Transfer Switch Withstand Ratings: 3-cycle or 30-cycle?

How and when to apply “3-cycle rated” vs. “30-cycle rated” transfer switches should be a rather important consideration for a design engineer.  A coordination study is a critical element that must go hand-in-hand with the design and specification of a safe and reliable emergency power system.

Some basic definitions

Interrupting ratings  – the highest current at rated voltage that a device is intended to interrupt under standard test conditions.

Withstand ratings – 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 their normal current.

The criteria
Testing agencies utilize the following withstand test criteria when reviewing products for approval under the UL-1008 standard:

  1. the door of the enclosure shall not have blown off,
  2. the conductors shall not have come loose from their connecting terminals, and
  3. the mounting of the live parts shall not have fallen apart.

The “time” factor
Under UL guidelines, transfer switch manufacturers are free to publish any withstand rating for which they have tested successfully.  Furthermore, UL allows manufacturers to publish that a transfer switch capable of withstanding a fault for three cycles can be placed in series with any circuit breaker upstream, and carry that withstand rating.

Considering that upstream insulated-case circuit breakers typically have, and 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?  It seems clear that, in this instance, a transfer switch with a 30-cycle withstand rating is required to allow proper coordination with upstream protection.

Coordination before product selection
I often see coordination studies conducted after the relevant electrical equipment has been selected (and sometimes even delivered!).  I can’t help but think that this must lead to difficulties in the commissioning phase of a project.  My recommendation would be to obtain withstand rating data for any automatic transfer switches as early as possible in the design process.  This is especially important if transfer switches will be protected by multiple levels of upstream protective devices.  With this withstand rating data available upfront, transfer switches can be properly specified and, when the time comes, successfully incorporated into a coordination study.

Have you encountered difficulties in this area of design?  How much participation do you have in the development of a coordination study?  What are the obstacles to the preparation of a preliminary coordination study ahead of final equipment selection?  Please share your questions or comments below!

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