Component Protection
Transfer Switches
Transfer switches are designed to transfer power sources under load in order
to feed a system, typically an emergency system, consisting of critical loads.
These devices are tested to meet basic short circuit testing requirements.
Transfer switches are often tested per UL Standard 1008.
Transfer switches should always be evaluated on the basis of the maximum
available short-circuit currents. The automatic transfer switch must withstand:
a. The magnetic stresses imposed by the instantaneous peak current available at
the point of application, and
b. The thermal stresses imposed by the available RMS short-circuit current.
The short-circuit current withstand rating of the transfer switch must be equal
to or greater than the available short-circuit current at the point of application.
When properly coordinated with current-limiting devices, automatic transfer
switches can be used on circuits having available short-circuit currents greater
than their unprotected withstand short-circuit current rating. Modern currentlimiting fuses, when properly sized, limit the short-circuit current to within the
withstand rating of a transfer switch.
Transfer switches must withstand minimum short-circuit currents at specified
power factors, as listed in UL Standard 1008, until the overcurrent protective
devices open.
Transfer switch manufacturers generally publish the withstand rating data for
their products. When the available short-circuit current exceeds the withstand
rating of the transfer switch, current-limitation is required. Properly sized
modern current-limiting fuses ahead of the transfer switch limit the available
short-circuit current to within the withstand rating of a transfer switch, thereby
protecting the transfer switch.
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At the option of the transfer switch manufacturer, their transfer switches can
be evaluated and if successfully pass, listed for available short-circuit currents
greater than the values in table “UL 1008 Minimum Withstand Test
Requirement.” This can be achieved with fuse protection. The transfer switch
manufacturer will mark the equipment with the fuse class and rating required
to achieve these higher short-circuit current ratings (withstand rating) as well
as the withstand rating without current-limiting fuses (three-cycle withstand
rating). Below is an example of a typical transfer switch label. For instance,
the 100A transfer switch has a 100,000A withstand rating with LPJ-300SP
fuses (Class J) and only a 22,000A three cycle withstand rating (without
current-limitation).
Switch
Size (Amps)
Max. Fuse
Amp/Class
Withstand
RMS Sym.
3-Cycle
Withstand
RMS Sym.
100
300A Class J
100,000A
22,000A
400
600
800
800A Class J
1200A Class L
1200A Class L
200,000A
200,000A
200,000A
35,000A
42,000A
42,000A
1000
1200
2000A Class L
2000A Class L
200,000A
200,000A
65,000A
65,000A
If a transfer switch has been listed or labeled for a maximum short-circuit
current with specific overcurrent protective devices, it can not be used with
those protective devices where there are greater available fault currents. If a
transfer switch utilizes circuit breakers for the transfer mechanism, it can not
be used where the available short-circuit current exceeds its short-circuit
current rating.
©2005 Cooper Bussmann