Interrupting ratings are the highest current at rated voltage that a device is intended to interrupt under standard test conditions. Withstand ratings are the highest current at rated voltage that a device is intended to carry under standard test conditions.
When applied to automatic transfer switches, these ratings become the highest rated current that the transfer switch can either withstand or interrupt, under standard test conditions. UL-1008, the industry standard for automatic transfer switches in the United States, requires that transfer switches be able to withstand (or interrupt) up to 20 times its normal current. The passing criteria for this test are:
- that the door of the enclosure shall not have blown off,
- the conductors shall not have come loose from their connecting terminals, and
- the mounting of the live parts shall not have fallen apart.
Lake Shore Electric IC-Series transfer switches are built with automatic and non-automatic insulated case switches. Independent of whether the transfer switch is equipped with overcurrent protection (as in the case of service entrance rated equipment), all of these transfer switches contain high interrupting trips, which easily allow for interrupting of available fault currents to meet the UL-1008 criteria. In fact, after a fault is cleared, a Lake Shore Electric automatic transfer switch can generally be returned to service (note: proper tests should be performed any time a circuit breaker is subjected to extreme high current faults).
Under UL guidelines, transfer switch manufacturers are free to publish whatever withstand ratings for which they have tested successfully. UL also allows a transfer switch that has withstood the fault for three cycles to publish that it can be placed in series with any circuit breaker upstream, and carry that withstand rating. How to apply these “3-cycle rated” transfer switches is a rather important consideration for a design engineer.
Considering that upstream insulated case circuit breakers are often set with instantaneous trip settings of up to 5 cycles, what happens to life safety and equipment integrity when a 3-cycle transfer switch is exposed to a 5-cycle fault?
Lake Shore Electric utilizes circuit breakers which carry a 30-cycle withstand rating. In practice, this product allows true coordination with upstream breakers without concern for the “2-cycle gap” described above.