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Applying Transfer Switch Withstand and Closing Ratings

When specifying transfer switches, it is important to understand their Withstand and Closing Ratings so the most appropriate models can be selected. Selecting switches with ratings below those required for the application can negatively impact reliability and availability. Selecting higher-than-needed ratings can result in excessive equipment costs. The following sections briefly explain what the ratings represent and describe the information needed to make the best selection.

The Need to Withstand and Close On Fault Currents

Clearing faults and protecting circuits is the function of breakers and fuses, not transfer switches. The sole purpose of a transfer switch is to connect electrical load to a power source. Providing power is the reason for using the switch and the rest of the backup power system. Consequently, a transfer switch that is connected to or closing on a power source must withstand and close on a short-circuit current until it is cleared by an overcurrent protection device. Withstand and Closing Ratings provide the information needed to specify transfer switches that can do the job.

In North America and certain other regions, UL 1008 Standard for Safety – Transfer Switch Equipment prescribes the tests that are used to verify these transfer switch ratings. IEC 60947-6-1 – Low-voltage switchgear and controlgear – Part 6-1: Multiple function equipment – Transfer switching equipment provides similar ratings that are used in Europe, Asia, and other parts of the world. The ratings are verified through testing according to these standards.

Transfer Switch Testing

The time duration for which a transfer switch can withstand a short circuit current is inversely proportional to the amount of current. To demonstrate withstand capability, UL and IEC testing subject transfer switches to prescribed amounts of current for prescribed durations. Under UL 1008, the current requirements scale with the ampacity of the transfer switch, where the available short circuit current must equal or exceed (1) 5000 Amps for switches rated up to 100 Amps, (2) 10,000 Amps for switches rated up to 400 Amps; and (3) 20,000 Amps (but not < 10,000 Amps) for switches rated above 400 Amps.

Using UL 1008, the Withstand Test and the Closing Test are conducted under these conditions. For the Withstand Test, the procedure begins with the switch contacts closed, and the switch must carry this current until an upstream overcurrent device opens. The power factor must not exceed 0.50 for currents up to 10,000 Amps, 0.30 for currents up to 20,000 Amps, and 0.20 for currents exceeding 20,000 Amps. An optional time-based test may also be conducted to provide additional short circuit ratings for a minimum of 0.050 seconds. During the Closing Test, the switch is subject to the same conditions, except that the test is started with the contacts in the open position.

Following these tests, UL1008 requires that the transfer switch shows:
  • the ability to close on untested contacts of the opposite power source circuit
  • the ability to operate using whatever automatic or manual controls are provided
  • no continuity between the terminals of the normal and alternate power source circuits
  • no opening of doors on transfer switch enclosures
  • cables cannot pull away from lugs and connectors
Following the Withstand and Closing Tests, the switch must also pass a Dielectric Voltage Withstand Test to verify that the test currents did not compromise circuit isolation in the switch. The testing is used to specify time-based ratings, which stipulate how much current can be held for the time required by the test, regardless of the type of overcurrent protection used. Because fast-acting circuit breakers and fuses shorten the duration of overcurrent events, UL 1008 offers manufacturer’s the ability to test and rate switches for use with specific fuses and circuit breakers.

Selective coordination schemes may require transfer switches to withstand current for longer durations than used for Withstand and Closing Tests. (For more information about selective coordination, review tech brief and white paper listed at the end of this article.) UL1008 thus specifies testing measures to verify optional short-time ratings. Verification requires the performance of a Short-Time Current Rating Test. This, in turn, requires a Temperature Rise Test to verify that materials in the switch are not compromised by the heat produced by the currents in ways that could present a safety hazard.

The Resulting Ratings

The ratings produced by UL 1008 testing include values for withstand and closing by overprotection type. They include ratings for fuses and for generic unspecified circuit breakers, and optional ratings for specific circuit breakers and Short-Time Ratings for holding currents for durations beyond the minimum time required by UL 1008. The structure and relationship of the ratings is shown in the following diagram.
The test results translate to ratings published by manufacturers. For example, the following table shows each type of rating for select ASCO transfer switches.
To select a switch rating, one must know the amount of short current that could flow at the transfer switch location as well as the characteristics of the circuit’s overcurrent protection devices, including their opening times. This is typically produced by a coordination study of the power distribution system in question. An appropriate transfer switch can then be selected to handle the necessary amount of current for the required timeframe. For more information about transfer switch selection and the most up-to-date ratings for ASCO switches, refer to ASCO Publication 1128 - Engineering Application Information - Withstand and Closing Ratings for Transfer Switch Equipment.


ASCO White Paper: UL1008 Transfer Switch Withstand and Closing Ratings

International Electrical Commission: IEC 60947-6-1 – Low-voltage switchgear and controlgear – Part 6-1: Multiple function equipment – Transfer switching equipment

ASCO Tech Brief: Selective Coordination

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