Download Switches and Over Current Protective Devices

Switches and
Over Current Protective
Devices
Class Instructor
ALPHER E. DE VERA, uap
DEVICES
Switches
Fuse
Circuit Breakers
Panelboard
Unit Sub – Station
Switchboard
Receptacles
Outlet and Boxes
Emergency / Stand By Power Equipment
A. SWITCHES
Switches are
electrical devices
intended for on / off
control of an electrical
circuit.
Rated for current,
voltage, duty, poles
and throw, including
fusibility and
enclosure.
A. SWITCHES
The current rating is the
amount of current that the
switch can carry
continuously and
interrupt safely .
Switches are rated 250V,
600V or 5KV.
Switches for normal use
in light and power circuits
are called general use
safety switches and are
rated ND ( normal duty ).
A. SWITCHES
Switches intended for
frequent interrupting are
rated HD ( heavy duty ).
Switches intended to be
opened only occasionally,
such as service switches,
are rated LD ( light duty ).
Switch may be
constructed with ( fusible
) or without ( non – fusible
) fusing.
A. SWITCHES
NEC states that “ No
grounded neutral
conductor should be
broken, thus most
switches must carry
the neutral through
unbroken, by means
of solid link within the
switch, known as “
Solid Neutral .
SWITCH POLES AND THROWS
Single Pole – Single
Throw Switch
Two Pole – Single Throw
Switch
Three Pole – Solid
Neutral Switch
Single Pole – Double
Throw Switch
* Center Type – With center “
off “ position or hand off control
automatic switch
* 3 – Way Switch – Two different
locations
SPECIAL SWITCHES
Time Controlled Devices
Precise low speed miniature drive motor or timer
permitting electric connection. Useful for areas
frequented by power outages.
Remote Control Switches
Contactor that latches mechanically useful for lighting
control and for energy conservation techniques.
Automatic Transfer Switch
Essential part of stand by power arrangement, a double
throw switch generally 3 – pole to restore power in case
of power failure.
CIRCUIT PROTECTIVE DEVICES
To protect the insulation, wiring, switches
and other apparatus from overload and
short circuit currents.
Acts as automatic means for opening
circuits as well as protective devices.
Two most common devices are :
1. Fuse
2. Circuit Breaker
B. 1. FUSE
Simple device
consisting of fusible
link or wire of low
melting temperature.
Two kinds are
cartridge fuse (
enclosed in insulating
fiber tube ) and plug
fuse ( enclosed in
porcelain cup ).
B.1. FUSE
When the fuse is
subjected to excess
current, the energy loss in
the link generates heat
and melt it.
Plug fuse are normally
used in residences all
rated 5 – 30 amp while
cartridge fuse are 5 – 600
amp.
B.2. CIRCUIT BREAKERS
Safety shut-off in electric
circuit: a device that can
automatically stop the flow of
electricity in a circuit if there is
too much current to operate
safely. Circuit breakers are
easier to use than fuses
because they just need to be
reset instead of replaced.
Circuit is a route for
electricity: a route around
which an electrical current can
flow, beginning and ending at
the same point.
B.2. CIRCUIT BREAKERS
An electro mechanical
device with the same
protective function as
a fuse and in addition,
acts as a switch.
It can be used in lieu
of a switch and fuse
combination to both
protect and
disconnect circuit.
B.2. CIRCUIT BREAKERS
Most breakers are equipped with both thermal and
magnetic trips.
Thermal trip – causes an element to move when heat
generation is excessive and trip the latching mechanic of
the breaker, opening the breaker contacts along with the
circuit.
Magnetic Trip – actuate the core which trips the circuit
breaker latch, where thermal and magnetic action have
inverse time characteristics like heavier load means
faster trip of action.
Two types of circuit breakers are molded case breaker
and large air breaker.
FUSE AND CIRCUIT BREAKERS
A. FUSE
Instantaneous in
operation
Energy limiting
device causing self
distraction
B. CIRCUIT BREAKERS
Provided with time delay
avoiding immediate
power failure.
Can be reset after
tripping by merely
operating its handle.
D.1. SWITCHBOARDS
Free standing assemblies
of switches, fuses and
circuit breakers which
provide switching and
feeder protection to a
number of circuits
connected to a main
source.
Represented in a single
line diagram.
D.2. UNIT SUB - STATIONS
Also called “ transfer load
centers “ , an assembly of fuse
and circuit breaker combined
with step – down transformer,
meter controls, bus work and
secondary switchgear.
Available for indoor or outdoor
use.
Useful in supplying power from
a primary voltage line to any
large facility, usually located at
the basement.
D.3. PANELBOARDS
Serve basically the same as “
switchboard “ except on a
smaller scale.
Accepts relatively large blocks
of power and distributes it in
smaller blocks.
Compose of main fuses to
which hare connected in circuit
protective devices ( breakers
and fuses ) which feed to
smaller circuits.
D.3. PANELBOARD
Compose of buses and
breakers mounted on
insulating board that in
turn is mounted inside an
enclosing cabinet.
The line terminal of each
circuit is connected to the
bus bars of the
panelboard.
The load terminal of the
device then feeds the
outgoing branch circuit.
D.3. PANELBOARD
Panelboards are described by specific
type, bus arrangement, branch breakers,
main breaker, voltage and mounting
Panelboards are classified as :
a. Flush type
b. Surface type
E. RECEPTACLES
Identified by the number
of poles and wires and
whether or not designed
for connection of a
separate grounding wire.
The equipment grounding
pole is not counted in the
number of poles but is
counted in wires.
E. RECEPTACLES
Poles must not be confused
with the system ground or
neutral.
Available from 10 – 400
amp., 2- 4 poles and
125V – 600V.
F. OUTLETS AND BOXES
Galvanized stamped
sheet metal provided
with conduit
knockouts
conveniently placed
around the periphery
or at the back.
F. OUTLETS AND BOXES
Most common is the 4”
square sand rectangular
boxes used for fixtures,
junctions, and devices.
Junction box are
enclosure for housing and
protecting electric wires
or cables that are joined
together in connecting or
branching electric circuits
G. EMERGENCY / STAND BY
POWER EQUIPMENT
A. Emergency System
Provide power and
illumination essential for
safety and security of
property.
Emergency power for exit
lighting, elevators, fire alarm,
fire pumps especially for
health care facilities.
B. Stand By System
Provide power to selected
loads in the event of failure of
normal source.
Intended to protect against
property damage and financial
loss not involved in safety
considerations.
Useful for water and sewage
treatment plants and industrial
facilities.