AUTOMATIC TRANSFER SWITCH
MANUAL OPERATION
ENGINEERING BRIEF
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MANUAL OPERATION
The ability to manually operate an automatic transfer switch has obvious merits.
Since many transfer switch failures can be attributed to control failures, a manual
operating means may allow an operator to maintain electrical supply to the load
with a minimum of disruption, even during utility power failures.
Further,
maintenance procedures are greatly simplified.
It is important, however, to ensure that the design of a manually operated
transfer switch does not unnecessarily degrade system performance, integrity, or
safety. The following points should be considered:
(1)
The transfer switch should not suffer a reduction of rating when operated
manually, as opposed to electrically. For instance, it should be capable of
interrupting the full rated load, as well as closing on to a faulted system
where a fault current level up to the withstand rating of the transfer switch
may occur. Failure to meet these performance levels could lead to
destruction of the transfer switch should it be operated under these
conditions.
It must be recognized that, even if a transfer switch is not intended for
manual switching while carrying load, there is no assurance that it will not
happen.
The simple truth is that transfer switches are regularly operated manually
while carrying current. This happens during commissioning, servicing, and
due to operator error. Transfer switches with slow operating contacts (eg.
solenoid-operated "contactor" type) are distinctly inferior when operated
under these conditions. Consider what could happen if a transfer switch
was slowly closed onto a bolted fault.
UL Standard 1008 places additional requirements on some automatic
transfer switches, which have "slow operating" manual operation, in
recognition of the contact arcing problem. These requirements exceed
CSA's Standard 178.
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MANUAL OPERATION
To ensure that damage does not take place during switching, quick-make,
quick-break contacts are required. These require a "stored-energy"
mechanism, and although more costly to produce, guarantee fast
operation under all conditions. All Thomson Technology transfer switches
have stored-energy contacts. Solenoid operated contactor type transfer
switches don't have stored-energy contacts simply because the solenoid
isn't strong enough to charge the springs.
(2)
The transfer switch must be safe for operating personnel. If manual
transferring is attempted without the control circuits being disabled, there
should be no possibility of injury. This may be accomplished by a means
which automatically disables the operating mechanism when access for
manual operation is gained,
or by a manual mechanism with an overriding device which will allow the
manual handle to remain stationary while the mechanism is electrically
operated.
(3)
The manual operating means must be obvious, simple, and secure. A
manual handle which is small, not labeled, or not permanently attached in
the operating position (ie. one which may be lost) is not acceptable.
Further, it should not be necessary for inexperienced personnel to refer to
the instruction manual to be able to quickly effect a manual transfer, since
transfer switches are emergency devices, and a need for manual
operation would likely occur under adverse conditions.
To address these critical performance requirements, Thomson Technology
transfer switches provide:
*
stored-energy contacts which always operate quickly, whether manually or
electrically initiated,
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MANUAL OPERATION
*
a safety switch or other means to automatically prevent the mechanism
from harming an operator while performing manual switching without
having disconnected power, and
*
a clearly labelled and permanently attached manual handle, which is very
easy to operate.
Conclusion
The bottom line is that you have a transfer switch because there is a critical need
to maintain power to a load. To achieve that result under all conditions, a manual
handle which is safe and easy to find, and which cannot cause contact damage,
is the best.
NOTICE: This information is the property of Thomson Technology Inc. Permission is hereby
given to reproduce this document in this exact form only, without modification of any kind.
© THOMSON TECHNOLOGY INC
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