.1 SWITCHGEAR DEFINITION.
Switchgear is a general term covering switching and
interrupting devices that control, meter and protect
the flow of electric power. The
component parts include circuit breakers, instrument
transformers, transfer switches,voltage regulators,
instruments, and protective relays and devices.
Switchgear includes associated interconnections
and supporting or enclosing structures.
.The Switchgear subdivides large blocks of electric
power and performs the
following missions:
1) Distributes incoming power between technical
and non-technical loads.
2) Isolates the various loads.
3) Controls auxiliary power sources.
4) Provides the means to determine the quality and
status of electric power.
5) Protects the generation and distribution systems
2. TYPES OF SWITCHGEAR.
Voltage classification: Low voltage and medium
voltage
* equipment rated up to 1000 volts AC is classed
as low voltage
* Equipment equal to or greater than 1000 volts
but less than 100,000 volts AC is classed as
medium voltage.
a). LOW VOLTAGE ELEMENTS.
Air circuit breakers
* are used with low voltage switchgear. Usually the air circuit
breakers have draw-out construction.This feature permits
removal of an individual breaker from the switchgear
enclosure for inspection or maintenance without de-
energizing the main bus.
* are usually used for heavy-duty.low voltage applications.
Heavy-duty circuit breakers are capable of handling higher
power loads than molded-case units and have higher
current-interrupting capacity.
* feature actuation of contacts by stored energy which is
either electrically or manually applied.
.
* It contain power sensor overcurrent trip devices that detect
an overcurrent to the load and initiate tripping or opening of
the circuit breaker.
* FAST AND POSITIVE action prevents unnecessary arcing
between the movable and stationary contacts. This results in
longer contact and breaker life.
b) MEDIUM VOLTAGE ELEMENTS.
CIRCUIT BREAKERS. Medium voltage switchgear uses oil, air-
blast, or vacuum circuit breakers. Usually the circuit breakers
have draw-out construction to permit removal of an individual
breaker from the enclosure for inspection or maintenance without
deenergizing the main bus. All of these circuit breakers can
quickly interrupt and extinguish the electric arc that occurs
between breaker contacts when the contacts are separated.
Types of MV circuit breakers
* OIL CIRCUIT BREAKERS
When the contacts are separated in oil, the interrupted voltage
and current can be greater as compared to contact separation in
air at room temperature.
ARC INTERRUPTION is better in oil than air because the dielectric
strength of oil is much greater than air. Also, the arc generates hydrogen
gas from the oil The gas is superior to air as a cooling medium.
USUALLY THE CONTACTS and the arc are enclosed in a fiber arcing
chamber, with exhaust ports on one side, to increase the capacity.
* VACUUM CIRCUIT BREAKERS.
Vacuum arc interruption is the newest and quickest
method of extinguishing an electric arc. This type of
breaker is oil-less, fireproof and nearly maintenance free.
Service life is very long.
Arc interruption is very rapid, usually in the first current zero. High
dielectric strength of a small vacuum gap contributes to the rapid
interruption of the arc. Short contact travel permits the mechanism to part
the contacts much faster than for oil breakers.
