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TM 5-685/NAVFAC MO-912
(1) Automatic. Automatic transfer switches
(a) Circuit breaker type. Circuit breaker
have voltage sensing relays for each phase. The
transfer switches are mechanically held devices us-
sensing relays are connected to the normal power
ing two circuit breakers. Usually the breaker han-
bus, behind the protecting devices.
dles are operated by a transfer mechanism which
(a) The transfer switch is connected to the
provides double-throw switching action connecting
normal power source under normal conditions.
one circuit terminal to either of two others. The
When the sensing relays detect a sustained drop in
transfer mechanism is operated electrically by a
the voltage of the normal power source, the relays
unidirectional gear motor (motor and integral
will automatically start the auxiliary generator. The
speed-reducing gearbox) or by dual motor operators
transfer switch operates upon a sustained drop in
with all parts in positive contact at all times. These
voltage in any phase of the normal source (approxi-
switches can also be operated manually and have
mately a 30 percent drop and delay of about two
provisions for disengaging the generator when nec-
seconds) to start the auxiliary generator.
(b) Neutral position. Some transfer switches
(b) When voltage and frequency of the auxil-
have a neutral position. However, the switch is me-
iary generator are at rated values, and the normal
chanically and electrically interlocked so that a neu-
power source is still below normal, the automatic
tral position is not possible during electrical opera-
control will transfer the load to the emergency
tion. Also, load circuits cannot be connected by the
switch to normal and emergency sources simulta-
(c) Upon return of normal power to within 10
neously whether the switch is operated electrically
percent of rated voltage on all phases and after a
or manually.
preset time delay, the switch automatically trans-
(c) Contactor type. Contactor type transfer
fers the load to the normal source. Usually the aux-
switches have mechanically or electrically held
iliary generator will run unloaded for about five
contactors with a command load bus. The switches
minutes after the transfer, before it shuts down.
are mechanically and electrically interlocked so that
The controls automatically reset for the next emer-
a neutral position is not possible under normal elec-
gency start.
trical operation. Additionally, the load circuits can-
(d) Usually the controls of a power transfer
not be connected to normal and emergency sources
system have a test switch. This permits simulation
of failure of the normal power source and test of
(2) Bypass function. An electrically operated
transfer switch operation.
transfer switch can be provided with a bypass func-
(e) Power transfer indicators are provided in
tion. The bypass function manually transfers the
most automatic transfer systems to indicate the cur-
power around the automatic transfer switch. The
rently used power source. Usually an amber light
electrically operated switch can then be tested, re-
marked "Emergency Power" shows that the system
moved, and repaired. The bypass function may or
is on emergency power when illuminated. A white
may not cause a momentary interruption to the load
light marked "Normal Power" shows that the sys-
depending upon the manufacturer. The bypass is
tem is receiving power from its normal source when
purely a manual function, therefore, if the source to
which the bypass is connected fails the bypass must
(2) Nonautomatic. In nonautomatic operation,
be manually transferred to the alternate source.
an operator is needed to manually transfer to or
Bypass transfer switches are only used in the most
from the emergency power source. The operator can
critical applications where the load is operational
usually make the transfer without opening an en-
closure. The transfer is usually based on instrument
(3) Manually operated. Manual  transfer
indications and is made by placing the transfer
switches are mechanically held devices using two
switch in the required emergency or normal posi-
circuit breakers operated by a handle. All parts are
in positive contact at all times. The switch is me-
(a) Power transfer indicators are provided
chanically interlocked; it is impossible for the load
for the operator. An amber light (Emergency Power)
circuits to be connected to normal and emergency
shows that the system is on emergency power when
sources simultaneously. Manually operated transfer
illuminated. A white light (Normal Power) shows
switches are available with single or dual operating
that the system is receiving power from its normal
handles. A common operating mechanism across the
source when illuminated.
two breakers mechanically connects and operates
(b) The operator is usually provided with an
the breakers.
override switch which bypasses the automatic
b. Operation. Transfer switches have two operat-
transfer controls. This feature permits indefinite
ing modes: automatic and non-automatic.


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