Download Circuit Breakers

Operator Generic Fundamentals
Components – Breakers, Relays, and Disconnects
© Copyright 2016
Operator Generic Fundamentals
2
Terminal Learning 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%
on the following areas:
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.
© Copyright 2016
TLOs
Operator Generic Fundamentals
3
Circuit Protection
TLO 1 – Explain the purpose, safety precautions, and operation of
electrical circuit interrupting and circuit switching devices.
1.1 Explain the principles of circuit protection and their application,
including selective tripping.
1.2 Describe the protection provided by each of the following: fuses,
protective relays, circuit breakers, and overload devices.
1.3 Describe the function of the following types of switches: disconnect
switch, automatic transfer switch and manual transfer switch.
1.4 Describe the personnel safety and equipment protection precautions
associated with circuit interrupting devices and relays.
1.5 Interpret symbols for breakers, relays, and disconnects in a simple
one-line diagram, and explain operation of the control circuit.
1.6 Explain the purpose and function of normal and power seeking
automatic transfer switches.
1.7 Describe the functions, operation, and protective features of motor
controllers.
© Copyright 2016
TLO 1
Operator Generic Fundamentals
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Circuit Protection
ELO 1.1 – Explain the principles of circuit protection and their application,
including selective tripping.
• Designed to de-energize circuit to protect equipment and circuit from
electrical faults
• Uses various types of fault sensors
– Overcurrent, undervoltage, underfrequency, etc.
• Uses various types of circuit interrupting devices
– Fuses, breakers, etc.
• Designed to maximize system reliability
– Avoid unnecessary trips
– Isolate only portion of circuit necessary, continuing service to
remainder
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ELO 1.1
Operator Generic Fundamentals
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Selective Tripping
• Protective device closest to fault operates to remove fault from
system
– Maintaining largest possible portion of system energized
• In example below, fuses open circuit at 50 amps to Load 1
• Output breaker for generator set to trip at 500 amps
– Ensures power to Load 2
2
75A
Fuses
1
Figure: Selective Tripping
© Copyright 2016
ELO 1.1
Operator Generic Fundamentals
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Circuit Protection Example
Power Plant Electrical System
• Isolates faults to prevent damage
• Maintains as much of the system energized as possible, maximizes
plant reliability
• Functions as selective tripping
Four Condensate Booster Pumps
• Motor Control Center (MCC) 1 supplies two pumps through
individual breakers
• Motor Control Center (MCC) 2 supplies two pumps through
individual breakers
• Ability to isolate faults at individual pump or power supply levels
© Copyright 2016
ELO 1.1
Operator Generic Fundamentals
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Circuit Protection Example
Fault on Individual Pump Motor
• Isolate the pump to prevent damage
• Maintain three pumps available and the plant online
• Maintain full electrical system availability
Fault at Power Supply Level
• Isolate the power supply at the MCC
• Maintain other MCC available
• Maintain two pumps available and possibly the plant online
© Copyright 2016
ELO 1.1
Operator Generic Fundamentals
8
Circuit Interrupting Devices
ELO 1.2 – Describe the protection provided by each of the following:
fuses, protective relays, circuit breakers, and overload devices.
Fuses
• Device containing fusible link that protects electrical circuit from
overcurrent condition only
– Fusible link directly heated and destroyed by excessive current
passing through it
• Element sized so heat generated by normal current flow does not
melt element
• Overcurrent or short-circuit current flows through fuse
– Fusible link melts to open circuit (blown fuse)
– May be time-delayed
– Vital to replace safety-related fuses with correct type
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Types of Fuses
Plug Fuse
• Consists of zinc or alloy strip
– Fusible element enclosed in
porcelain or Pyrex™
housing
– Screw base
• Normally used on circuits rated
at 125V or less to ground
Figure: Typical Fuses
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ELO 1.2
Operator Generic Fundamentals
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Types of Fuses
Cartridge Fuse
• Constructed with zinc or alloy
fusible element
– Enclosed in cylindrical tube
– Element ends attached to
metallic contact piece at
ends of tube
• Normally used on circuits rated
between 250 volts and 600 volts
• Maximum continuous currentcarrying capacity of 600 amps
Figure: Typical Fuses
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Circuit Breaker Protective Relays
Relays
• Varied types of protective relays detect fault conditions
– Send signals to trip one or more circuit breakers to isolate the
fault
– Protect equipment from damage and personnel from injury
• Different parameters are monitored to
– Provide prompt response to a fault condition
– Also avoid unnecessary system interruptions
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Circuit Breaker Protective Relays
Overload Relay Devices (also known as overcurrent)
• Breakers usually provided with three overcurrent tripping devices
– Provides breaker with long-time, short-time, and instantaneous
tripping capabilities
• Long-time delay trip (also known as 51)
– Device reacts to light overloads and trips breaker after a time
delay
– Trips breaker on overload condition slightly higher than normal full
load
o Example: gradual bearing failure
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Circuit Breaker Protective Relays
Overload Relay Devices
• Short-time delay trip (also known as 51)
– Device reacts to slightly higher current and trips breaker in a
shorter period of time
– Allows for motor starting currents without tripping breaker unless
current level has not decayed within certain time frame after
motor start
o Example: Locked/seized rotor
• Instantaneous trip (also known as 50)
– Device reacts quickly to trip breaker due to high currents
o Example: Short-circuit
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Circuit Breaker Protective Relays
Undervoltage Relay (also known as 27)
• Used in large power systems with many induction motors
• Induction motors draw more current when voltage drops
• Challenges entire power system
• Isolating the cause of undervoltage protects the system
Underfrequency Relay (also known as 81)
• Trips breaker when frequency drops below a preset value
• Protects loads on a system that cannot tolerate a significant change
in frequency
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Circuit Breaker Protective Relays
Lockout (also known as 86)
• Fault should be isolated by breaker actuation
– Appears as if fault cleared, could allow breaker to reclose on fault
• Lockout relays prevent automatic reclosure
• Ensures the system is not re-energized before the fault is isolated
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ELO 1.2
Operator Generic Fundamentals
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Circuit Breaker Protective Relays
Reverse-Power Relay (also known as 32)
• Senses a change in normal direction of current, indicating an
abnormal condition
– A change in direction of power flow through breaker
– Power flowing into source versus power flowing out of source
• Usually used to protect electrical generator from damage
– Due to motoring
– Trips generator output breaker
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Circuit Interrupting Devices
Thermal Overloads
• Heat sensitive element and an
overload heater connected in
series with motor load circuit
• When motor current is
excessive and sustained
– Heat from heater causes
heat sensitive element to
open the motor breaker or
motor line contacts
Figure: Three-Phase Magnetic Controller
With Thermal Overloads
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ELO 1.2
Operator Generic Fundamentals
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Circuit Interrupting Devices
• Molded case breakers with larger current ratings also have magnetic
trip element to supplement thermal trip element
• Magnetic unit utilizes magnetic force surrounding conductor to
operate circuit breaker tripping linkage
• After activation, must manually reset an overload device to resume
motor operation
• Can reset magnetic overload devices immediately
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Resetting Overload Devices
• An activated overload device must be reset for motor operation
– NOTE: Thermal overloads must cool before they can be reset
– Manual Reset
o Located in controller enclosure which contains overload device
o Usually has a hand-operated rod, lever, or button that returns
device tripping mechanism to its original position and resets
interlocks
– Automatic Reset
o Usually uses a spring or gravity operated device to reset
overload device without operator action
o Only after condition causing overload has cleared
– Electrical Reset
o Actuated by an electromagnet controlled by a push button
o Used when desired to reset an overload device remotely
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ELO 1.2
Operator Generic Fundamentals
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Circuit Interrupting Devices
Knowledge Check – NRC Exam Bank
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. Undervoltage
B. Underfrequency
C. Time-delayed overcurrent
D. Instantaneous overcurrent
Correct answer is C.
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Transfer and Disconnect Switches
ELO 1.3 – Describe the function of the following types of switches:
disconnect switch, automatic transfer switch, and manual transfer switch.
• Provide flexibility within an electrical distribution system
• Used to change lineup of system or power source for loads within
system
• Provide direct visual indication that a circuit is broken
– High-voltage disconnects used in switchyard operation
© Copyright 2016
ELO 1.3
Operator Generic Fundamentals
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Disconnect Switches
• Two-position switches used for isolation of power supplies from one
or more loads or motor control centers
• May be used in pairs to transfer power supplies from one source to
another
Figure: Typical Disconnect Switch
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ELO 1.3
Operator Generic Fundamentals
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Disconnect Switch Design & Operation
• Disconnects differ from breakers
– Operated manually
– Not designed to be opened under load
• Design does not include arc chutes or any other means to extinguish
arc drawn when disconnect opened
– Cannot be opened under load
• When energizing circuit with disconnect switch:
– Close disconnect switch first, then close breaker
• When de-energizing circuit with disconnect switch:
– Open breaker first, then open disconnect
© Copyright 2016
ELO 1.3
Operator Generic Fundamentals
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Disconnect Switch Design & Operation
• Disconnects may contain fuses, which provide overcurrent protection
for loads supplied by disconnect
– If not equipped with fuses, provide isolation for circuit only
– Separate fuses or breakers would be required elsewhere in circuit
to provide protection for loads
© Copyright 2016
ELO 1.3
Operator Generic Fundamentals
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Disconnect Switch Design & Operation
• Safety Switches
– Low voltage (less than or equal to 600VAC) switches that are
enclosed
– May be locked in OFF
– Used in isolation points for electrical maintenance
• Special precautions required operating disconnects to protect
personnel from potential arcs
– Leather gloves and safety glass should be worn
– Operator should stand to side of disconnect and look away during
operation
o Protects operator's eyes and face from arc that may occur
during switch operation
© Copyright 2016
ELO 1.3
Operator Generic Fundamentals
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Disconnect Switch Design & Operation
• Before disconnect can be opened, all electrical loads fed by
disconnect must be verified off, or not operating
• Opening disconnect under load can result in damage to disconnect
and injury to personnel
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ELO 1.3
Operator Generic Fundamentals
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Transfer Switches
• Used to make and break electrical circuits in order to provide smooth
power transfer from one source of power to another
Manual Transfer Switch
• Similar to disconnect switches
– Except, have three positions to allow power supply for an
electrical component to be transferred from one source to another
• Manual transfer switches may contain internal fuse protection
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ELO 1.3
Operator Generic Fundamentals
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Transfer and Disconnect Switches
Knowledge Check
What is an advantage of using high-voltage disconnect switches
instead of breakers to isolate main power transformers?
A. Disconnect switches can be operated either locally or remotely.
B. Disconnect switches provide direct visual indication that the
circuit is broken.
C. Disconnect switches are cheaper and provide the same
automatic protection as a breaker.
D. Disconnect switches are capable of interrupting a higher current
flow with less heating than a breaker.
Correct answer is B.
© Copyright 2016
ELO 1.3
Operator Generic Fundamentals
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Transfer and Disconnect Switches
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 (reenergizing)
B. Open disconnect first (de-energizing); shut breaker first (reenergizing)
C. Open breaker first (de-energizing); shut breaker first (reenergizing)
D. Open disconnect first (de-energizing); shut disconnect first (reenergizing)
Correct answer is A.
© Copyright 2016
ELO 1.3
Operator Generic Fundamentals
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Safety and Equipment Protection
ELO 1.4 – Describe the personnel safety and equipment protection
procedures and precautions associated with circuit interrupting devices
and relays.
Personnel should always observe electrical safety precautions and
PPE requirements:
• Do not open a disconnect switch under load
• 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
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ELO 1.4
Operator Generic Fundamentals
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Safety and Equipment Protection
• 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
• Perform the following before racking out circuit breakers:
– Ensure circuit breaker is open
– Ensure control power is removed when applicable
– Tag or lockout applicable electrical sources
– Utilize Personal Protective Equipment as specified for the voltage
and current involved
• Always strip loads prior to reenergizing a dead bus
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ELO 1.4
Operator Generic Fundamentals
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Precautions
The following electrical safety precautions are good work practices:
• Have a person stand by to deenergize 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
© Copyright 2016
ELO 1.4
Operator Generic Fundamentals
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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. Use insulated tools to prevent inadvertent contact with adjacent
equipment.
B. Cover exposed energized circuits with insulating material to
prevent inadvertent contact.
C. Attach a metal strap from your body to a nearby neutral ground
to ensure that you are grounded.
D. Have a person standing by with the ability to remove you from
the equipment in the event of an emergency.
Correct answer is C.
© Copyright 2016
ELO 1.4
Operator Generic Fundamentals
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Electrical Drawings
ELO 1.5 – Interpret symbols for breakers, relays, and disconnects in a
simple one-line diagram, and explain the operation of the control circuit.
• Common symbols include contacts, fuses, breakers, indicating lights,
trip coil, and closing coil
Fuse
Indicating Lights
Closing Coil,
Trip Coil
Breaker
Open Contact
© Copyright 2016
Overloads
ELO 1.5
Closed Contact
Operator Generic Fundamentals
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Electrical Drawings
Common Symbols
• Overloads, relays, switches, rectifier bridge, and transformer
Relays
Switch
Transformer
Switches
Closed Switch
Open Switch
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Operator Generic Fundamentals
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Electrical Drawings
• Usually a legend on first sheet of drawing series
• Different suppliers have differences in their conventions, so operator
should review drawing legend when in doubt
• "a" contacts are open when the relay controlling it is de-energized,
and closed when it is energized
• “b" contacts are closed when the relay controlling it is de-energized,
and open when it is energized
• Drawing representation shows contacts in their de-energized state
© Copyright 2016
ELO 1.5
Operator Generic Fundamentals
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Circuit Breaker Control
• To operate circuit breakers from
remote location
– Electrical control circuit must
be incorporated
– Control power supplied by
AC source rectified to DC
o Some vital power sources
use DC as control power
Figure: Breaker Control Circuit
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Circuit Breaker Control
Major Components:
• Rectifier unit
• Closing relay
• Closing coil
• Tripping coil
• Auxiliary contacts
• Circuit breaker control switch
Figure: Breaker Control Circuit
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Circuit Breaker Control
• Control circuit can be designed
with protective features:
– Overcurrent
– Underfrequency
– Undervoltage
• Fault conditions cause
associated contact to close
– Energize tripping coil
– Trip circuit breaker
Figure: Breaker Control Circuit
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Closing a Remotely Operated Circuit Breaker
• To close circuit breaker control
switch, turn to CLOSE position
– Provides path to energize
closing relay (CR)
• Energized closing relay shuts
an auxiliary contact
– Energizes closing coil (CC)
– Closing coil shuts circuit
breaker
Figure: Breaker Control Circuit
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Closing a Remotely Operated Circuit
Breaker
• Once breaker is shut
– It is latched in the closed
position
– "b" contact associated with
the closing replay opens deenergizing
o Closing relay
o Closing coil (anti-pumping
feature)
Figure: Breaker Control Circuit
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Operator Generic Fundamentals
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Closing a Remotely Operated Circuit
Breaker
• When breaker closes
– "a" contact closes
– Enables trip circuit
• Circuit breaker control switch
returns to neutral position when
released
Figure: Breaker Control Circuit
© Copyright 2016
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Operator Generic Fundamentals
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Opening a Remotely Operated Circuit
Breaker
• Control switch turned to TRIP
position
• Provides path to energize trip
coil (TC)
• Releases latching mechanism
– Circuit breaker will open
– "a" contact will open
– deenergized tripping coil
• "b" contact will close
– Sets up next remote closure
• Control switch may be released
© Copyright 2016
Figure: Breaker Control Circuit
ELO 1.5
Operator Generic Fundamentals
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Opening a Remotely Operated Circuit
Breaker
• Circuit breaker is equipped with
– Underfrequency trip
– Undervoltage trip
• Both/Either energize the breaker's trip coil opening breaker
© Copyright 2016
ELO 1.5
Operator Generic Fundamentals
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Analyzing Valve Control Circuits
Valve Fully Open
ELO 1.5
• Valve open limit switch contact
is shut
• A1 relay energized A1(a)
contact shut; red, open lamp
energized
• Valve shut limit switch open
• A2 relay deenergized and A2(b)
contact is shut
Figure: Valve Fully Open Circuit
• Both A1(b) and A2(a) contacts
open; green shut light out
© Copyright 2016
ELO 1.5
Operator Generic Fundamentals
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Analyzing Valve Control Circuit
Valve in Intermediate Position
ELO 1.5
• Both valve open and valve
closed limit switches open
• Both relays deenergized; (b)
contacts closed, (a) contacts
open
• Both lamps energized
Figure: Valve Intermediate Position
© Copyright 2016
Operator Generic Fundamentals
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Analyzing Valve Control Circuits
Valve Closed
• Valve shut limit switch contact
closed
• A2 relay energized; A2(a)
contact closed, green shut lamp
energized
• Valve open limit switch is open;
A1 relay deenergized, A1 (b)
contact is shut
Figure: Valve Closed Circuit
• Both A2(b) and A1(a) contacts
open; red open light out
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Operator Generic Fundamentals
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Electrical Drawings Example 1
Opening and Closing the Valve
• As you can see, energizing K1 relay opens valve, deenergizing K1
relay closes valve
Figure: Valve Control Circuit
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Electrical Drawings Example 1
• To manually open the valve
– Push PB2
– Energizes K3 contact, which in turn energizes K1 relay
• There is no other means to open valve through control circuit
• After valve is opened, K1 relay has a seal-in K1 contact which
maintains power to K1 relay and keeps valve open
Figure: Valve Control Circuit
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Operator Generic Fundamentals
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Electrical Drawings Example 1
• To close the valve
– Push PB1
– Interrupts power to K1 relay
– Causes the K1 seal-in contact to open when K1 relay is
deenergized
– Valve closes and stays closed
Figure: Valve Control Circuit
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Operator Generic Fundamentals
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Electrical Drawings Example 1
Valve response to loss of control power
• K1 relay must remain energized for valve to stay open
– If control circuit loses power, effect is the same as pushing PB1;
power to K1 relay is interrupted and valve closes
– K1 seal in contact opens; when power is restored, valve stays
closed
Figure: Valve Control Circuit
© Copyright 2016
ELO 1.5
Operator Generic Fundamentals
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Electrical Drawings Example 1
Alarm Light Function
• As you can see from the drawing, the alarm lights when K2 time
delay relay picks up and closes K2 contact
– Occurs after K2 relay has been energized for a 10-second period
– If deenergized before time delay completes, they reset to zero
Figure: Valve Control Circuit
© Copyright 2016
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Operator Generic Fundamentals
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Electrical Drawings Example 1
• K2 time delay relay is energized if the following sequence occurs:
– PB2 is pushed to open valve
o energizes K3 relay
o closes K3 contact, energizing K1 relay and closing K1 seal in
contact
• Power supplied to K2 time delay relay only if LS1 (limit switch) is
made up, which occurs when valve is fully closed
Figure: Valve Control Circuit
© Copyright 2016
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Operator Generic Fundamentals
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Electrical Drawings Example 1
• Circumstances required for alarm to light are:
– Button is pushed to open the valve and 10 seconds later, with an
open signal still applied, valve is fully closed
• If valve is partially or fully open, LS1 will be open and alarm will not
sound
• If operator depresses PB1 to close the valve, K1 relay will deenergize
and its seal in contact will open, alarm will not sound
Figure: Valve Control Circuit
© Copyright 2016
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Operator Generic Fundamentals
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Electrical Drawings Example 2
• Simple Motor Control Circuit
• Control power is taken from
termination L1
• Returns at termination L2
• Requires START pushbutton to
be depressed
• Energizes MAIN coil
• Closes maintaining contacts
• Starting resistors in circuit
– Until Time Delay coil
energizes
Figure: Control Power Circuit
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Electrical Drawings Example 2
• Time Delay energizes
Accelerating Coil
• Closes contacts to bypass
resistors
– Motor up to speed and
starting current drops off
• When motor stopped with
STOP coil (or overload)
– MAIN coil deenergizes
– Time Delay coil deenergizes
– Accelerating coil
deenergizes
– Starting resistors ready for
next motor start
Figure: Control Power Circuit
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Electrical Drawings
Knowledge Check - NRC
Question
Refer to the drawing of a typical valve
control circuit. What is the purpose
of depressing the S1 pushbutton?
A. To de-energize the K3 relay
after the initiating condition
has cleared.
B. To prevent energizing the K3
relay when the initiating
condition occurs.
C. To manually energize the K3
relay in the absence of the
initiating condition.
D. To maintain the K3 relay
energized after the initiating
condition has cleared.
© Copyright 2016
ELO 1.5
Correct answer is A.
Operator Generic Fundamentals
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Electrical Drawings
Knowledge Check – NRC Question
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.
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Electrical Drawings
Knowledge Check (continued)
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 pushbutton PB2 is depressed.
B. Alert the operator when the valve has not moved off its closed
seat within 10 seconds of depressing pushbutton PB2.
C. Alert the operator that the valve is opening by sounding the
alarm for 10 seconds after PB2 is depressed.
D. Alert the operator if the valve has not reached full open within 10
seconds of depressing pushbutton PB2.
Correct answer is B.
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ELO 1.5
Operator Generic Fundamentals
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Automatic Transfer Switches
ELO 1.6 – Explain the purpose and function of normal and power seeking
automatic transfer switches.
• Used in electrical distribution systems to
– quickly disconnect a deenergized electrical load from one power
supply
– and connect it to a backup power supply such as an emergency
diesel generator
• Ensure source of power available to essential electrical loads at all
times
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Automatic Transfer Switches
• Grouped into two categories based on operation
– Power-seeking
o Auto to alternate power supply
o Manual back to normal supply
– Normal-seeking
o Auto to alternate power supply
o Auto back to normal power supply
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Automatic Transfer Switches
• Many normal-seeking ATSs equipped with time delay
– prevents them from shifting back to normal source of power until it
has been restored for a pre-set period of time (e.g. five seconds)
– Ensures ATS will not shift its electrical loads back to an unreliable
source of power when normal source restored
• Power-seeking ATSs do not make a distinction between power
sources
– It will stay connected to new source of power until that source of
power is lost, or
– until the ATS is manually shifted back to original source of power
© Copyright 2016
ELO 1.6
Operator Generic Fundamentals
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Motor Controllers
ELO 1.7 – Describe the functions, operation, and protective features of
motor controllers.
• Range from a simple toggle switch to a complex system using
solenoids, relays, and timers
• Basic function to control and protect the operation of a motor includes
– Starting and stopping motor
– Protecting motor from overcurrent, undervoltage, and overheating
conditions that would cause motor damage
• Two basic categories of motor controllers
– Manual controller
– Magnetic controller
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Manual Motor Controllers
• Operated by hand
• Provided with thermal and direct acting overload units
– Protect motor from overload conditions
• ON-OFF switch with overload conditions
• Could be push buttons
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Operator Generic Fundamentals
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Manual Motor Controllers
• Used on small loads such as
– Machine tools
– Fans/blowers
– Pumps
– Compressors
• Simple design and operation
• Provide quiet operation
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Magnetic Motor Controller
• Master switch frequently
operated automatically
– Float switch
– Pressure switch
– Thermostat
• Manually operated master
switches for these types of
controllers include
– Push buttons
– Drum switches
Figure: Three-Phase Magnetic Controller
With Thermal Overloads
– Knife switches
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Magnetic Motor Contactor
• Operated by electromagnet
• Electromagnet and movable
iron armature on which movable
and stationary contacts are
mounted
• When no current flowing
through electromagnetic coil
– armature held away by
spring
• When coil energized
Figure: Magnetic Contactor Assembly
– electromagnet attracts
armature and closes
electrical contacts
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Motor Controller Types and Operation
• Three major types of AC across-the-line controllers:
– Low-voltage protection (LVP)
– Low-voltage release (LVR)
– Low-voltage release effect (LVRE)
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Low Voltage Protection
• Main purpose is to
– Deenergize motor during
low voltage
– Prevent restarting
automatically upon return of
normal voltage
Figure: Low-Voltage Protection Controller
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Low-Voltage Protection Controller
• When START button pushed
– Contactor M coil is
energized
– Closing M and Ma contacts
• When START button released
– M “a” contact remains
closed, completing circuit
Figure: Low-Voltage Protection Controller
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Low-Voltage Protection Controller
• When low voltage condition
occurs
– M coil will drop out at some
pre-determined value of
voltage
– Usually 70-80% of rated
voltage
– M and Ma contacts open
• To restart motor, START button
must be pushed
© Copyright 2016
Figure: Low-Voltage Protection Controller
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Low-Voltage Protection Controller
• Depressing STOP button
deenergizes M coil
– Opens M and Ma contacts
– Stops motor
Figure: Low-Voltage Protection Controller
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Low-Voltage Release Controller
• General purpose
– Deenergize motor in low
voltage condition
– Restart motor when normal
voltage is restored
• Used primarily on small and/or
critical loads
– Cooling water pumps
required for safety related
equipment
© Copyright 2016
Figure: Low-Voltage Release Controller
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Low-Voltage Release Controller
• Placing START switch in run
– Energizes M coil
– Closing M contacts
– Starting motor
• When low-voltage condition
occurs
– M coil drops out
– Opening M contacts
Figure: Low-Voltage Release Controller
– Deenergizing motor
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Low-Voltage Release Controller
• When normal voltage restored
– M coil is reenergized
– Closing M contacts
– Restarting motor
Figure: Low-Voltage Release Controller
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Low-Voltage Release Effect
• Maintains motor circuit at all times
• Manual controller found mostly on small loads that must start
automatically upon restoration of voltage
• May contain overloads
• If overloads are used, they are placed in lines to load
Figure: Low-Voltage Release Effect Controller
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Knowledge Check
Knowledge Check
Refer to the drawing of a typical valve control circuit for a 480 VAC
motor-operated valve below.
The valve is currently open with the contact configuration as shown. If
the S1 pushbutton is depressed, the valve will ____________ and
when the S1 pushbutton is subsequently released, the valve will
____________.
A. remain open; close
B. remain open; remain open
C. close; open
D. close; remain closed
Correct answer is D.
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Circuit Breakers
TLO 2 – Explain the construction, operation, and indications for electrical
circuit breakers.
2.1 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.
2.2 Describe the following associated with racking out circuit breakers:
purpose for racking out circuit breakers, effect of racking out breakers on
control and indicating circuits, removal of control power on breaker
operation.
2.3 Describe the indications provided for each of the following: local circuit
breaker position indications, control room circuit breaker status indications,
circuit breaker and protective relay trip indications.
2.4 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).
© Copyright 2016
TLO 2
Operator Generic Fundamentals
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Circuit Breakers
ELO 2.1 – 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.
• Circuit breakers used to
– Isolate circuits
– Circuit protection in the event of faults
– Switching during normal operation
• Circuit breakers do not sense faults
– Relays or overload devices that sense faults often contained in
same cabinet as circuit breaker they signal to trip
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Circuit Breakers
Breaker Classifications
• High voltage: above 15,000 volts
• Intermediate or medium voltage: 600-15,000 volts
• Low voltage: less than 600 volts
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Circuit Breakers
Low-Voltage Air Circuit Breaker
• For circuits rated at 600 volts or
lower
• Applications:
– Molded case breakers
– Small motor control center
(MCC) breakers
Figure: Molded Case Circuit Breaker
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Circuit Breakers
Molded Case Breaker Operation
• Turning ON or OFF position will
connect or disconnect a circuit
• All breakers, except very small
ones, have a linkage that
allows for a quick make (quick
break) contact action
• If circuit breaker opens under
fault condition, handle goes to
"trip-free" position
Figure: Cutaway View of Molded Case
Circuit Breaker
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83
Circuit Breakers
Automatic Tripping
• Circuit breaker automatically
trips when current through it
exceeds a pre-determined value
• In lower current ratings, thermal
tripping devices provide the
means of automatic tripping
Figure: Cutaway View of Molded Case
Circuit Breaker
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Thermal Tripping Elements
• Consists of bimetallic element
calibrated so normal current
heat does not cause deflection
• High current will cause element
to deflect and trip linkage that
holds circuit breaker shut
– Short circuit
– Overload
• Circuit breaker opened by
spring action
Figure: Thermal Tripping Element
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Thermal Tripping Elements
• Bimetallic element is responsive
to heat produced by current
flowing through it
• Inverse-time characteristic
– If an extremely high current
is developed, circuit breaker
will be tripped very rapidly
– For moderate overload
currents, it will operate more
slowly
Figure: Thermal Tripping Element
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Magnetic Tripping Elements
Arc Chutes
• When the separable contacts of an air circuit breaker open, an arc
develops between contacts
• Different designs and arrangements of contacts and their surrounding
chambers
• Most common design places moving contacts inside an arc chute
– Construction allows this arc chute to magnetically draw arc
formed as contacts open
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Large Air Circuit Breaker
• Large distribution systems
require much larger air circuit
breakers
• Breakers have current ratings
as high as 4,000 amps, and
interrupting ratings as high as
150,000 amps
• Requires stronger mechanism
to “make” and “break” contact
Figure: Large Air Breaker Front
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Large Air Circuit Breaker
Operation
• Closing device, stored energy
mechanism
• Uses large compressed coil
springs
• Springs compressed
– Manually
– Small charging motor
Figure: Large Air Circuit Breaker
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Large Air Circuit Breaker
• Closing breakers compresses
tripping spring
• Tripping spring and trip latch trip
open breaker
• Trip latch operation
– Manually
– Remotely by trip coil
• Electrically-operated circuit
breakers used when
– Operated at frequent
intervals
– Remote operation required
© Copyright 2016
ELO 2.1
Figure: Large Air Circuit Breaker
Operator Generic Fundamentals
90
Large Air Circuit Breaker
• When electrically operated
breaker tripped
– Closing spring recharged by
spring charging motor
– Makes breaker ready for
next closing operation
Figure: Large Air Circuit Breaker
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Large Air Circuit Breaker
• Closing springs are
compressed by pulling
downward on large operating
handle on front of breaker
• Closing springs may also be
charged by an electric motor
– Motor powered from breaker
control power
– Will charge springs after
breaker closing attempt
– If control power lost, can
charge motor manually
using a lever or hand crank
Figure: Large Air Breaker Front
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High-Voltage Breaker Classifications
• Circuits with voltage ratings higher than 600 volts use high-voltage
circuit breakers (including breakers rated at intermediate voltage)
• Standard voltage ratings are from 4,160 to 765,000 volts with threephase interrupting ratings of 50,000 to 50,000,000 kVA
– Early design of high-voltage circuit breakers were oil circuit
breakers
– Newer designs are magnetic or compressed air
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High-Voltage Breaker Classifications
Magnetic Air Circuit Breakers
• Rated up to 750,000 kVA at
13,800 volts, interrupts air with
aid of magnetic blowout coils
• When contacts separate during
a fault condition (see figure),
magnets draw arc out
horizontally and transfer it to a
set of arcing contacts
• Blowout coil provides a
magnetic field to draw arc
upward into arc chutes
• Arc accelerates upward into arc
chute where it is elongated and
divided into many small
segments
© Copyright 2016
Figure: Magnetic Air Circuit Breaker
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Operator Generic Fundamentals
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High-Voltage Breaker Classifications
Compressed Air Circuit Breakers
• Uses compressed air stream directed toward separable contacts to
interrupt arc
• Air-blast circuit breakers developed for extra high-voltage
applications
Figure: Compressed Air Arc Chute
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High-Voltage Breaker Classifications
Oil Circuit Breakers
• Have contacts immersed in oil,
arc cooled and quenched
• Oil tanks in oil circuit breakers
are sealed
• Electrical connections between
contacts and external circuits
through porcelain bushings
Figure: Oil Circuit Breaker
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GE Magne-Blast Breaker
• GE Magne-Blast breaker is a
medium-voltage breaker (see
figure) with wide use in power
plant switchgear application
• Early designs were an air
circuit breaker, with a solenoid
operated mechanism
• Latest designs outfitted with
vacuum type contacts
Figure: GE Magne-Blast Breaker
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GE Magne-Blast Breaker
• Chassis contains primary
contact assembly and
bushings, interlocks, and
ground strap
• Primary contact assembly is
the main current carrying part
of the breaker
• Assembly consists of all
barriers, arc chutes, and air
puffer system
• Some breaker designs include
a manual bar to charge closing
springs locally at breaker
© Copyright 2016
Figure: GE Magne-Blast Breaker
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Charging Motor
• Normally, charging motor charges closing spring(s)
• Located below operating mechanism on front left side and connected
to a drive fitting and levers to ratchet wheel
• Spring discharge interlock
– Discharges the closing springs when the breaker is rolled in or out
of switchgear cubicle
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Local Circuit Breaker Operation
• Some circuit breakers are designed for local rather than remote
operation
• Locally operated breakers may be designed to be opened and closed
electrically or manually
Figure: Circuit Breaker
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Local Circuit Breaker Operation
• Electrically operated
– Provided with a local breaker control switch
– Uses breaker control power to electrically operate circuit breaker
– Normally located in associated switchgear
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Local Circuit Breaker Operation
• Control switch has three
positions
– TRIP, breaker opens
– Midposition, normal "at rest"
position
– CLOSE, breaker closes if
interlocks met
Figure: Breaker Control Switch
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Local Circuit Breaker Operation
• Logic circuitry checks breaker interlocks are satisfied before allowing
closing
• Loss of control power breaker fails as is
• Breaker control switch may have a PULL OUT (Pull-To-Lock) position
that prevents automatic closing
– Standard with this type of control switch and does not imply
breaker has automatic closing features
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Local Circuit Breaker Operation
• Electrically operated breakers may also have two different position
indicators
– Circuit breaker position indicating flags
o Red flag indicates breaker was last positioned closed
o Green flag indicates breaker was last positioned open
o “Flag mismatch” if breaker positions differently from switch
– Amber or white light
– Circuit breaker position indicating lights
o Illuminated red indicates closed
o Illuminated green indicates open
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Local Circuit Breaker Operation
Manually Operated Breakers
• Some MCC or load center circuit breakers do not have capability of
being operated electrically
• Manually operated through use of a "TRIP" and a "CLOSE" push
button
• Do not have:
– Circuit breaker control switches
– Position indicating lights
– Logic circuits to check for interlocks
• Must be operated per procedure
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Local-Remote Transfer Switch
• Some components provided with REMOTE/LOCAL transfer switch
– DG output breaker
– Remote Shutdown Panel components
• When REMOTE selected:
– control power available for trips and breaker can be operated
remotely from control room
• When LOCAL selected:
– control power available for trips, but breaker can only be operated
locally
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Circuit Breaker Protective Relays
• Circuit breaker control circuit can be designed so that any one of a
number of protective features may be incorporated
• If conditions exists while circuit breaker is closed
– Relay will close its associated contact
– Energize breaker tripping coil
– Circuit breaker trips open
• Various protective relays presented earlier (for example)
– Undervoltage, underfrequency, reverse power
– Long-term, short-term, or instantaneous overcurrent
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Circuit Breakers
Knowledge Check – NRC Question
A typical 120 VAC manual circuit breaker has tripped due to overload.
To close this circuit breaker, the breaker handle must be moved from
the...
A. OFF position directly to the ON position; trip latch reset is not
required.
B. midposition directly to the ON position; trip latch reset is not
required.
C. OFF position to the midposition to reset the trip latch, and then
to the ON position.
D. midposition to the OFF position to reset the trip latch, and then
to the ON position.
Correct answer is D.
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Circuit Breakers
Knowledge Check – NRC Question
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.
Correct answer is C.
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Racking Circuit Breakers
ELO 2.2 – Describe the following associated with racking out circuit
breakers: purpose for racking out circuit breakers, effect of racking out
breakers on control and indicating circuits, and removal of control power
on breaker operation.
• Breakers may be racked to three positions
– Connect (racked in): normal position, breaker supplies power to
load
– Test: breaker will not connect to load, but control power is
supplied to breaker
– Disconnect (racked out): deenergizes the load for maintenance
(control power must be removed to completely deenergize)
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Large ACB Racking Operation
• To completely deenergize an electrical component and its associated
control and indication circuits
– breaker should be racked out with control power fuses removed
• There are many types of ACB circuit breakers
– Method of rackout will be specific to type of breaker
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Large ACB Racking Operation
• Draw out circuit breakers disconnected by moving breakers
physically away from bus
• Accomplished with racking wrench
Figure: Draw out Breaker
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Large ACB Racking Operation
• Breaker moves from
– CONNECT position to
– TEST position to
– DISCONNECT position
• Some plants have breaker
racking tools that rack the
breaker with the operator at a
distance
• Interlock prevents circuit
breaker operation at any point
during racking operation
© Copyright 2016
Figure: Breaker Racking Tool
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Operator Generic Fundamentals
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Large ACB Racking Operation
• Remote rackout device
minimizes danger associated
with potential arc blast as
breaker is racked out
• Ensure plant procedures and
required PPE are used during
breaker racking operation
• Accidents when racking
breakers can be fatal!
Figure: Racking Arc Blast
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Large ACB Racking Operation
• CONNECT position allows
energizing/ deenergizing load
• TEST position allows operation
of breaker with control power
while not connecting load to
bus
• DISCONNECT allows removal
of breaker from switchgear
Figure: Breaker Racked Out
© Copyright 2016
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Large ACB Racking Operation
• To perform a verification of breaker position and operability, following
must be ensured:
– Indication on floor of circuit breaker housing corresponds to
markings on the circuit breaker
– Racking release lever is fully in CONNECT position (extreme
counterclockwise position)
– Closing spring motor toggle switch is in ON position and closing
spring is charged (or fuses installed)
– Control power breaker is closed and/or control power fuses in
place (not blown), ensuring power is available
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Racking Circuit Breakers
Knowledge Check
To completely deenergize an electrical component and its associated
control and indication circuits, the component breaker should be…
A. open with the control switch in Pull-To-Lock.
B. open with the control switch tagged in the open position.
C. racked out and tagged in racked-out position.
D. racked out with control power fuses removed.
Correct answer is D.
© Copyright 2016
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Operator Generic Fundamentals
117
Circuit Breaker Position Verification
ELO 2.3 – Describe the indications provided for each of the following:
local circuit breaker position indications, control room circuit breaker
status indications, and circuit breaker and protective relay trip indications.
• Circuit breaker status can be determined by a number of means
– Breaker's OPEN/CLOSE mechanical indicators
– Breaker position indication lights (control power)
– Load-side voltage/current
– Physical breaker position
• All can be used to positively determine breaker position
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Operator Generic Fundamentals
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Circuit Breaker Position Verification
• Control indications include:
– Red (energized/closed) indicating light
– Green (deenergized/open) indicating light
– Amber (mismatch) indicating light
o On some breakers
– Breaker control switch position
– Load ammeter
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Operator Generic Fundamentals
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Circuit Breaker Position and Control
Switch Mismatch
• Amber “mismatch” light indication
– When light is ON
o Physical breaker position (open or closed) does not match
current control switch position
– Provides additional control room indication of potential problem
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Circuit Breaker Trip Flags
• Automatic circuit breaker trip may be determined by local device
protection flag indication
• Once protective trip occurs, the trip coil energizes and the circuit
breaker opens
– Indicated by local mechanical flag
• If any protective devices are actuated
– local mechanical trip flags must be manually reset once the
condition clears
o However, relay reset not required to close breaker
– Unless it trips an 86 lockout device
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Operator Generic Fundamentals
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Circuit Breaker Position Verification
• Use all available indications to determine circuit breaker condition
• Best indications are those solely dependent on breaker position
– Load side voltmeter readings
– Local OPEN/CLOSE mechanical flags
• Protective devices (relays) not good indicators
– Breaker may indicate tripped by a protective trip mechanical flag
but may actually be closed
– Circuit breaker could actually be closed but the trip flag may still
indicate tripped because protective device was not reset
(manually) from previous trip
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Circuit Breaker Position Verification
• Circuit breaker position indicating lights are not necessarily a good
indicator of actual breaker position
– Without breaker control power, breaker may indicate open (red
light is not illuminated) when actually closed
– Indicating bulbs may also be burned out
• Load side ammeter readings cannot be counted on to give an
accurate indication of breaker position
– Unless breaker directly Starts/Stops a motor
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Operator Generic Fundamentals
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Circuit Breaker Position Verification
Knowledge Check
Breaker local overcurrent trip flag indicators, when actuated, indicate
that…
A. a breaker trip will occur unless current is reduced.
B. a breaker overcurrent condition is responsible for a breaker trip.
C. an overcurrent condition has cleared and the breaker can be
closed.
D. the associated breaker has failed to trip open during an
overcurrent condition.
Correct answer is B.
© Copyright 2016
ELO 2.3
Operator Generic Fundamentals
124
Circuit Breaker Position Verification
Knowledge Check
While remotely investigating the condition of a normally-open 480 VAC
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 lit.
• MCC voltmeter indicates 480 VAC.
• MCC ammeter indicates zero amperes.
Based on these indications, the operator should report that the feeder
breaker is __________ and racked __________.
A. open; in
B. closed; in
C. open; to the TEST position
D. closed; to the TEST position
Correct answer is B.
© Copyright 2016
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Operator Generic Fundamentals
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Circuit Breaker Position Verification
Knowledge Check
While remotely investigating the condition of a typical normally-open
motor control center (MCC) feeder breaker, an operator observes the
following indications:
• Green breaker position indicating light is lit.
• Red breaker position indicating light is out.
• MCC voltmeter indicates zero volts.
• MCC ammeter indicates zero amperes.
Based on these indications, the operator can accurately report that the
breaker is open and racked to __________ position.
A. the OUT
B. the IN
C. the TEST
D. an unknown
Correct answer is D.
© Copyright 2016
ELO 2.3
Operator Generic Fundamentals
126
Circuit Breaker Control Power
ELO 2.4 – Describe the effects of losing circuit breaker control power on
breaker operation and indications.
• To operate breakers using a control switch, remotely or locally, a
control circuit must be provided
• Control power is usually taken from line contacts, rectified, and
provided to the control circuit through fuses
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Operator Generic Fundamentals
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Circuit Breaker Control Power
• Rectifier unit
• Closing relay
• Closing coil
• Trip coil
• Auxiliary contacts
• Breaker control switch
Figure: Breaker Control Circuit
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Operator Generic Fundamentals
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Circuit Breaker Control Power
• Protective features can be
added to trip circuit breaker
• Control circuit shown has
protective features that will
close contacts to energize trip
coil
– Underfrequency
– Undervoltage
Figure: Breaker Control Circuit
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Closing Breaker
• Control switch to close
– Energizes closing relay
– Energizes closing coil
– Closes breaker
– Closes "a" contact to enable
the trip coil
• No automatic closures
Figure: Breaker Control Circuit
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Opening Breaker
• Control switch to "trip"
– Trip coil releases latch,
allowing breaker to open
– "a" contact opens,
deenergizing trip coil
– "b" contact closes, enabling
closing relay
• Automatic opening features
– Underfrequency
– Undervoltage
– When relays actuate,
contact energize trip coil
© Copyright 2016
Figure: Breaker Control Circuit
ELO 2.4
Operator Generic Fundamentals
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Loss of Control Power
• Effects of losing circuit breaker control power
– Lose local and remote breaker indication lights
– Lose remote breaker control (open/close)
– Lose capability to trip open automatically from a protection trip
device or electrical fault
– Lose ability for breaker closing spring to electrically recharge after
local closing of breaker (charging motor not energized)
– Can still OPEN breaker locally/manually
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Operator Generic Fundamentals
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Circuit Breaker Control Power
Knowledge Check
Loss of breaker control power will cause...
A. breaker line voltage to indicate zero regardless of actual breaker
position.
B. the remote breaker position to indicate open regardless of actual
breaker position.
C. inability to operate the breaker locally and remotely.
D. failure of the closing spring to charge following local closing of
the breaker.
Correct answer is D.
© Copyright 2016
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Operator Generic Fundamentals
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Circuit Breaker Control Power
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. removed from; connected to
C. available to; isolated from
D. available to; connected to
Correct answer is C.
© Copyright 2016
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Paralleling AC Sources
TLO 3 – Describe the conditions that must be met prior to paralleling two
generators including effects of not meeting these conditions.
3.1 Describe the conditions required to properly parallel two AC power
sources, including voltage, frequency, and phase.
3.2 Describe the effects of paralleling two AC sources under the
following conditions: current out of phase, frequencies not matched,
high voltage differential, and low current or too much load.
© Copyright 2016
TLO 3
Operator Generic Fundamentals
135
Paralleling AC Sources – Conditions
ELO 3.1 – Describe the conditions required to properly parallel two AC
power sources, including voltage, frequency, and phase.
Three conditions must be met prior to paralleling or synchronizing AC
sources.
• Terminal voltages almost equal
– Minimize VAR loading
• Frequency of incoming machine slightly higher than grid
– Synchroscope rotating slowly in the FAST (clockwise) direction
– Ensures incoming machine picks up some load instead of
becoming a load (motorizing)
• Output voltages in phase
– Minimize current surge through breaker
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Paralleling AC Sources
• During paralleling operations, voltages of the generators to be
paralleled are shown on voltmeters
• Frequency matching is accomplished through the use of output
frequency meters
• A synchroscope is a device that senses two frequencies
– indicates phase differences between generators
– allows phase matching of two generators
• Once breaker closed, synchroscope locks in
at 12 o’clock
• Most plants have “sync check relays”
– Doesn’t allow breaker closure out of phase
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Operator Generic Fundamentals
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Abnormal Conditions During Paralleling
Operations
ELO 3.2 – Describe the effects of paralleling two AC sources under the
following conditions: current out of phase, frequencies not matched, high
voltage differential, and low current or too much load..
• Recall that three conditions must be met prior to paralleling or
synchronizing AC sources
– Following these three minimize current through breaker being
paralleled
o Minimum VAR loading
o Minimum real load picked up
o Minimum current from phase differences
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Paralleling Terminal Voltages
• Voltages of running and incoming should be matched
– Some plants allow incoming voltage to be slightly higher
• Voltage indication might be “stepped down” to 120 volts on meters
• Once generator loaded, voltage setting can be changed to carry
desired VAR loading
– Usually raise voltage in morning to pick up anticipated VARs out
– Usually lower voltage in evening to lower anticipated VARs out
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Operator Generic Fundamentals
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Paralleling Frequencies
• Frequency of incoming machine slightly higher
– Ensures some load picked up by incoming generator
– Operator usually goes to RAISE on speed control to pick up
additional load
• Frequency of incoming machine slightly lower
– Synchroscope rotating in SLOW (counterclockwise) direction
o Known as "motoring or reverse powering“
– Breaker could stay closed if speed control immediately taken to
RAISE to pick up load
• If frequencies matched
– Synchroscope stays motionless at given clock position
o Could be in-phase or out-of-phase
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Operator Generic Fundamentals
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Paralleling Output Voltages in Phase
• Incoming (generator) and running (grid) “in-phase” as synchroscope
passes through 12 o’clock
• When breaker closed, generator gets “locked” in phase with grid
– Synchroscope goes to 12 o’clock and stays there
• If breaker closed “out-of-phase”, current surge could damage breaker
or generator
• Sync check relays might allow closure
– Between 11 and 1 o’clock
– Basically between +30 degrees and -30 degrees phase
differential
– Therefore, anything > 30 degrees could be damaging
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Operator Generic Fundamentals
141
Powering a Deenergized Bus
• Closing the output breaker of a three-phase generator onto a
deenergized bus can produce an overcurrent condition on the
generator
– Occurs if the bus was not first unloaded
– Due to instantaneous flow of starting current for all loads running
when power was lost
• If DG provided sole power to a vital bus
– Bus usually “load shed” first (all breakers open)
– DG comes up to speed
– DG output breaker closes
– Sequencer loads bus in timed sequence
o Allows starting currents to diminish before starting next load
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Operator Generic Fundamentals
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Paralleling Operations
Knowledge Check
A main generator is about to be connected to an infinite power grid.
Generator voltage is equal to grid voltage and the synchroscope is
rotating slowly in the counterclockwise direction. The generator breaker
is closed just prior to the synchroscope pointer reaching the 12 o'clock
position.
Which one of the following is most likely to occur after the breaker is
closed?
A. The breaker will remain closed and the generator will supply
only MW to the grid.
B. The breaker will remain closed and the generator will supply
both MW and MVAR to the grid.
C. The breaker will open due to overcurrent.
D. The breaker will open due to reverse power.
Correct answer is D.
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Operator Generic Fundamentals
143
Paralleling Operations
Knowledge Check
If a main generator output breaker is closed when the generator output
voltage is 5 degrees out of phase with the power grid voltage, the main
generator will experience a __________ stress; if the breaker remains
closed and no additional operator action is taken, the main generator
voltage will __________ with the grid voltage.
A.
B.
C.
D.
minor; remain out of phase
minor; become locked into phase
potentially damaging; remain out of phase
potentially damaging; become locked into phase
Correct answer is B.
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Operator Generic Fundamentals
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NRC KA to ELO Tie
KA #
KA Statement
Purpose of racking out breakers (de-energize components and associated control and
K1.01 indication circuits)
RO SRO
2.6
2.8
2.2
K1.02 Local indication that breaker is open, closed or tripped
Loss of power supply circuit breaker indicator lights and capability in remotely open and
K1.03 close
Operation of various push buttons, switches and handles and the resulting action on
K1.04 breakers
2.8
2.9
2.3
2.9
3.1
2.4
2.9
3.0
2.1
K1.05 Function of thermal overload protection device
2.3
2.4
1.2
K1.06 Interpretation of symbols for breakers, relays and disconnects in a one-line diagram
Safety procedures and precautions associated with breakers, including MCC bus
K1.07 breakers, high, medium and low voltage breakers, relays and disconnects
Effects of closing breakers with current out of phase, different frequencies, high voltage
K1.08 differential, low current, or too much load
Effect of racking out breakers on control and indicating circuits and removal of control
K1.09 power on breaker operation
2.3
2.6
1.5
3.0
3.3
1.4
3.3
3.5
3.2
2.8
3.1 2.3, 2.4
K1.10 Function, control, and precautions associated with disconnects
2.7
3.1
1.3
K1.11 Control room indication of a breaker status
3.1
3.3
2.3
K1.12 Trip indicators for circuit breakers and protective relays
2.9
2.9 1.2, 2.3
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ELO
Operator Generic Fundamentals