to prevent or minimize damage.
Time Delay Overcurrent
An overcurrent relay can also be set to monitor current and allow a specific
value of current to exist for a specified amount of time before tripping.
These are called time delay relays (also known as a 51). They can be set for
either a short or long time delay depending on its application.
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Power plant and industrial systems need overcurrent delay relays to allow
for starting large loads. When a large induction motor starts, it draws a
starting current that is several times larger than its normal running current.
This starting current persists until the motor is up to its normal running
speed. High initial starting current is unavoidable; it is required to start the
motor. If the high initial starting current persists for more than a few
seconds, the motor overheats and damages the motor.
A short time delay can allow the starting current of a motor to exist for a
preset time, allowing the motor to come up to speed. At the end of this
time, the current should have decreased to normal running current, below
the overcurrent relay setting. If, after the time delay, the current is still too
high, the relay will initiate trip signals to isolate the motor and prevent
damage.
A long time delay overcurrent relay can allow light overloads to exist for a
period of time but will trip to protect overcurrent conditions caused by
smaller fault conditions, such as bearing failures or other mechanical
interferences causing more resistance and drawing more current.
Undervoltage
Large power systems use undervoltage relays (also known as a 27) because
an undervoltage condition can damage the components of the entire system.
Most large systems have many induction motors. Induction motors draw
more current to produce the same power if voltage falls below the motor's
design voltage. When one of these motors draws more current, the
increased current causes excessive heat and challenges the entire power
system.
The undervoltage relay can isolate the portion of the system that is causing
the undervoltage and protect the system from increased current flow and
potential damage.
Undervoltage relays are generally adjustable and usually set to trip a
breaker when voltage drops to 60 to 70 percent of normal value.
Underfrequency Relay
Frequency should be stable on a large grid. If it varies beyond specified
limits, the underfrequency relays (also known as an 81) will trip circuit
breakers to protect the equipment. Common under-frequency trip
components are reactor coolant pumps (RCPs). RCPs are designed to ALL
trip at the same under-frequency setpoint (assists in development of natural
circulation conditions). Any reduction in frequency (speed) results in a
reduction of mass flow rate, which is undesirable from a heat removal
standpoint.
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Lockout Relay
Lockouts are relays (also known as an 86) that prevent quick re-energizing
of a system after detection of a fault condition. When a relay detects a fault,
circuit breakers open to isolate the fault, and the relay that detected the fault
may be isolated from the fault. If that occurs, the relay no longer senses the
fault condition, and system conditions would appear to be normal (to the
relay). If the relay were to automatically reset, and the circuit breaker to
close, the fault would be reconnected to the system.
Lockout relays do not allow a signal to reset until the operator has
investigated the fault condition and manually resets the lockout.
Reverse-Power Relay
Reverse power relays (also known as 32) sense a change in the normal
direction of current indicating a change in the direction of power flow
coming from the breaker (out of the generator) to power flowing into the
breaker (into the generator). This device protects a generator from damage
due to “motoring” by tripping the generator output breakers.
Circuit Interrupting Devices
Circuit interrupting devices function for both for circuit protection in the
event of faults and for switching during normal operation of the electrical
system. Circuit breakers do not sense faults, but relays or overloads that
sense them are often contained in the same cabinet as the circuit breaker
that they signal to trip. A later section will cover circuit breakers in detail.
Thermal Overloads
Thermal overloads, shown in the figure below, consist of a heat sensitive
element and an overload heater connected in series with motor load circuit.
When motor current is excessive and sustained, heat from the breaker
thermal heater causes a bimetallic strip to deflect, activating a linkage that
opens the motor break or motor line contacts. Since it takes time for the
heat to build up, the thermal overloads (OL) have an inherent time delay.
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Figure: Three-Phase Magnetic Controller with Thermal Overloads
Magnetic Overloads
Often used to supplement thermal overloads in larger molded-case breakers,
magnetic overload devices consist of a coil connected in series with motor
load circuit. If motor current exceeds a preset value, the magnetic field
induced in the coil will move the armature and open the overload device
contact, tripping the motor breaker. Some magnetic overloads operate
instantly when motor current exceeds the tripping current, and some have
short time delays.
Resetting Overload Devices
After activation, it is necessary to reset an overload device to resume motor
operation. An operator can reset magnetic overload devices immediately
after tripping as directed by station procedures. Thermal overload relays
must cool before resetting.
There are three ways to reset an overload:
1. Manual reset — manual resets are often located in controller enclosure
that contains the overload device. The reset usually has a handoperated rod, lever, or button that returns the device-tripping
mechanism to its original position and resets interlocks.
2. Automatic reset — automatic resets typically use a spring or gravity
operated device to reset the overload device without operator action,
only after the condition that caused overload has cleared.
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3. Electrical reset — an electromagnet controlled by a push button
actuates an electrical reset. Overload devices are necessary to operate
the device remotely.
In designing the protective features for an electrical system, there are
several different options to consider. The options for overcurrent protection
are overloads, fuses and overcurrent relays that signal circuit breakers to
trip. The option chosen depends on the nature and importance of the
component.
Fuses and overloads protect only against overcurrent. They have no
capability to detect frequency, voltage, or direction of power flow.
However, overcurrent is the protection needed in most cases. Frequency
and voltage are system-wide parameters. As a result, most frequency and
voltage monitoring occurs at system-wide levels. Reverse power is only
monitored on generator output breakers. Therefore, most loads only require
overcurrent protection.
Induction motors draw high initial currents when started. Therefore,
whatever overcurrent we choose for an induction motor, it must satisfy the
following requirements:


Allow the starting current surge for several seconds.
Interrupt the current flow quickly enough to prevent or minimize
damage to the component.
 Interrupt the current flow quickly enough to contain any damage to
that component alone, and protect the rest of the electrical system.
Fuses and thermal overloads have an inherent time delay while they heat up,
but the delay varies. Time delay relays and magnetic overloads can have
precisely set time delays, so large induction motors often use these for
protection.
Smaller motors and other components frequently use fuses, because they are
relatively inexpensive, require a low level of maintenance, and are reliable.
No signal is required from another component to trip a fuse. No complex
linkage is required. It has a fusible link, which melts if too much current
passes through it, making it inherently reliable.
Many circuit breakers have overloads installed, making them an easy and
inexpensive choice.
11
Knowledge Check (Answer Key)
Fuses will detect and isolate which of the following fault
conditions:
A.
Underfrequency
B.
Overcurrent
C.
Undervoltage
D.
Phase imbalance
Knowledge Check (Answer Key)
Which one of the following breaker trip signals will trip
the associated motor breaker if a motor bearing seizes
while the motor is running?
A.
Time-delayed overcurrent
B.
Undervoltage
C.
Underfrequency
D.
Instantaneous overcurrent
Knowledge Check (Answer Key)
Which of the following protective relays would sense
and isolate a light but persistent overload condition?
A.
Overcurrent—long time delay relay
B.
Undervoltage relay
C.
Instantaneous overcurrent relay
D.
Overcurrent—short time delay relay
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ELO 1.3 Disconnect Switches
Introduction
Transfer and disconnect switches provide flexibility within an electrical
distribution system. They can change the lineup of the system or change
the power source for the loads within the system. They provide direct
visual indication that the circuit is broken.
Manual Disconnect Switches
Disconnect switches, also referred to as disconnects, are two-position
switches used for the isolation of power supplies from one or more loads or
motor control centers. When used in pairs, they facilitate the transfer of
power supply from one source to another.
Disconnects differ from breakers because disconnects are manually
operated and are not usually designed to be opened under load. Breakers
are designed to be opened under load, thereby protecting the electrical load.
The disconnect switch design does not usually include arc chutes or any
other means to extinguish the arc drawn when opened.
Before opening a disconnect switch, verify that all electrical loads fed by
the disconnect switch are off, or not operating. The figure below shows
front and side views of a typical disconnect switch.
Figure: Typical Disconnect Switch
Opening a disconnect switch under load can result in damage to the
disconnect switch and injury to personnel. The figure below shows a 480
volt disconnect switch fault. Disconnects may contain fuses which provide
overcurrent protection for the loads supplied by the disconnect switch.
Disconnect switches that are not equipped with fuses provide isolation for
the circuit only.
13
High Voltage Disconnects
High voltage disconnect switches are used in transmission lines and provide
additional electrical protection for switchyard circuits. High voltage
disconnects are not designed to operate under load. They are manually
operated usually with a hand crank or pole. Since they do not have any sort
or arc extinguishing feature they cannot carry any current when opened or
closed.
Operation is similar to the manual disconnect switch described above. High
voltage disconnects are normally opened last when de-energizing and
normally closed first when re-energizing.
In most cases, high voltage disconnects also provide visual indication that
they are opened. Since they are not enclosed in a breaker cabinet each
phase of a disconnect switch is visible as connection from the line side to
the load side is broken.
Knowledge Check (Answer Key)
Which one of the following statements describes the use
of high-voltage disconnects?
A.
Disconnects must be closed with caution when under
load because of possible arcing.
B.
Disconnects may be used to isolate transformers in an
unloaded network.
C.
Disconnects trip open like circuit breakers, but must be
manually closed.
D.
Disconnects should be limited to normal load current
interruption.
14
Knowledge Check (Answer Key)
Refer to the simplified drawing below of an electrical
distribution system showing 7.2 kilovolt (KV)
switchgear, step-down transformers, and 480 volt motor
control centers (MCCs). The high-voltage side of each
step-down transformer has a remote-operated disconnect.
The control circuit for each disconnect is positioninterlocked with the associated MCC feeder breaker.
Which one of the following describes the interlock
operating scheme that will provide the greatest protection
for the disconnect?
A.
Permits opening the disconnect only if the feeder breaker
is open.
B.
Permits opening the disconnect only if the feeder breaker
is closed.
C.
Permits opening the feeder breaker only if the disconnect
is closed.
D.
Permits opening the feeder breaker only if the disconnect
is open.
15
Knowledge Check (Answer Key)
A 480 volt AC motor control center supplies a load
through a breaker and a manual disconnect. Which one
of the following sequences will provide the greatest level
of personnel safety when de-energizing the load for
maintenance and when re-energizing the load after the
maintenance?
A.
Open breaker first (de-energizing); shut disconnect first
(re-energizing)
B.
Open disconnect first (de-energizing); shut breaker first
(re-energizing)
C.
Open breaker first (de-energizing); shut breaker first (reenergizing)
D.
Open disconnect first (de-energizing); shut disconnect
first (re-energizing)
ELO 1.4 Safety and Equipment Protection
Introduction
After completing this section, the student will be able to describe the
personnel safety and equipment protection procedures and precautions
associated with circuit interrupting devices and relays.
Safety and Equipment Protection Guidelines
Personnel should observe the following safety measures during the
operation of protective devices, circuit breakers, and switches:






Do not open a disconnect switch under load. Before opening a
disconnect switch, verify that all of the equipment the disconnect
feeds is off.
Disconnects should not be used to start and stop equipment.
Follow all precautions for working on energized equipment when
checking voltages on breakers, relays, and switches with test
equipment.
Each power station will have procedures governing the operation of
circuit interrupting devices, including required protective clothing and
equipment. Ensure that you learn and follow the procedures for
operating circuit-interrupting devices at your facility.
Do not remove or replace any fuse under load.
Never replace a fuse with one that has a different voltage or current
rating than that of the intended circuit.
16

Perform the following before racking out circuit breakers:
a. Ensure the circuit breaker is open.
b. Ensure control power is off when applicable.
c. Tag or lockout applicable electrical sources.
d. Utilize personal protective equipment as specified for the
voltage and current involved.
 Always strip loads prior to re-energizing a dead bus. Restart loads
one at a time to avoid starting currents for all loads concurrently.
Safety and Equipment Protection Example
When preparing to work on or around energized electrical equipment, there
are required precautions to make the work area safer for all concerned.
Consider a job that will require workers to work in an open switchgear
cubicle, with energized circuits inside the cubicle, in a high-traffic area of
the plant. Consider taking the following precautions:
Precautions

Have a person stand by to de-energize the equipment in the event of
an emergency
 Stand on insulating rubber material to increase the electrical resistance
of the body to ground
 Cover exposed energized circuits with insulating material to prevent
inadvertent contact
 Use insulated tools to prevent inadvertent contact with adjacent
equipment
Knowledge Check (Answer Key)
Which one of the following is an unsafe practice if
performed when working on or near energized electrical
equipment?
A.
Attach a metal strap from your body to a nearby neutral
ground to ensure that you are grounded.
B.
Use insulated tools to prevent inadvertent contact with
adjacent equipment.
C.
Cover exposed energized circuits with insulating material
to prevent inadvertent contact.
D.
Have a person standing by with the ability to remove you
from the equipment in the event of an emergency.
17
Knowledge Check (Answer Key)
A 480 volt AC motor is supplied power via an electrical
disconnect in series with a circuit breaker. Which one of
the following describes the proper operations to isolate
power to the motor?
A.
Open the disconnect first, then the breaker.
B.
Sequence is not important as long as the motor is
operating.
C.
Open the device that is closest to the power source first.
D.
Open the breaker first, then the disconnect switch.
Knowledge Check (Answer Key)
The primary reason for isolating emergency electrical
loads from their power supply bus prior to energizing the
bus via the emergency diesel generator is to prevent an...
A.
underfrequency condition on the loads.
B.
overcurrent condition on the generator.
C.
underfrequency condition on the generator.
D.
overcurrent condition on the loads.
18
ELO 1.5 Electrical Drawings
Introduction
After completion of this section, you will be able to interpret symbols for
breakers, relays, and disconnects in a simple one-line diagram, as well as
explain the operation of the control circuit.
Type
Symbol
Breakers
Trip Coil
Closing Coil
Open (A) Contact
Closed (B) Contact
Fuses
Indicating Lights
19
Type
Symbol
Overloads
Rectifier Bridge
Relays
Switches
Transformer
When using drawings, there is usually a legend on the first sheet of the
drawing series that provides a guide to symbols. Each supplier has
differences in their conventions, so the operator should review the drawing
legend when in doubt about the meaning of symbols.
20
Recall that “a” contacts are open when the controlled circuit breaker is
open, and closed when it is closed. “b” contacts are the opposite. “b”
contacts are open when the breaker is closed, and closed when the breaker is
open.
Typically, contacts in valve control circuits will be labeled, but if they are
not, then “a” contacts are open when de-energized, and closed when
energized. “b” contacts are closed when de-energized and open when
energized. The above drawing representation shows contacts in their deenergized state for both “a” and “b” contacts.
Circuit Breaker Control
To operate circuit breakers from a remote location, use an electrical control
circuit, such as the one shown below in the figure.
Figure: Breaker Control Circuit
Control power for the breaker is AC taken from the line side and then
rectified to supply direct current (DC) control power. Note that the control
power circuit is fused. The major components of the control circuit are:




Rectifier unit
Closing relay
Closing coil
Tripping coil
21


Auxiliary contacts
Circuit breaker control switch
The control circuit can incorporate protective features. Those commonly
included are:



Overcurrent
Underfrequency
Undervoltage
Protective relays sense each of these fault conditions, which close a contact
to energize the breakers trip coil, tripping the breaker. Breakers with
multiple trips will have multiple contacts in parallel, any of which can
energize the trip coil and trip the breaker.
Closing a Remotely Operated Circuit Breaker
To close the circuit breaker, the operator takes the control switch turned to
CLOSED position. This provides a current path to energize the closing
relay (CR), which closes the auxiliary contact designated on the schematic.
Closing this auxiliary contact energizes the closing coil (CC), which closes
the circuit breaker.
Once the breaker is closed, it latches in the closed position. The “b” contact
associated with the closing relay opens de-energizing the closing relay.
This prevents repeated closing attempts if the breaker trips open (antipumping feature). When the breaker closes, the “a” contact closes and
enables the trip circuit. The circuit breaker control switch returns to a
neutral position when released.
Opening a Remotely Operated Circuit Breaker
To open the circuit breaker, turn the control switch to the TRIP position.
This provides a current path to energize the trip coil (TC), which releases
the latching mechanism, and the circuit breaker trips open. When the
breaker opens, the “a” contact opens, de-energizing the trip coil. The “b”
contact closes, allowing the next remote closure when turning the control
switch to the close position.
The control circuit can incorporate:


Underfrequency
Undervoltage
Both or either energize the breaker’s trip coil, opening the breaker.
22
Valve Control Circuits
When the valve is fully open or fully shut, either one of the open or shut
lights will light while the other will be off. When the valve is midposition,
both lamps will light. This differentiates between mid-position and a loss of
control power. With loss of control power, both lamps will be off. The
figure below shows a valve open circuit. The “a” contacts are open when
the relay is de-energized. The “b” contacts are closed when the relay is deenergized.
Figure: Valve Open Circuit
Note that the valve open limit switch contact is closed, so the A1 relay is
energized. The “a” contact from the A1 relay will be energized, and closed.
The “b” contact from the A1 relay will be energized and open. Therefore,
the energized A1 relay provides a current path to light the open light, but
not the closed light. The valve shut limit switch contact is open, because
the valve is open, so the A2 relay is de-energized. The A2 relay’s “a”
contact is open, and its “b” contact is closed. This also provides a current
path to light the open lamp, and does not light the closed lamp.
When the valve is in the intermediate position, neither the open nor the shut
limit switches will trigger, so the contacts for relays A1 and A2 will be
open, and neither relay will energize. The figure below shows a valve
midposition circuit.
23
Figure: Valve Midposition Circuit
Since neither the A1 nor the A2 relay is energized, the “b” contacts for both
will be closed, and both the open and closed indicating lamps will be
illuminated.
When the valve is closed, the limit switch (LS) for the closed valve contact
will trigger, energizing relay A2. The open limit switch contact will be
open, so relay A1 will be de-energized. This results in the valve shut
indicator lighted and the valve open light being off. The figure below
shows a valve closed circuit.
Figure: Valve Closed Circuit
24
Electrical Drawings Example
Consider the following drawing:
Figure: Valve Control Circuit
Opening and Closing the Valve
As you can see from the figure above, energizing the K1 relay opens the
valve, and de-energizing the K1 relay closes the valve. To manually open
the valve, depress push button 2 (PB2), which energizes the K3 relay and
closes the K3 contact, then energizes the K1 relay. There is no other means
to open the valve through the control circuit. After the valve opens, the K1
relay has a seal in K1 contact that maintains power to the K1 relay and
keeps the valve open.
To close the valve, depress push button 1 (PB1), which interrupts power to
the K1 relay. This also causes the K1 seal in contact to open upon deenergizing the K1 relay, so the valve closes and stays closed. There is also
no other means to close the valve through the control circuitry. This is a
manually opened and closed valve with no automatic features.
Valve Response to Loss of Control Power
The K1 relay must remain energized for the valve to stay open. If the
control circuit loses power, the effect is the same as pushing PB1—an
interruption of power to the K1 relay and the valve closes. The K1 seal in
contact opens, so upon power restoration, the valve stays closed.
Alarm Light Function
As shown in the figure below, the alarm lights when the K2 time delay relay
picks up and closes the K2 contact. This occurs after energizing the K2
relay for a 10-second period. Time delay relays of this type energize, timeout, and then cause their respective contacts to close. If the relays deenergize before the time delay completes, they reset to zero.
25
The K2 time delay relay is energized when PB2 is pushed to open the valve.
This energizes the K3 relay and closes the K3 contact, energizing the K1
relay and closing the K1 seal in contact. This is important, because the K3
relay receives energy only momentarily when PB2 is depressed. If the K1
seal in relay is not closed, the K1 or K2 relays remain de-energized.
Power is supplied to the K2 time delay relay only if LS1 (limit switch) is
made up, which occurs when the valve is fully closed. The alarm
illuminates when the button is pushed to open the valve. Ten seconds later,
with an open signal still applied, the valve is fully closed. If the valve is
partially or fully open, LS1 will be open and the alarm will not sound. If
the operator depresses PB1 to close the valve, the K1 relay will de-energize
and its seal in contact will open, so the alarm will not sound.
Electrical Drawings Example
Consider the following drawing:
Figure: Control Power Circuit
Analyze the Circuit
Refer to the control power circuit above and identify the different ways the
motor could be de-energized. The control power source is from one phase
of line current at termination L1; it returns to a different phase at
termination L2.
26
To energize (or start) the motor, the START pushbutton is depressed.
Assuming the overloads are not open (activated), this energizes the MAIN
coil, which closes the main motor contacts as well as the maintaining
contact. When the START pushbutton is released, the maintaining contact
allows current to continue to flow through the circuit, keeping the main
contacts closed and the motor running. Closing of the main contacts
supplies current to the time delay coil. The time delay keeps the starting
resistors in the circuit until the motor is running and starting current
diminishes at which time the time delay contact closes and energizes the
accelerating coil. The accelerating coil closes the A contacts taking the
starting resistors out of the circuit.
Following the control power loop from termination L1, the first component
that can interrupt control power is the stop pushbutton. If the stop push
button is depressed, the maintain contact is de-energized and the line
contacts will de-energize, stopping power supply to the motor. Continuing
through the maintain contact, the next components that could de-energize
the motor are the overloads. They sense current on two phases. If either
phase senses an overcurrent condition, the overload relay will then open the
line contacts, de-energizing the motor. When the motor is stopped by either
the STOP pushbutton or an overload condition, the MAIN coil deenergizes, de-energizing the time delay coil (and opening the accelerating
contacts) and putting the starting resistors back in the circuit for the next
motor start.
27
Knowledge Check (Answer Key)
Refer to the drawing of a typical valve control circuit
below. What is the purpose of depressing the S1 push
button?
A.
To de-energize the K3 relay after the initiating condition
has cleared.
B.
To maintain the K3 relay energized after the initiating
condition has cleared.
C.
To prevent energizing the K3 relay when the initiating
condition occurs.
D.
To manually energize the K3 relay in the absence of the
initiating condition.
28

Knowledge Check (Answer Key)
Refer to the drawing of a typical valve control circuit for
a 480 volt AC motor-operated valve below. The valve is
currently open with the contact configuration as shown.
If the S1 push button is depressed, the valve will
____________ and when the S1 push button is
subsequently released, the valve will ____________.
A.
remain open; close
B.
remain open; remain open
C.
close; open
D.
close; remain closed
29
Knowledge Check (Answer Key)
Refer to the drawing of a valve control circuit below.
Note that limit switch (LS) contacts are shown open
regardless of valve position, but relay contacts are shown
open/closed according to the standard convention for
control circuit drawings. Which one of the following
describes the purpose of the alarm?
A.
Alert the operator when the valve motor circuit has been
energized for 10 seconds after push button 2 (PB2) is
depressed.
B.
Alert the operator that the valve is opening by sounding
the alarm for 10 seconds after PB2 is depressed.
C.
Alert the operator when the valve has not moved off its
closed seat within 10 seconds of depressing push button
PB2.
D.
Alert the operator if the valve has not reached full open
within 10 seconds of depressing push button PB2.
30
Knowledge Check (Answer Key)
Refer to the drawing of a valve motor control circuit
below for a valve that is currently fully open and has a
10-second stroke time. Limit switch LS2 has failed
open. Note that limit switch (LS) contacts are shown
open regardless of valve position, but relay contacts are
shown open/closed according to the standard convention
for control circuit drawings.
Which one of the following describes the valve response
if the control switch is taken to the closed position for 2
seconds and then released?
A.
The valve will begin to close and then stop moving.
B.
The valve will not move.
C.
The valve will begin to close and then open fully.
D.
The valve will close fully.
31
Knowledge Check (Answer Key)
Refer to the drawing of a motor and its control circuit
below. Note that relay contacts are shown open and
closed, according to the standard convention for control
circuit drawings.
The motor has been operating for several hours when it is
decided to stop the motor. What is the status of the
starting resistors before and after the motor STOP push
button is depressed?
A.
Initially bypassed; bypass is removed immediately after
the STOP push button is depressed.
B.
Initially inserted in the motor circuit; bypassed
immediately after the STOP push button is depressed.
C.
Initially bypassed; bypass is removed following a preset
time delay after the STOP push button is depressed.
D.
Initially inserted in the motor circuit; bypassed following
a preset time delay after the STOP push button is
depressed.
32
TLO 1 Summary
During this lesson, you learned about circuit protection principles including:
interrupting devices, protection methods, simple electric diagrams, and
safety precautions. The information below provides a summary of
information in this TLO.
1. Explain the principles and applications of circuit protection,
including selective tripping.
o Selective tripping is arranging circuit breakers, fuses, and
other protective devices in an electrical distribution
system so the device closes to a faulted component
operates first, in order to keep the largest portion of the
distribution system energized.
2. Describe the protection provided by each of the following:
o Fuse — protects a component from overcurrent.
o Protective relays — designed to protect generating
equipment and electrical circuits from any undesirable
condition, such as undervoltage, and underfrequency.
Circuit breaker — the purpose of a circuit breaker
provides a means for connecting and disconnecting
circuits of relatively high capacities without damaging
them. The three most commonly used automatic trip
features for a circuit breaker are:
 Overcurrent
 Underfrequency
 Undervoltage
3. Describe the function of the following types of switches:
o Disconnect switches — two-position switches used for
the isolation of power supplies from one or more loads or
motor control centers.
o Safety switches — low voltage (600 volts AC and below)
disconnect switches that are enclosed and may be locked
in the off position. These types of disconnects are often
used as isolation points for electrical maintenance.
4. Describe personnel safety and equipment protection precautions
associated with circuit interrupting devices and relays.
5. Interpret symbols for breakers, relays and disconnects in a
simple one-line diagram, and explain the operation of the control
circuit.
33
Now that you have completed this lesson, you should be able to describe
circuit interrupting and switching devices as well as identify appropriate
applications for each. You should also be familiar with required safety
precautions in operating electrical circuits.
1. Explain the principles and applications of circuit protection, including
selective tripping.
2. Describe the protection provided by each of the following:
a. Fuses
b. Protective relays
c. Circuit breakers
d. Overloads
3. Describe the function of the following types of switches:
a. Disconnect switch
b. Automatic transfer switch
c. Manual transfer switch
4. Describe the personnel safety and equipment protection procedures
and precautions associated with circuit interrupting devices and relays.
5. Interpret symbols for breakers, relays and disconnects in a simple oneline diagram, and explain the operation of the control circuit.
TLO 2 Circuit Breakers
Overview
Circuit breakers are the primary means of switching and circuit interruption
in power systems. Understanding how the construction, operation, and
indications for circuit breakers is essential to plant operators.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. Explain the constructions, and operation of circuit breakers, the
different types of circuit breakers and their applications, and the
protective features incorporated into circuit breakers.
2. Describe the following associated with racking out circuit breakers:
a. Purpose for racking out circuit breakers
b. Effect of racking out breakers on control and indicating circuits
c. Removal of control power on breaker operation
3. Describe the indications provided for each of the following:
a. Local circuit breaker position indications
b. Control room circuit breaker status indications
c. Circuit breaker and protective relay trip indications
4. Describe the effects of losing circuit breaker control power (to include
circuit breaker indicator lights and the ability to open and close a
circuit breaker remotely).
34
ELO 2.1 Circuit Breaker Construction and Function
Introduction
On completion of this section, you will be able to explain the construction
and functions of circuit breakers, the different types of circuit breakers and
their applications, and the protective features incorporated into circuit
breakers. Each facility has specific types and models of circuit breakers
that are unique to their location, plant specific vendor information,
operating procedures, and in some cases task qualification are required to
operate and rack out these breakers. We will review characteristics and
operation of some common breakers used in many nuclear plant facilities.
There is significant operating experience where both breaker failure and
improper operation have contributed to station events and personnel injury,
up to and including fatalities.
Circuit Breaker Description
A circuit breaker is a device that has three fundamental purposes: first,
providing circuit protection; second, performing normal switching
operations, third, isolating power from a circuit to allow safe maintenance
or repair. Circuit breaker design is flexible because designs allow any
undesirable condition to actuate a circuit breaker. For example, a circuit
breaker can automatically disconnect a circuit completely when any
abnormal condition exists.
The circuit breaker opens the circuit and stops the current flow when the
abnormal condition exceeds a predetermined value, without damaging the
circuit or the circuit breaker. Circuit breakers commonly replace fuses and
sometimes function as switches.
A circuit breaker differs from a fuse because it trips to break the circuit and
may be reset, while a fuse melts and must be replaced. Air circuit breakers
are breakers where interruption of the breaker’s contacts takes place in an
air environment. Oil circuit breakers use oil to quench the arc drawn when
the breaker contacts open.
Higher voltage and current demands led to the development of magnetic air
circuit breakers, compressed air circuit breakers, vacuum circuit breakers,
SF6 circuit breakers, and other types. All circuit breakers perform the same
three basic functions, but higher voltage and current demands require larger
and more expensive breakers.
35
Voltage in distribution systems falls into one of three groups: high-voltage,
intermediate-voltage, or low-voltage. Circuit breaker classifications use the
same three groups.
1. High-voltage is voltage that is above 15,000 volts.
2. Intermediate- or medium-voltage is voltage between 600 volts and
15,000 volts.
3. Low-voltage is voltage less than 600 volts.
Low-Voltage Air Circuit Breakers
A low-voltage circuit breaker is one that is suited for circuits rated at 600
volts or lower. One of the most commonly used low-voltage air circuit
breakers is the molded case circuit breaker below.
Figure: Molded Case Circuit Breaker
The figure below shows a cutaway view of the molded case circuit breaker.
Figure: Cutaway View of Molded Case Circuit Breaker
36
Operating a Molded Case Circuit Breaker
Manually moving the operating handle to the ON or OFF position connects
or disconnects a circuit using a circuit breaker. All breakers, with the
exception of some small ones, have a linkage between the operating handle
and the electrical contacts that allows for a quick make (quick break)
contact action, regardless of the movement speed of the operating handle.
In a short-circuit or overload condition, the design of the breaker precludes
holding the handle shut. If the circuit breaker opens under one of these
conditions, the handle will go to the trip-free position. The trip-free
position is midway between the ON and OFF positions. In this condition,
the circuit breaker moves to a midposition and cannot be closed without
moving the handle to the OFF position to reset the trip latch, and then to the
ON position to close the breaker.
Automatic Tripping
A circuit breaker automatically trips when the current through it exceeds a
predetermined value. In lower current ratings, thermal tripping devices
provide the means of automatic tripping of the circuit breaker.
Thermal Tripping Elements
Thermal trip elements consist of a bimetallic element calibrated so that the
heat from normal current through it does not cause it to deflect. An
abnormally high current, possibly caused by a short-circuit or overload
condition, will cause the element to deflect and trip the linkage that holds
the circuit breaker shut. Spring action will open the circuit breaker. The
bimetallic element, which is responsive to the heat produced by current
flowing through it, has an inverse-time characteristic. If an extremely high
current is developed, the circuit breaker will trip rapidly. For moderate
overload currents, it trips more slowly.
37
Figure: Thermal Tripping Element
Magnetic Tripping Elements
Molded case breakers with larger current ratings also have a magnetic trip
element to supplement the thermal trip element. The magnetic unit utilizes
the magnetic force surrounding the conductor to operate the circuit breaker
tripping linkage.
Arc Chutes
When the separable contacts of an air circuit breaker open, an arc develops
between the contacts. Different manufacturers use many designs and
arrangements of contacts and their surrounding chambers. The most
common design places the moving contacts inside an arc chute. The
construction of this arc chute allows the arc chute magnetically draw the arc
formed as the contacts open. When the arc enters the arc chute, it divides
into small segments, and cools. This action extinguishes the arc rapidly,
which minimizes the chance of a fire and minimizes damage to the breaker
contacts.
38
Ratings
Molded case circuit breakers come in a wide range of sizes and current
ratings. There are six frame sizes available: 100, 225, 400, 600, 800, and
2,000 amps. The size, contact rating, and current interrupting ratings are
the same for all circuit breakers of a given frame size. The trip element
rating governs the continuous current rating of a breaker. Breakers are
available in voltages from 120 to 600 volts, and interrupting capacity ranges
as high as 100,000 amps.
Large Air Circuit Breakers
Large commercial and industrial distribution systems require larger air
circuit breakers. The circuit breakers in the figure below are available in
higher continuous current and interrupting ratings than the molded case
circuit breaker. Large air circuit breakers have current ratings as high as
4,000 amps, and interrupting ratings as high as 150,000 amps.
Figure: Large Air Breaker Front
Operation
Most large air circuit breakers use a closing device, known as a stored
energy mechanism, for fast, positive closing action. Energy is stored by
compressing large powerful coil springs attached to the contact assembly of
a circuit breaker. Once these springs compress, operating a latch releases
the springs, and spring pressure shuts the circuit breaker.
Two means of compressing circuit breaker closing springs are manually or
using a small electric motor. Classification of this type of circuit breaker
depends on its spring compression method: either a manually or electrically
operated circuit breaker.
39
Figure: Closing Spring Compression Methods
When a large air circuit breaker is closed, the latch secures its operating
mechanism. Closing the circuit breaker compresses a set of tripping springs
or coils, and a trip latch serves as the trip for the circuit breaker. The trip
latch mechanism is operated by two methods: manually or remotely using a
solenoid trip coil.
Designers choose electrically operated circuit breakers when circuit
breakers operate at frequent intervals or when remote operation is required.
On tripping the electrically operated stored energy circuit breaker, the
spring charging motor recharges the spring so that the breaker is ready for
the next closing operation. Manually operated circuit breakers require
manually compressing the closing springs (usually with a hand crank)
before operating the breaker.
Pistons normally puff air across the contacts to help extinguish the arc
during breaker operation.
The following figure shows a large manually operated air circuit breaker
that is a stored energy circuit breaker. Pulling downward on the large
operating handle on the front of the breaker compresses the closing springs.
To close this circuit breaker, manually depress the small closing lever. To
trip this circuit breaker, operate the tripping lever, located at the bottom
front of the breaker.
40
Figure: Large Air Circuit Breaker
High-Voltage Circuit Breakers
Circuits with voltage ratings higher than 600 volts use high-voltage circuit
breakers, including breakers rated at intermediate voltage. Standard voltage
ratings for these circuit breakers are from 4,160 volts to 765,000 volts with
three-phase interrupting ratings of 50,000 kilo Volts Amperes (kVA) to
50,000,000 kVA.
In the early stages of electrical system development, the majority of highvoltage circuit breakers were oil circuit breakers. However, circuit breaker
development has also resulted in magnetic and compressed air types; all
three types are in use today.
Magnetic Air Circuit Breaker
Ratings for magnetic air circuit breakers go up to 750,000 kVA at 13,800
volts. This type of circuit breaker interrupts in air between two separable
contacts with the aid of magnetic blowout coils.
As the current-carrying contacts separate during a fault condition shown on
the following page, magnets draw the arc out horizontally and transfer it to
a set of arcing contacts. Simultaneously, the blowout coil provides a
magnetic field to draw the arc upward into the arc chutes. The arc, aided by
the blowout coil magnetic field and thermal effects, accelerates upward into
the arc chute, where it is elongated and divided into many small segments.
41
Figure: Magnetic Air Circuit Breaker
While the construction of this type of circuit breaker is similar to that of a
large air circuit breaker used for low-voltage applications, electricity
operates the magnetic air circuit breakers.
Compressed-Air Circuit Breakers
Compressed-air circuit breakers, or air-blast circuit breakers, depend on a
stream of compressed air directed toward the separable contacts of the
breaker to interrupt the arc formed when opening the breaker. Recent
developments produced air-blast circuit breakers used in extra high-voltage
applications with standard ratings up to 765,000 volts.
Figure: Compressed Air Arc Shute
42
Oil Circuit Breakers
Oil circuit breakers, shown in the figure below, are circuit breakers that
have their contacts immersed in oil. Current interruption takes place in oil,
which cools the arc developed and thereby quenches the arc. The poles of
small oil circuit breakers can be in one oil tank; however, the large highvoltage circuit breakers have each pole in a separate oil tank. The oil tanks
in oil circuit breakers are normally sealed. The electrical connections
between the contacts and external circuits are through porcelain bushings.
Figure: Oil Circuit Breaker
GE Magne-Blast Breaker
The GE Magne-Blast breaker, below, is a medium voltage breaker that is
widely used in power plant switchgear application. The early designs were
an air circuit breaker, with a solenoid-operated mechanism; while the latest
designs were outfitted with vacuum type contacts, which can be retrofitted
to the earlier design breakers.
A mechanical counter, visible on the breaker’s front window and primarily
used for maintenance, indicates the number of open and closed cycles on
the breaker. The breaker manually closes or trips at the switchgear.
Figure: GE Magne-Blast Breaker
43
The Magne-Blast Breakers are also manufactured in a horizontal draw out
design. The chassis contains the primary contact assembly and bushings,
interlocks, and ground strap. The primary contact assembly is the main
current-carrying part of the breaker. The assembly consists of all the
barriers, arc chutes, and air puffer system.
The charging motor charges the closing spring(s). The charging motor is
located below the operating mechanism on the front left side and connected
to a drive fitting and levers to the ratchet wheel. Some breaker designs
include a manual bar to charge the closing springs locally at the breaker.
The spring discharge interlock discharges the closing springs if they are
charged when the breaker is rolled in or out of the switchgear cubicle.
Local Circuit Breaker Operation
Some circuit breakers are designed for local rather than remote operation
and may be designed to be operated either manually or electrically.
Electrically operated breakers are usually located in associated switchgear,
are provided with a local breaker control switch that uses breaker control
power for operation.
Figure: Circuit Breaker
Electrically Operated Breakers
Electrically opened and closed locally operated breakers have a local
breaker control switch. The control switch uses breaker control power to
operate the circuit breaker (electrically) that is normally located in the
associated switchgear or motor control center (MCC). The control switch
has three positions including, TRIP, midposition, and CLOSED. When
taken to TRIP, the breaker opens.
Midposition is the normal resting position of the control switch. After
operation and release, a spring returns the breaker switch to the midposition.
44
When taken to CLOSED, the breaker closes, depending on satisfaction of
interlock(s) conditions.
Electrically operated circuit breakers have logic circuitry, which
automatically checks that breaker interlocks are satisfied before allowing
the circuit breaker to close. On a loss of control power, this type of breaker
will fail as is. The breaker control switch may also have a PULL OUT
(Pull-to-Lock) position that prevents the automatic closing of the breaker.
The PULL OUT feature is standard with this type of control switch and
does not imply that the breaker has any automatic closing features.
Indicating lights show position of the breaker as either closed (red indicator
illuminated) or open (green indicator illuminated).
Manually Operated Breakers
Some MCC or load center circuit breakers do not have the capability of
electrical operation. The TRIP and a CLOSE push buttons located on the
front of the breaker allow manual operation of these breakers. These
breakers do not have circuit breaker control switches or position indicating
lights like the electrically operated breakers, nor do they have logic circuits
to check for interlocks.
Local-Remote Transfer Switches
Some remotely operated breakers may include a control power transfer
switch, which allows switching breaker operation between LOCAL and
REMOTE. Selecting LOCAL restricts the breaker to only local operation.
Selecting REMOTE enables operation of the breaker from a remote location
such as the control room. In both positions, control power is available for
automatic protective trips.
Protective Relays
Design of a circuit breaker control circuit incorporates any of a number of
protective features. The list below includes the most commonly used
automatic trip features for a circuit breaker. If any one of the conditions
exists while the circuit breaker is closed, it closes its associated contact and
energizes the tripping coil, which, in turn, trips the circuit breaker.
Undervoltage relay — an adjustable device generally set to trip the
breaker when voltage drops to 60 to 70 percent of normal value.
 Underfrequency relay — an adjustable device that will trip the breaker
when frequency drops below a preset value to protect the loads on a
system that cannot tolerate a significant change in frequency.

45






Reverse-Power relay - this device senses a change in the normal
direction of current indicating an abnormal condition in the system.
This is actually a change in the direction of power flow from power
coming out of the breaker to power flowing into the breaker. This
device protects a generator from damage from motoring by tripping
the generator output breaker.
Trip relay - this device releases the breaker operating mechanism and
allows it to open when the appropriate conditions exist or upon
receiving either an automatic or manual signal. This device is
normally de-energized but requires energizing to activate the trip
latch.
Overload relay devices (also known as overcurrent devices) - a circuit
breaker must be able to detect and react to above normal currents as
well as short-circuit currents. By providing the breaker with three
over current tripping devices, the breaker can sense and act on three
values of current. This provides the breaker with long-time, shorttime, and instantaneous tripping abilities. The three trips all sense the
same current; however, they each react differently to this current.
Long-Time trip - device reacts to light overloads to trip the breaker
after a time.
Short-Time trip - device reacts to a slightly higher current and trips the
breaker in less time than a long-time trip.
Instantaneous trip - device quickly reacts to trip the breaker due to
high-currents produced by short-circuits.
Knowledge Check (Answer Key)
A typical 120 volt AC manual circuit breaker has tripped
due to overload. To close the circuit breaker, move the
breaker handle from the...
A.
midposition to the OFF position to reset the trip latch,
and then to the ON position.
B.
OFF position directly to the ON position; trip latch reset
is not required.
C.
midposition directly to the ON position; trip latch reset is
not required.
D.
OFF position to the midposition to reset the trip latch,
and then to the ON position.
46
Knowledge Check (Answer Key)
How is typical breaker operation affected when the
associated breaker control power transfer switch is
placed in the LOCAL position?
A.
Control power will be available to provide protective
trips, and the breaker can be electrically operated only
from the control room.
B.
Control power will be removed from both the open and
close circuits, and the breaker can be electrically
operated only from the control room.
C.
Control power will be available to provide protective
trips, and the breaker can be electrically operated only
from the breaker cabinet.
D.
Control power will be removed from both the open and
close circuits, and the breaker can be electrically
operated only from the breaker cabinet.
Knowledge Check (Answer Key)
Which one of the following would cause a loss of ability
to remotely trip a circuit breaker and a loss of remote
breaker position indication?
A.
Failure of the breaker control switch
B.
Mechanical binding of the breaker tripping bar
C.
Loss of control power for the breaker
D.
Racking the breaker to the test position
47
Knowledge Check (Answer Key)
Loss of breaker control power will cause...
A.
inability to operate the breaker locally and remotely.
B.
failure of the closing spring to charge following local
closing of the breaker.
C.
breaker line voltage to indicate zero regardless of actual
breaker position.
D.
the remote breaker position to indicate open regardless of
actual breaker position.
ELO 2.2 Racking Circuit Breakers
Introduction
After completing this section, you will be able to describe the following
associated with racking circuit breakers:



Purpose of racking out circuit breakers
Effect of racking out breakers on control and indicating circuits
Effect of removing control power on breaker operation
Circuit Breaker Racking
It is possible to rack circuit breakers to three different positions, each of
which provides specific features needed to support power plant operation
and maintenance. The three positions are connect (racked in), disconnect
(racked out), and the test position.
Connect (Racked In)
Racked in (connected) is the normal position. In this position, there are two
breaker options. The first option closed supplies power to the load. The
second option opened isolates the load. Essentially, in the racked in
position, the circuit breaker is performing its design function.
Disconnect (Racked Out)
When in the racked out position, the circuit breaker will not supply power
to the load. Set a circuit breaker to the racked out position to de-energize
the load, allowing safe working conditions. Note that racking out alone
may not completely de-energize the load. Usually, control power must also
be isolated.
48
Test
When in the racked to test position, the breaker will not supply the load, but
receives control power, which allows opening and closing the breaker to
test the breaker itself.
Large ACB Racking Operation
To de-energize an electrical component as well as its associated control and
indication circuits completely, rack out the component breaker and remove
control power fuses. There are many types of circuit breakers. The
following circuit breaker racking discussion is general in nature.
Draw Out
You can disconnect circuit breakers by moving the breakers physically
away from the bus, see below figure. Perform this with a racking tool that
is manually connected to the breaker.
Figure: Draw Out Breaker
The racking tool may be a manual device, but some plants have breaker
racking tools that move the breaker with the operator at a distance, see
below figure.
The breaker moves from the CONNECT position to the TEST position, and
then to the DISCONNECT position. Plants usually provide interlocks to
prevent operation of the circuit breaker at any point between these three
discrete positions.
49
Figure: Breaker Racking Tool
The interlocks also mechanically control the breaker tripping mechanism.
That is, when the racking operation begins, a mechanism trips the breaker
mechanically, preventing closing of the breaker. Movement of the breaker
releases the trip mechanism.
Ensure that plant procedures and required personal protective equipment
(PPE) are used during breaker racking operation. Accidents when racking
breakers can be fatal!
Figure: Racking Arc Blast
50
A separate connecting device supplies control power to the circuit breaker.
This device usually consists of a sliding contact strip with one part mounted
on the back of the cubicle at the floor and the other part mounted on the
circuit breaker. When racking the breaker back in, contact is made between
the two parts when the circuit breaker is moved to the TEST position (from
DISCONNECT) and continues as the breaker is moved to the CONNECT
position.
Consequently, the status light indication observed in the control room or on
the front of the breaker is not a conclusive indication of breaker position or
operability. To perform a verification of breaker position and operability,
verify all four of the following items:

Indication on the floor of the circuit breaker housing corresponds to
the markings on the circuit breaker.
 Racking release lever is fully in the CONNECT position (extreme
counterclockwise position).
 The closing spring motor toggle switch is in the ON position and the
closing spring is charged (or fuses installed).
 The control power breaker is closed and/or control power fuses are in
place (not blown) ensuring power is available.
Figure: Breaker Racked Out
51
Knowledge Check (Answer Key)
To completely de-energize an electrical component and
its associated control and indication circuits, the
component breaker should be...
A.
open with the control switch tagged in the open position.
B.
racked out with control power fuses removed.
C.
open with the control switch in pull-to-lock.
D.
racked out and tagged in racked-out position.
Knowledge Check (Answer Key)
When a typical 4,160-volt breaker is racked to the test
position, control power is __________ the breaker and
the breaker is __________ the load.
A.
removed from; isolated from
B.
available to; isolated from
C.
removed from; connected to
D.
available to; connected to
ELO 2.3 Circuit Breaker Indications
Introduction
After completing this section, the student will be able to describe the
indications provided for each of the following:
1. Local circuit breaker position indications
2. Control room circuit breaker status indications
3. Circuit breaker and protective relay trip indications
Circuit Breaker Position Verification
A number of means can determine circuit breaker status and physical
position: They include:
 The breaker’s OPEN/CLOSE mechanical indicators
 Breaker position indication lights (control power)
 Load-side voltage/current
 Physical breaker position
52
All of these can be used to positively determine breaker position.
Control Indications
1. Breaker mechanical indicator’s status of OPEN or CLOSED.
2. Breaker position indication lights:
a. Red lamp illuminated indicates closed.
b. Green lamp illuminated indicates open.
c. Amber lamp illuminated (if supplied), indicates a mismatch
between breaker position and the last position placement of the
breaker control switch. If the last manual positioning of the
control switch was to close the breaker, and the breaker is now
open, this indicates a mismatch (probably a protective trip of the
breaker), and the amber lamp should be illuminated. The
breaker control switch has red and green flag indicators to
indicate the last position placement of the switch. The red flag
indicates that the last breaker position was CLOSED. The green
flag indicates that the last breaker position was OPEN.
d. All lamps out indicates a loss of control power to breaker.
Load-Side Voltage and Current
Voltage indicated on the load side verifies that the breaker is closed. No
voltage indicates the breaker is open, but double-check that the voltmeter is
reading accurately.
Current indication on the load side indicates breaker is closed, but no
current could indicate that breaker is in the closed position with no load, or
the breaker is open. No current is an inconclusive indication.
Circuit Breaker Trip Flags
Personnel may observe an automatic circuit breaker trip by a local device
(relay) protection flag indication. Once a circuit breaker protective trip
occurs, the trip coil energizes causing the circuit breaker to trip open. A
local mechanical flag indicates actuation of a circuit breaker protective trip
device. A mechanical flag for each individual device indicates multiple
protective device trips.
Conditions such as overcurrent, undervoltage, underfrequency, and reverse
power usually have protective devices. If activation of any of these
protective devices causes the circuit breaker to trip, personnel must reset the
associated local mechanical trip flags manually once the condition clears.
Operator Responsibilities
An operator should use all available indications to determine the actual
condition of a circuit breaker. The best indications to use are those that are
solely dependent on breaker position like load side voltmeter readings and
the local OPEN/CLOSED mechanical flags.
53
Protective devices (relays) are less conclusive indicators of breaker position
indication because a breaker may indicate tripped by a protective trip
mechanical flag but may actually be closed. It is necessary to reset a
protective relay manually once the condition that caused it is clear. If not
reset, the trip flag may still indicate tripped from the previous trip, yet the
circuit breaker is closed.
Circuit breaker position indicating lights are also less conclusive indicators
of the breaker’s actual position. For example, without breaker control
power, a breaker may indicate open (red light not illuminated) when in
reality it is closed. A burned-out indicating lamp may indicate a condition
that is incorrect.
Likewise, load side ammeter readings are not a conclusive indication of
breaker position, because the breaker can be in the closed position with no
load (zero [0] amps) indicated. If load side amps are greater than zero (0), it
is an indication that the breaker is in the closed position and loaded.
Using all of these indicators together allows the operator to establish
breaker position accurately.
Knowledge Check (Answer Key)
Which one of the following describes the normal
operation of a local breaker overcurrent trip flag
indicator?
A.
Actuates when a breaker overcurrent trip has occurred;
can be manually reset when the overcurrent condition
clears.
B.
Actuates when a breaker has failed to trip on an
overcurrent condition; can be manually reset when the
overcurrent condition clears.
C.
Actuates to cause a breaker trip when the overcurrent trip
setpoint is reached; can be remotely reset when the
overcurrent condition clears.
D.
Actuates when no lockout is present; satisfies an
electrical interlock to remotely close a breaker.
54
Knowledge Check (Answer Key)
Breaker local overcurrent trip flag indicators, when
actuated, indicate that...
A.
the associated breaker has failed to trip open during an
overcurrent condition.
B.
a breaker overcurrent condition is responsible for a
breaker trip.
C.
an overcurrent condition has cleared and the breaker can
be closed.
D.
a breaker trip will occur unless current is reduced.
Knowledge Check (Answer Key)
Given the following indications for an open 4,160 volt
AC breaker:




The local OPEN/CLOSED mechanical flag
indicates open.
A breaker overcurrent trip flag is actuated on one
phase.
The line-side voltmeter indicates 4,160 volt AC.
The load-side voltmeter indicates zero (0) volts.
Assuming no operator actions were taken since the
breaker opened, which one of the following could have
caused the breaker to open?
A.
An operator opened the breaker locally.
B.
An operator opened the breaker from a remote location.
C.
A ground fault caused an automatic breaker trip.
D.
A loss of control power caused an automatic breaker trip.
55
Knowledge Check (Answer Key)
The following remote indications are observed for a 480
volt AC load center supply breaker. (The breaker is
normally open.)




Red indicating light is on.
Green indicating light is off.
Load center voltage indicates zero (0) volts.
Breaker incoming voltage indicates 480 volts.
What is the condition of the breaker?
A.
Open and racked in
B.
Closed and racked to test position
C.
Open and racked to test position
D.
Closed and racked in
Knowledge Check (Answer Key)
The following indications are observed for a motor
breaker in the control room:



Red position indicating light is off.
Green position indicating light is off.
Load amps indicate normal load current.
Assuming one of the indicating lights is burned out, what
is the condition of the breaker?
A.
Open and racked in
B.
Closed and racked to test position
C.
Open and racked to test position
D.
Closed and racked in
56
Knowledge Check (Answer Key)
The following indications are observed in the control
room for a normally-open breaker that directly
starts/stops a 480 volt AC motor:




Red position indicating light is on.
Green position indicating light is off.
Load current indicates 50 amps.
Supply voltage indicates 480 volts.
What is the condition of the breaker?
A.
Closed and racked in
B.
Open and racked to test position
C.
Open and racked in
D.
Closed and racked to test position
Knowledge Check (Answer Key)
While remotely investigating the condition of a
normally-open 480 volt AC motor control center (MCC)
feeder breaker, an operator observes the following
indications:




Green breaker position indicating light is out.
Red breaker position indicating light is
illuminated.
MCC voltmeter indicates 480 volt AC.
MCC ammeter indicates zero (0) amperes.
Based on these indications, the operator should report
that the feeder breaker is __________ and racked
__________.
A.
closed; to the test position
B.
closed; in
C.
open; in
D.
open; to the test position
57
ELO 2.4 Circuit Breaker Control Power
Introduction
After completing this section, the student will be able to describe the effects
of losing circuit breaker control power, including circuit breaker indicator
lights and the ability to open and close a circuit breaker remotely.
Circuit Breaker Control Power
An electrical control circuit must be incorporated to operate circuit breakers
remotely, or by using the control switch at the breaker. Any source can
supply control power, but the typical source is AC supplied from the
breaker line side, rectified to DC for the control circuit. The power source
is usually fused and sometimes has switches in the circuit to isolate control
power.
The figure below shows a typical breaker control circuit:
Figure: Breaker Control Circuit
Major Components of a Control Circuit
The major components of the control circuit are:
1.
2.
3.
4.
5.
Rectifier unit
Closing relay
Closing coil
Trip coil
Auxiliary contacts
58
6.
Breaker control switch
Control circuits include their own protective features. The circuit shown
has contacts driven by underfrequency and undervoltage relays. If either of
these relays energizes, its contact in this control circuit will close,
energizing the trip coil, and tripping the circuit breaker.
To close the breaker, position the control switch to CLOSED. This
energizes the closing relay, which closes a contact energizing the closing
coil, and shuts the breaker. When the breaker closes, the “b” contact opens,
de-energizing the closing relay, which then de-energizes the closing coil.
The “a” contact closes, enabling the trip coil to energize, either through the
control switch, or through protective relay contacts.
To open the breaker, position the control switch to TRIP. This energizes
the trip coil, releasing the latch and allowing the breaker to open. After the
breaker opens, the “a” contact opens, de-energizing the trip coil. The “b”
contact closes, allowing the control switch to energize the closing relay,
preparing for the next closure.
When breaker control power is lost, the following capabilities are lost:
1. Local and remote breaker indication lamps are out.
2. Remote breaker control (open/close) is lost.
3. Breaker closing springs will not recharge after a local closing of the
breaker because the charging motor does not have power.
4. Capability to trip open on a protective relay trip is lost (can still
open breaker locally/manually).
Note that overloads or fuses, if provided, are still functional in an overload
condition. They are not dependent on control power.
Knowledge Check (Answer Key)
Loss of breaker control power will cause...
A.
the remote breaker position to indicate open regardless of
actual breaker position.
B.
breaker line voltage to indicate zero regardless of actual
breaker position.
C.
failure of the closing spring to charge following local
closing of the breaker.
D.
inability to operate the breaker locally and remotely.
59
Knowledge Check (Answer Key)
Which one of the following would cause a loss of ability
to remotely trip a circuit breaker and a loss of remote
breaker position indication?
A.
Racking the breaker to the test position
B.
Loss of control power for the breaker
C.
Failure of the breaker control switch
D.
Mechanical binding of the breaker tripping bar
Knowledge Check (Answer Key)
Which one of the following will cause a loss of
indication from the remote breaker position indicating
lights associated with a typical 480 VAC load supply
breaker?
A.
Removing the breaker control power fuses
B.
Locally opening the breaker
C.
Burnout of the local breaker position indicating lights
D.
Loss of breaker line voltage
60
Knowledge Check (Answer Key)
The following indications exist for an open 4,160 volt
AC breaker:




All phase overcurrent trip flags are reset.
The control power fuses indicate blown.
The line-side voltmeter indicates 4,160 volt AC.
The load-side voltmeter indicates zero (0) volts.
Assuming no operator actions occurred since the breaker
opened, which one of the following could have caused
the breaker to open?
A.
A ground fault caused an automatic breaker trip.
B.
An operator tripped the breaker manually at the breaker
cabinet.
C.
An operator tripped the breaker manually from a remote
location.
D.
A loss of control power caused an automatic breaker trip.
TLO 2 Summary
In this section, you learned the functions of circuit breakers, how to operate
them, their indications, and the effects of losing control power.
Circuit breakers are multi-use components. They provide circuit protection,
as well as the normal means of switching and for isolation of components
for maintenance.
Now that you have completed this lesson, you should be able to explain the
operation modes of circuit breakers, determine the position of a breaker
from the indications available, and predict the impact of losing control
power or racking a circuit breaker to different positions.
Specifically you should be able to do the following:
1. Explain the construction, functions, and operation of circuit breakers,
the different types of circuit breakers and their applications, and the
protective features incorporated into circuit breakers.
2. Describe the following associated with racking out circuit breakers:
a. Purpose for racking out circuit breakers
b. Effect of racking out breakers on control and indicating circuits
c. Removal of control power on breaker operation
3. Describe the indications provided for each of the following:
a. Local circuit breaker position indications
61
b. Control room circuit breaker status indications
c. Circuit breaker and protective relay trip indications
4. Describe the effects of losing circuit breaker control power on breaker
operation and indications.
TLO 3 Paralleling AC Sources
Overview
Describe the required conditions prior to paralleling two generators,
including the effects of not meeting these conditions.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. Describe the conditions required to properly parallel two AC power
sources, including the following:
a. Voltage
b. Frequency
c. Phase
2. Describe the effects of paralleling two AC sources under the following
conditions:
a. Current out of phase
b. Frequencies not matched
c. High-voltage differential
d. Low-current or too much load
ELO 3.1 Paralleling AC Sources
Introduction
Most electrical power grids and distribution systems have more than one
AC generator operating at one time. Normally, plants operate two or more
generators in parallel to increase the available power. Understanding the
proper method of paralleling AC sources is important to proper and
sustained plant operation. Closing a disconnect switch or overriding and
closing a breaker without first isolating electrical loads or proper
synchronization can result in equipment damage and pose an extreme
hazard to personnel.
After completing this section, the student will be able to describe the
conditions required to parallel two AC power sources properly, including
the following three items, voltage, frequency, and phase.
62
Paralleling AC Sources
Generally, there are three required prior to paralleling or synchronizing
multiple AC sources.

Terminal voltages must be almost equal. This minimizes VAR
loading.
 Frequencies must be almost equal. Frequency of the incoming
machine slightly higher than the grid (synchroscope rotating slowly in
the fast direction). This ensures that the incoming machine picks up
some load rather than becoming a load (motorizing).
 Output voltages must be in phase to minimize current surge through
the breaker
During paralleling operations, voltages of the generators to be paralleled
are displayed on voltmeters and frequency matching is accomplished
using a synchroscope that senses the frequencies of each generator and
allows matching of them.
Figure: Synchroscope
Once the breaker is closed, the synchroscope will lock in at 12 o’clock
indicating that the two power sources are running in parallel.
Most plants have “sync check relays” that prevent closing the breakers
while out of phase while paralleling to the grid.
63
Knowledge Check (Answer Key)
A main generator is about to be connected to an infinite
power grid. If personnel close the generator output
breaker with generator and grid voltages matched, but
with generator frequency 0.1 Hertz (Hz) higher than grid
frequency, this will initially result in the generator...
A.
picking up a portion of the grid real load.
B.
experiencing overspeed conditions.
C.
experiencing reverse power conditions.
D.
picking up a portion of the grid reactive load.
Knowledge Check (Answer Key)
A main generator is being paralleled to the power grid.
Generator voltage has been properly adjusted and the
synchroscope is rotating slowly in the clockwise
direction.
The generator breaker must be closed just as the
synchroscope pointer reaches the 12 o'clock position to
prevent...
A.
motoring of the generator, due to unequal frequencies.
B.
excessive megawatt (MW) load transfer to the generator,
due to unequal frequencies.
C.
excessive MW load transfer to the generator, due to outof-phase voltages.
D.
excessive arcing within the generator output breaker, due
to out-of-phase voltages.
64
ELO 3.2 Abnormal Conditions During Paralleling Operations
Introduction to Abnormal Conditions During Paralleling
Operations
After completing this section, you will be able to describe the effects of
paralleling two AC sources under the following conditions:
1.
2.
3.
4.
Voltages not matched
Improper frequency relationship
Out of phase
Improper generator loading
Remember there are three required conditions prior to paralleling or
synchronizing AC sources. Failure to meet these conditions can result in
adverse equipment operation and continuity of operation.
Voltages Not Matched
If the voltages of the generator and grid are not close to equal when the
breaker is closed, the generator will either pick up reactive load or act as a
reactive load. This causes additional current load on the generator as well
as through its output breaker when closing.
Voltages of the running and incoming sources should be matched (some
plants allow incoming voltage to be slightly higher). Once the generator is
loaded, the voltage setting can be adjusted to carry the desire VAR loading.
Improper Frequency Relationship
The frequency of the incoming machine (generator) should be slightly
higher than the running machine (grid). This is indicated by the
synchroscope rotating “slowly” in the “clockwise” direction. This ensures
that a small amount of load is picked up by the generator. If the generator
frequency was less than grid frequency, the generator output breaker could
trip on reverse power since the generator would be acting like a motor. The
amount of real load (watts) transfer is relative to the frequency difference
between the generator and grid.
Usually plant procedures have the operator go to RAISE on speed control as
soon as the breaker is closed to pick up some additional load. This also
minimizes the possibility of motoring the generator.
Voltages Out of Phase
A mismatch in the phases causes development of large opposing voltages in
the two sources. The worst case mismatch would be 180 degrees out of
phase, resulting in an opposing voltage between the two generators of twice
the output voltage. This high-voltage can cause damage to the distribution
system due to extremely high-currents and large mechanical torque exerted
on both of the generators.
65
The greater the phase mismatch, the greater the damage that will occur to
the generator output breaker because of excessive arcing when the circuit
breaker is closed.
Synchronizing (closing the incoming generator output breaker) should
occur as the synchroscope passes through 12 o’clock while moving slowly
in the “fast” direction. If the breaker is closed out of phase, the current surge
could damage the breaker or the generator. The sync check relays usually
allow closure between 11 and 1 o’clock (between ~ +30 degrees and -30
degrees phase differential). Anything greater than 30 degrees could be
damaging.
Improper Generator Loading
Normally overloading a generator when paralleling is not a concern because
the minimum load picked up is a function of the incoming machine being
slightly greater then grid frequency. If the synchroscope was moving FAST
in the CLOSKWISE direction larger amounts of current will carried
through the breaker when closing.
Overloading of a generator when supplying a bus could occur if the
generator output breaker was being closed onto a de-energized bus without
first opening the breakers on that bus. This condition is prevented when the
diesel generator is started and tied to the vital bus on a loss of all AC by:

shedding all the loads on the bus on the loss of power

allowing the diesel generator to come up to speed

closing the generator output breaker

sequencing the loads onto the bus
This allows the starting currents of one load to diminish before another
load is brought online.
66
Knowledge Check (Answer Key)
Consider paralleling a main generator to the power grid.
Assume that generator voltage equals the grid voltage
and the synchroscope is rotating slowly in the clockwise
direction.
Close the generator breaker just as the synchroscope
pointer reaches the 12 o'clock position to prevent...
A.
excessive megawatt (MW) load transfer to the generator,
due to out-of-phase voltages.
B.
excessive arcing within the generator output breaker, due
to out-of-phase voltages.
C.
motoring of the generator, due to unequal frequencies.
D.
excessive MW load transfer to the generator, due to
unequal frequencies.
Knowledge Check (Answer Key)
Closing the output breaker of a three-phase generator
onto a de-energized bus can...
A.
produce an overcurrent condition on the generator if the
bus was not first unloaded.
B.
produce an overvoltage condition on the bus.
C.
result in a reverse power trip of the generator circuit
breaker if generator frequency is low.
D.
result in large reactive currents in the generator.
67
TLO 3 Summary
Now that you have completed this lesson, you should be able to do the
following:
1. Describe the conditions required to properly parallel two AC power
sources, including the following:
a. Voltage
b. Frequency
c. Phase
2. Describe the effects of paralleling two AC sources under the following
conditions:
a. Current out of phase
b. Frequencies not matched
c. High-voltage differential
d. Low-current or too much load
Breakers, Relays, and Disconnects Summary
This module covered circuit protection devices including breakers, relays,
and disconnects, their applications and advantages, proper methods for
operation, and system responses.
After completing this training session, the trainee will demonstrate mastery
of this topic by passing a written exam with a grade of 80 percent or higher
on the following TLOs:
1. Explain the purpose, safety precautions, and operation of electrical
circuit interrupting and circuit switching devices.
2. Explain the construction, operation, and indications for electrical
circuit breakers.
3. Describe the conditions that must be met prior to paralleling two
generators including effects of not meeting these conditions.
68
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check Answer Key
ELO 1.2 Circuit Interrupting Devices
Knowledge Check
Fuses will detect and isolate which of the following fault
conditions:
A.
Underfrequency
B.
Overcurrent
C.
Undervoltage
D.
Phase imbalance
Knowledge Check
Which one of the following breaker trip signals will trip
the associated motor breaker if a motor bearing seizes
while the motor is running?
A.
Time-delayed overcurrent
B.
Undervoltage
C.
Underfrequency
D.
Instantaneous overcurrent
Knowledge Check
Which of the following protective relays would sense
and isolate a light but persistent overload condition?
A.
Overcurrent—long time delay relay
B.
Undervoltage relay
C.
Instantaneous overcurrent relay
1
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
D.
Overcurrent—short time delay relay
ELO 1.3 Transfer and Disconnect Switches
Knowledge Check
Which one of the following statements describes the use
of high-voltage disconnects?
A.
Disconnects must be closed with caution when under
load because of possible arcing.
B.
Disconnects may be used to isolate transformers in an
unloaded network.
C.
Disconnects trip open like circuit breakers, but must be
manually closed.
D.
Disconnects should be limited to normal load current
interruption.
2
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
Refer to the simplified drawing below of an electrical
distribution system showing 7.2 kilovolt (KV)
switchgear, step-down transformers, and 480 volt motor
control centers (MCCs). The high-voltage side of each
step-down transformer has a remote-operated disconnect.
The control circuit for each disconnect is positioninterlocked with the associated MCC feeder breaker.
Which one of the following describes the interlock
operating scheme that will provide the greatest protection
for the disconnect?
A.
Permits opening the disconnect only if the feeder breaker
is open.
B.
Permits opening the disconnect only if the feeder breaker
is closed.
C.
Permits opening the feeder breaker only if the disconnect
is closed.
D.
Permits opening the feeder breaker only if the disconnect
is open.
3
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
A 480 volt AC motor control center supplies a load
through a breaker and a manual disconnect. Which one
of the following sequences will provide the greatest level
of personnel safety when de-energizing the load for
maintenance and when re-energizing the load after the
maintenance?
A.
Open breaker first (de-energizing); shut disconnect first
(re-energizing)
B.
Open disconnect first (de-energizing); shut breaker first
(re-energizing)
C.
Open breaker first (de-energizing); shut breaker first (reenergizing)
D.
Open disconnect first (de-energizing); shut disconnect
first (re-energizing)
ELO 1.4 Safety and Equipment Protection
Knowledge Check
Which one of the following is an unsafe practice if
performed when working on or near energized electrical
equipment?
A.
Attach a metal strap from your body to a nearby neutral
ground to ensure that you are grounded.
B.
Use insulated tools to prevent inadvertent contact with
adjacent equipment.
C.
Cover exposed energized circuits with insulating material
to prevent inadvertent contact.
D.
Have a person standing by with the ability to remove you
from the equipment in the event of an emergency.
4
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
A 480 volt AC motor is supplied power via an electrical
disconnect in series with a circuit breaker. Which one of
the following describes the proper operations to isolate
power to the motor?
A.
Open the disconnect first, then the breaker.
B.
Sequence is not important as long as the motor is
operating.
C.
Open the device that is closest to the power source first.
D.
Open the breaker first, then the disconnect switch.
Knowledge Check
The primary reason for isolating emergency electrical
loads from their power supply bus prior to energizing the
bus via the emergency diesel generator is to prevent an...
A.
underfrequency condition on the loads.
B.
overcurrent condition on the generator.
C.
underfrequency condition on the generator.
D.
overcurrent condition on the loads.
5
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
ELO 1.5 Electrical Drawings
Knowledge Check
Refer to the drawing of a typical valve control circuit
below. What is the purpose of depressing the S1 push
button?
A.
To de-energize the K3 relay after the initiating condition
has cleared.
B.
To maintain the K3 relay energized after the initiating
condition has cleared.
C.
To prevent energizing the K3 relay when the initiating
condition occurs.
D.
To manually energize the K3 relay in the absence of the
initiating condition.
6
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
Refer to the drawing of a typical valve control circuit for
a 480 volt AC motor-operated valve below. The valve is
currently open with the contact configuration as shown.
If the S1 push button is depressed, the valve will
____________ and when the S1 push button is
subsequently released, the valve will ____________.
A.
remain open; close
B.
remain open; remain open
C.
close; open
D.
close; remain closed
7
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
Refer to the drawing of a valve control circuit below.
Note that limit switch (LS) contacts are shown open
regardless of valve position, but relay contacts are shown
open/closed according to the standard convention for
control circuit drawings. Which one of the following
describes the purpose of the alarm?
A.
Alert the operator when the valve motor circuit has been
energized for 10 seconds after push button 2 (PB2) is
depressed.
B.
Alert the operator that the valve is opening by sounding
the alarm for 10 seconds after PB2 is depressed.
C.
Alert the operator when the valve has not moved off its
closed seat within 10 seconds of depressing push button
PB2.
D.
Alert the operator if the valve has not reached full open
within 10 seconds of depressing push button PB2.
8
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
Refer to the drawing of a valve motor control circuit
below for a valve that is currently fully open and has a
10-second stroke time. Limit switch LS2 has failed
open. Note that limit switch (LS) contacts are shown
open regardless of valve position, but relay contacts are
shown open/closed according to the standard convention
for control circuit drawings.
Which one of the following describes the valve response
if the control switch is taken to the closed position for 2
seconds and then released?
A.
The valve will begin to close and then stop moving.
B.
The valve will not move.
C.
The valve will begin to close and then open fully.
D.
The valve will close fully.
9
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
Refer to the drawing of a motor and its control circuit
below. Note that relay contacts are shown open and
closed, according to the standard convention for control
circuit drawings.
The motor has been operating for several hours when it is
decided to stop the motor. What is the status of the
starting resistors before and after the motor STOP push
button is depressed?
A.
Initially bypassed; bypass is removed immediately after
the STOP push button is depressed.
B.
Initially inserted in the motor circuit; bypassed
immediately after the STOP push button is depressed.
C.
Initially bypassed; bypass is removed following a preset
time delay after the STOP push button is depressed.
D.
Initially inserted in the motor circuit; bypassed following
a preset time delay after the STOP push button is
depressed.
10
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
ELO 2.1 Circuit Breaker Construction and Function
Knowledge Check
A typical 120 volt AC manual circuit breaker has tripped
due to overload. To close the circuit breaker, move the
breaker handle from the...
A.
midposition to the OFF position to reset the trip latch,
and then to the ON position.
B.
OFF position directly to the ON position; trip latch reset
is not required.
C.
midposition directly to the ON position; trip latch reset is
not required.
D.
OFF position to the midposition to reset the trip latch,
and then to the ON position.
Knowledge Check
How is typical breaker operation affected when the
associated breaker control power transfer switch is
placed in the LOCAL position?
A.
Control power will be available to provide protective
trips, and the breaker can be electrically operated only
from the control room.
B.
Control power will be removed from both the open and
close circuits, and the breaker can be electrically
operated only from the control room.
C.
Control power will be available to provide protective
trips, and the breaker can be electrically operated only
from the breaker cabinet.
D.
Control power will be removed from both the open and
close circuits, and the breaker can be electrically
operated only from the breaker cabinet.
11
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
Which one of the following would cause a loss of ability
to remotely trip a circuit breaker and a loss of remote
breaker position indication?
A.
Failure of the breaker control switch
B.
Mechanical binding of the breaker tripping bar
C.
Loss of control power for the breaker
D.
Racking the breaker to the test position
Knowledge Check
Loss of breaker control power will cause...
A.
inability to operate the breaker locally and remotely.
B.
failure of the closing spring to charge following local
closing of the breaker.
C.
breaker line voltage to indicate zero regardless of actual
breaker position.
D.
the remote breaker position to indicate open regardless of
actual breaker position.
12
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
ELO 2.2 Racking Circuit Breakers
Knowledge Check
To completely de-energize an electrical component and
its associated control and indication circuits, the
component breaker should be...
A.
open with the control switch tagged in the open position.
B.
racked out with control power fuses removed.
C.
open with the control switch in pull-to-lock.
D.
racked out and tagged in racked-out position.
Knowledge Check
When a typical 4,160-volt breaker is racked to the test
position, control power is __________ the breaker and
the breaker is __________ the load.
A.
removed from; isolated from
B.
available to; isolated from
C.
removed from; connected to
D.
available to; connected to
13
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
ELO 2.3 Circuit Breaker Indications
Knowledge Check
Which one of the following describes the normal
operation of a local breaker overcurrent trip flag
indicator?
A.
Actuates when a breaker overcurrent trip has occurred;
can be manually reset when the overcurrent condition
clears.
B.
Actuates when a breaker has failed to trip on an
overcurrent condition; can be manually reset when the
overcurrent condition clears.
C.
Actuates to cause a breaker trip when the overcurrent trip
setpoint is reached; can be remotely reset when the
overcurrent condition clears.
D.
Actuates when no lockout is present; satisfies an
electrical interlock to remotely close a breaker.
Knowledge Check
Breaker local overcurrent trip flag indicators, when
actuated, indicate that...
A.
the associated breaker has failed to trip open during an
overcurrent condition.
B.
a breaker overcurrent condition is responsible for a
breaker trip.
C.
an overcurrent condition has cleared and the breaker can
be closed.
D.
a breaker trip will occur unless current is reduced.
14
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
Given the following indications for an open 4,160 volt
AC breaker:




The local OPEN/CLOSED mechanical flag
indicates open.
A breaker overcurrent trip flag is actuated on one
phase.
The line-side voltmeter indicates 4,160 volt AC.
The load-side voltmeter indicates zero (0) volts.
Assuming no operator actions were taken since the
breaker opened, which one of the following could have
caused the breaker to open?
A.
An operator opened the breaker locally.
B.
An operator opened the breaker from a remote location.
C.
A ground fault caused an automatic breaker trip.
D.
A loss of control power caused an automatic breaker trip.
Knowledge Check
The following remote indications are observed for a 480
volt AC load center supply breaker. (The breaker is
normally open.)




Red indicating light is on.
Green indicating light is off.
Load center voltage indicates zero (0) volts.
Breaker incoming voltage indicates 480 volts.
What is the condition of the breaker?
A.
Open and racked in
B.
Closed and racked to test position
C.
Open and racked to test position
D.
Closed and racked in
15
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
The following indications are observed for a motor
breaker in the control room:



Red position indicating light is off.
Green position indicating light is off.
Load amps indicate normal load current.
Assuming one of the indicating lights is burned out, what
is the condition of the breaker?
A.
Open and racked in
B.
Closed and racked to test position
C.
Open and racked to test position
D.
Closed and racked in
Knowledge Check
The following indications are observed in the control
room for a normally-open breaker that directly
starts/stops a 480 volt AC motor:




Red position indicating light is on.
Green position indicating light is off.
Load current indicates 50 amps.
Supply voltage indicates 480 volts.
What is the condition of the breaker?
A.
Closed and racked in
B.
Open and racked to test position
C.
Open and racked in
D.
Closed and racked to test position
16
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
While remotely investigating the condition of a
normally-open 480 volt AC motor control center (MCC)
feeder breaker, an operator observes the following
indications:




Green breaker position indicating light is out.
Red breaker position indicating light is
illuminated.
MCC voltmeter indicates 480 volt AC.
MCC ammeter indicates zero (0) amperes.
Based on these indications, the operator should report
that the feeder breaker is __________ and racked
__________.
A.
closed; to the test position
B.
closed; in
C.
open; in
D.
open; to the test position
ELO 2.4 Circuit Breaker Control Power
Knowledge Check
Loss of breaker control power will cause...
A.
the remote breaker position to indicate open regardless of
actual breaker position.
B.
breaker line voltage to indicate zero regardless of actual
breaker position.
C.
failure of the closing spring to charge following local
closing of the breaker.
D.
inability to operate the breaker locally and remotely.
17
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
Which one of the following would cause a loss of ability
to remotely trip a circuit breaker and a loss of remote
breaker position indication?
A.
Racking the breaker to the test position
B.
Loss of control power for the breaker
C.
Failure of the breaker control switch
D.
Mechanical binding of the breaker tripping bar
Knowledge Check
Which one of the following will cause a loss of
indication from the remote breaker position indicating
lights associated with a typical 480 VAC load supply
breaker?
A.
Removing the breaker control power fuses
B.
Locally opening the breaker
C.
Burnout of the local breaker position indicating lights
D.
Loss of breaker line voltage
18
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
The following indications exist for an open 4,160 volt
AC breaker:




All phase overcurrent trip flags are reset.
The control power fuses indicate blown.
The line-side voltmeter indicates 4,160 volt AC.
The load-side voltmeter indicates zero (0) volts.
Assuming no operator actions occurred since the breaker
opened, which one of the following could have caused
the breaker to open?
A.
A ground fault caused an automatic breaker trip.
B.
An operator tripped the breaker manually at the breaker
cabinet.
C.
An operator tripped the breaker manually from a remote
location.
D.
A loss of control power caused an automatic breaker trip.
ELO 3.1 Paralleling AC Sources
Knowledge Check
A main generator is about to be connected to an infinite
power grid. If personnel close the generator output
breaker with generator and grid voltages matched, but
with generator frequency 0.1 Hertz (Hz) higher than grid
frequency, this will initially result in the generator...
A.
picking up a portion of the grid real load.
B.
experiencing overspeed conditions.
C.
experiencing reverse power conditions.
D.
picking up a portion of the grid reactive load.
19
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
A main generator is being paralleled to the power grid.
Generator voltage has been properly adjusted and the
synchroscope is rotating slowly in the clockwise
direction.
The generator breaker must be closed just as the
synchroscope pointer reaches the 12 o'clock position to
prevent...
A.
motoring of the generator, due to unequal frequencies.
B.
excessive megawatt (MW) load transfer to the generator,
due to unequal frequencies.
C.
excessive MW load transfer to the generator, due to outof-phase voltages.
D.
excessive arcing within the generator output breaker, due
to out-of-phase voltages.
ELO 3.2 Abnormal Conditions During Paralleling Operations
Knowledge Check
Consider paralleling a main generator to the power grid.
Assume that generator voltage equals the grid voltage
and the synchroscope is rotating slowly in the clockwise
direction.
Close the generator breaker just as the synchroscope
pointer reaches the 12 o'clock position to prevent...
A.
excessive megawatt (MW) load transfer to the generator,
due to out-of-phase voltages.
B.
excessive arcing within the generator output breaker, due
to out-of-phase voltages.
C.
motoring of the generator, due to unequal frequencies.
D.
excessive MW load transfer to the generator, due to
unequal frequencies.
20
Breakers, Relays, and Disconnects
Knowledge Check Answer Key
Knowledge Check
Closing the output breaker of a three-phase generator
onto a de-energized bus can...
A.
produce an overcurrent condition on the generator if the
bus was not first unloaded.
B.
produce an overvoltage condition on the bus.
C.
result in a reverse power trip of the generator circuit
breaker if generator frequency is low.
D.
result in large reactive currents in the generator.
21