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 TM 5-685/NAVFAC MO-912
CHAPTER 5
SWITCHGEAR
5-1. Switchgear definition.
bus are typical heavy-duty conductors rated for use
between 601 volts AC and 38,000 volts AC, as re-
Switchgear is a general term covering switching
quired.
and interrupting devices that control, meter and
protect the flow of electric power. The component
5-3. Low voltage elements.
parts include circuit breakers, instrument trans-
a. Circuit breakers. Either molded-case or air cir-
formers, transfer switches, voltage regulators, in-
cuit breakers are used with low voltage switchgear.
struments, and protective relays and devices.
Usually the air circuit breakers have draw-out con-
Switchgear includes associated interconnections
struction. This feature permits removal of an indi-
and supporting or enclosing structures. The various
vidual breaker from the switchgear enclosure for
configurations range in size from a single panel to
inspection or maintenance without de-energizing
an assembly of panels and enclosures (see fig 5-l).
the main bus.
Figure 5-2 contains a diagram of typical switchgear
(1) Air circuit breakers. Air circuit breakers are
control circuitry. Switchgear subdivides large blocks
usually used for heavy-duty, low voltage applica-
of electric power and performs the following mis-
tions. Heavy-duty circuit breakers are capable of
sions:
handling higher power loads than molded-case
a. Distributes incoming power between technical
units and have higher current-interrupting capac-
and non-technical loads.
ity. Air circuit breakers feature actuation of contacts
b. Isolates the various loads.
by stored energy which is either electrically or
c. Controls auxiliary power sources.
manually applied. Accordingly, the mechanism is
d. Provides the means to determine the quality
powered to be put in a position where stored energy
and status of electric power.
can be released to close or open the contacts very
e. Protects the generation and distribution sys-
quickly. Closing or tripping action is applied man-
tems.
ually (by hand or foot power) or electrically (where
a solenoid provides mechanical force). The me-
5-2. Types of switchgear.
chanical force may be applied magnetically. Air
Voltage classification. Low voltage and medium
circuit breakers contain power sensor overcurrent
voltage switchgear equipment are used in auxiliary
trip devices that detect an overcurrent to the load
power generation systems. Switchgear at military
and initiate tripping or opening of the circuit
installations is usually in a grounded, metal enclo-
breaker.
sure (see fig 5-l). Per the Institute of Electrical and
(a) Manual circuit breakers employ spring-
Electronics Engineers (IEEE), equipment rated up
operated, stored-energy mechanisms for operation.
to 1000 volts AC is classed as low voltage. Equip-
Release of the energy results in quick operation of
ment equal to or greater than 1000 volts but less
the mechanism to open or close the contacts. Oper-
than 100,000 volts AC is classed as medium voltage.
ating speed is not dependent on the speed or force
a. Low voltage. Major elements of low voltage
used by the operator to store the energy.
switchgear are circuit breakers, potential trans-
(b) Fast and positive action prevents unnec-
formers, current transformers, and control circuits,
essary arcing between the movable and stationary
refer to paragraph 5-3. Related elements of the
contacts. This results in longer contact and breaker
switchgear include the service entrance conductor,
life.
main box, switches, indicator lights, and i nstru-
(c) Manual stored-energy circuit breakers
ments . The service entrance conductor and main
have springs which are charged (refer to the glos-
bus (sized as required) are typical heavy duty con-
sary) by operation of the insulated handle. The
ductors used to carry heavy current loads.
charging action energizes the spring prior to closing
b. Medium voltage. Medium voltage switchgear
or opening of the circuit breaker. The spring, when
consists of major and related elements as in low
fully charged, contains enough stored energy to pro-
voltage switchgear. Refer to paragraph 5-4 for de-
vide at least one closing and one opening of the
tails. Construction of circuit breakers employed in
circuit breaker. The charged spring provides quick
the two types of switchgear and the methods to
and positive operation of the circuit breaker. Part of
accomplish breaker tripping are the primary differ-
the stored energy, which is released during closing,
ences. The service entrance conductors and main
may be used to charge the opening springs.
5-1
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