Power Systems

How to apply transfer switch time duration and short-time WCR

Learn about UL 1008 short-circuit testing and passing criteria, and know how to apply the short-circuit withstand and closing rating in a power system

By Hassan Obeid January 12, 2022
Courtesy: Cummins Power Generation


Learning Objectives

  • Learn about the difference between the short-circuit withstand and closing rating and the short-time rating.
  • Explain how the time-based short-circuit WCR simplifies the selection of breakers.
  • Examine how the short-time withstand and closing rating enable selective coordination strategies.

When designing a power system, sizing the electrical distribution to handle the available fault current is critical. A major challenge design engineers face when specifying transfer switches is that the available fault current and the overcurrent protection devices, specifically circuit breakers, are not known at the time specifications are written.

This discusses how design engineers can take advantage of the high withstand and closing rating and short-time rating of transfer switches to simplify circuit breaker selection and enable effective selective coordination schemes.

Because transfer switches are applied in mission critical and life safety applications, superior reliability is paramount. Therefore, transfer switches are subjected to the most stringent testing requirements outlined by the leading standard in North America: UL 1008: Standard for Safety – Transfer Switch Equipment (UL 1008 is harmonized with Canadian standard CSA 22.2 178).

UL 1008 specifies robust testing requirements for verifying manufacturer ratings, including the WCR. The tests specified by UL 1008 are:

  • Temperature rise test.
  • Dielectric voltage-withstand test.
  • Overload test.
  • Contact opening test.
  • Endurance test.
  • Short-circuit test.
  • Dielectric voltage-withstand test (following short-circuit withstand/closing test).
  • Short-time current test (optional).

Most transfer switches don’t experience the extreme UL 1008 testing requirements during their life span. However, the positive outcome is that robust and highly reliable transfer switches are then deployed to mission critical and life safety applications.

Per UL 1008, transfer switches are not intended to interrupt fault current and therefore, they don’t have an ampere interrupting capacity rating. OCPDs, such as fuses and circuit breakers, clear faults and therefore have an AIC rating.

The AIC rating is the maximum available fault current that an OCPD will safely clear when a fault is applied at the load side of the OCPD. Transfer switches have short-circuit WCR: a level of fault current that the transfer switch can withstand and close onto without creating a hazardous condition. For a safe installation, the WCR of the transfer switch must be equal or higher than the available fault current at the point in the circuit where the transfer switch is installed. The WCR presumes that each source of the transfer switch is protected by OCPDs — so that the fault current persists for a limited period of time.

UL 1008 short-circuit WCR test requirements

UL 1008 Table25 lists the minimum current, power factor and time duration to which the transfer switch must be tested. Note that the current can be higher, the power factor can be lower and the time durations can be different. However, the accepted root mean square symmetrical fault currents and durations that are recognized by UL 1008 are listed in UL 1008 Tables 1 and 27.

After a fault (short-circuit) is cleared by the OCPD, the transfer switch must remain operable so that it can restore power to the critical loads from the alternate power source.

Transfer switch short-circuit WCR testing

Transfer switches are subjected to a withstand test and a closing test.

  • Withstand test (starts with contacts closed): A specified fault current is applied for either a specific duration or until a specific OCPD trips.
  • Closing test (starts with contacts open then closes): The same transfer switch must close onto the fault current under the same conditions used in the withstand test. Note: The same set of contacts are used for both tests: withstand and closing.

The short-circuit passing criteria defined by UL 1008 are:

  • Ability to operate the switch and close to the opposite source.
  • No breakage of switch base or any other internal parts.
  • Door must stay secure.
  • Cables stay connected to lugs without insulation damage.
  • No continuity between the normal and alternate source terminals.
  • Pass a dielectric voltage-withstand test.

Guidelines for applying WCR

If a transfer switch does not have a sufficient WCR, severe damage and a potential fire hazard could result from the fault current. One possible solution is to oversize the transfer switch to a larger frame to achieve sufficient WCR. However, this solution could lead to a larger footprint and possibly higher cost.

Good engineering practice requires adequately rated devices in the power distribution system. Therefore, the specified WCR for the transfer switch should be equal to or greater than the available fault current at the location of the transfer switch.

Quick steps to apply the WCR rating of a transfer switch in the power system:

  1. Determine the available fault current at the switch location from the short-circuit analysis.
  2. Select transfer switches with appropriate short circuit WCR: Transfer switches must be rated for the available fault current at their line side terminals and protected by an OCPD selected appropriately.
  3. Select appropriate OCPD:

a. Time based: generic UL 489 circuit breaker.

b. Specific breaker: tested with the transfer switch.

Note that UL 489 requires molded case circuit breakers above 400 amperes to clear a fault in no more than 0.050 seconds. Figure 1 shows the available fault currents at the different buses in the distribution system along with the minimum WCR of each transfer switch.

Figure 1: Transfer switch withstand and closing rating. Courtesy: Cummins Power Generation

Applying a specific breaker to protect the transfer switch

Effective November 2014, UL changed the requirements for adding circuit breakers to transfer switches’ specific breaker file without running additional short-circuit tests (UL 1008 seventh edition). In earlier versions of UL 1008, a new breaker could be added by comparing its maximum published trip time with the maximum published trip time of a previously tested breaker at the rated short-circuit current. If the trip time of the new breaker was less than or equal to the previously tested breaker, the new breaker could be added to the list.

With the introduction of the UL 1008 seventh edition, manufacturers are now required to compare the published trip time of the new breaker with the actual trip time of the breaker from a previous short-circuit test.

For example, if a tested breaker has a maximum published trip time 0.05 seconds, under the earlier versions of UL 1008, any breaker that has a maximum published trip time of 0.05 seconds or less could be added to the list regardless of how long it took that breaker to trip during the test. Under the UL 1008 seventh edition, if the tested breaker actually tripped in 0.03 seconds during the test, only the circuit breakers that have a maximum published trip time of 0.03 seconds or less can be added to the list without an additional test.

This change resulted in the removal of some listed breakers from the file and has made it more difficult to add new breakers to lists without performing additional short-circuit tests. Such tests are difficult to complete due to the limited number of facilities that are capable of performing short-circuit testing and the high cost associated with it.

This breaker qualification challenge may limit the original equipment manufacturer’s ability to provide a full range of breakers approved and listed to protect the transfer switch.

Using transfer switch high time duration WCR to simplify breaker selection

Because it is difficult to have all the variables needed to calculate the fault current, design engineers often take a conservative approach and specify high fault clearing time-based ratings.

Another major challenge engineers are faced with during the design phase is unknown switchgear/switchboard original equipment manufacturers and therefore unknown breaker selections that will be used to protect the transfer switch. Therefore, transfer switches with high time duration based WCR will present a perfect solution to these issues and speed up the design cycle.

In addition, by using the high WCR, which is time duration based, it drastically simplifies the issues presented by UL1008 seventh edition of needing to pre-select the appropriate breakers. Figure 2 shows an example of a transfer switch label with short-circuit WCR.

In Figure 3, different size frame transfer switches with high time duration WCRs are applied to the example shown in Figure 1 to simplify selecting the breakers ahead of the transfer switches.

Figure 2: Transfer switch short-circuit withstand and closing rating label. Courtesy: Cummins Power Generation

Figure 3: Applying time duration based withstand and closing rating. Courtesy: Cummins Power Generation

Short-time WCR

Transfer switch manufacturers can take the extra step and perform the UL 1008 optional short-circuit short-time withstand and closing test and subject the transfer switch to fault currents for durations longer than what is listed in UL 1008 Table 27. The passing criteria for the short-time rating are similar to the short-circuit passing criteria outlined before.

However, there is an additional test that the manufacturer must pass: a temperature rise test. Passing this test demonstrates that the transfer switch can still carry the rated current after a short-circuit occurs in the distribution system.

Selective coordination strategies may require transfer switches to withstand fault current for longer durations than what is shown in UL 1008 Table 27. UL 1008 thus specifies testing measures to verify short-time ratings. The short-time current rating test applies a current equaling the manufacturer’s short-time rating at the maximum rated voltage of the switch at the same power factors used for the WCR tests. Under this test, manufacturers can verify that a switch can hold a current for durations of their choosing.

It is important to note that the short-time rating must be withstand and closing rating. Transfer switches listed as withstand rating only violate UL 1008 — as the rating is not a true short-time rating and shall not be accepted.

Figure 4: Transfer switch short-time withstand and closing rating label. Courtesy: Cummins Power Generation

Circuit breaker short-time pickup

There are at least six basic adjustable trip functions on sophisticated circuit breakers that shape the time-current characteristic curve, allow proper protection and enable selective coordination. Short-time pickup and short-time delay determine the amount of current the breaker will carry for a short period of time, allowing downstream protective devices to clear faults without tripping the breaker.

When a breaker with short-time trip response element (such as UL 1066 breakers) is feeding a transfer switch, the transfer switch must have a short-time WCR. The short-time response of the circuit breaker must be coordinated with the short-time current rating of the transfer switch as listed on the transfer switch label.

For example, a transfer switch is fed by a UL 1558 switchgear and the breakers feeding the transfer switch are UL 1066 breakers equipped with a short-time element. In this case, the transfer switch must have a short-time rating and the short-time response of the circuit breaker must be coordinated with short-time current rating of the transfer switch (listed on the transfer switch nameplate).

Figure 5: High short-time withstand and closing rating simplifies selective coordination strategies. Courtesy: Cummins Power Generation

Using transfer switch high short-time WCR to enable selective coordination

Because selective coordination is achieved using fuses and circuit breakers, the transfer switches placed in these systems must support a selective coordination scheme by withstanding the full fault current for the duration of the fault.

For example, Figure 5 shows the RMS symmetrical short-time current rating of 125,000 amperes at 600 volts alternating current for a duration of 0.5 seconds. The design engineer can use these ratings to enable selective coordination throughout the distribution system (see Figure 6).

Figure 6: Short-time withstand and closing rating label. Courtesy: Cummins Power Generation

WCR ensures safety

Transfer switches must be designed, built and tested to an upmost standard of reliability due to their crucial role in a power system. For applications where human lives depend on infallible power, transfer switches that are built to include a high WCR are paramount for ensured power reliability.

UL 1008 specifies robust testing requirements for verifying manufacturer ratings, including the WCR. Designing around and specifying transfer switches with high time duration WCR drastically simplifies the selection of breakers to protect the transfer switches.

Hassan Obeid
Author Bio: Hassan Obeid is a global technical advisor for systems and controls — critical protection at Cummins Power Generation, focusing on technical vision, business strategy and solving a wide range of complex problems.