Using withstand current rating to improve electrical system designs

Understanding how withstand current rating affects an automatic transfer switch enables consulting engineers to design more effective electrical systems.

By Steve Ennesser and Allen Frederick, Kohler Power Systems November 8, 2013

The automatic transfer switch (ATS) is a critical part of an electrical power system. Understanding its withstand current rating (WCR) is essential. If a transfer switch does not have a sufficient WCR, severe damage can occur, and a potential fire hazard can exist. On the other hand, selecting an ATS with a higher rating than what is required results in over-specification and unnecessary expense. 

Transfer switch considerations

A transfer switch is typically the last distribution device feeding the critical loads of a facility (see Figure 1). As such, it has three crucial responsibilities:

  1. Close into high inrush currents, i.e., high inductive loads.
  2. Remain closed during faults, especially where very high fault current can result in significant voltage drop on affected phases. Failure here can result in arcing and heat damage that can lead to premature switch failure.
  3. Operate frequently, not only for power system failures, but for standby system testing.

While undesirable, short-circuit faults are common. When faults occur, the fault impedance can be quite low, resulting in short-circuit currents of significant magnitude. High fault currents can create heat energy and thermal stress (measured in I2t), which can affect all electrical system components. Extended fault currents can result in destructive heating of cable and transformer insulation, breakers, and switch contacts. And it’s not just electrical stress problems: fault currents can create mechanical stress by producing high magnetic forces that bend bus structures, separate switch contacts, and cause power cables to pull out and energize surrounding structures.

Specifying a transfer switch

To avoid catastrophic events, protective devices such as circuit breakers and fuses are used to isolate a fault from the power source if a fault occurs. A proper WCR rating ensures that a particular transfer switch can withstand the fault current until the immediately upstream protective device opens. The appropriate WCR also demonstrates that the switch will remain operational after the fault current passes. 

To minimize fault risk, which is typically caused by load-side cable failures, the transfer switch should be located as close as possible to the protected critical load it serves. After a transfer switch is exposed to a short-circuit fault, it must still be operable to restore power from the alternative power source. 

Prior to selecting a transfer switch, detailed knowledge is required about where the switch will be used. An updated power system study can provide this. This type of study shows computed fault currents at each system bus (for normal and contingency system configurations), evaluation of the capability of upstream and downstream devices, protective device coordination analysis and recommendations, and other computations such as available arc-fault energies at each bus. The study also allows engineers to select fuses and determine circuit-breaker trip settings. 

Proper protective device coordination ensures that an electrical fault is cleared as close to the point of occurrence as practical. Modern power system studies use computer modeling to consider all parameters including conductor sizes, quantities, and lengths; transformer ratings and impedances; and other relevant data. Using this information, the appropriate transfer switch and location can be selected. 

Testing transfer switches

To assign a WCR to a transfer switch, a short-circuit test is performed, which exposes the unit to its rated level of fault current. Then a dielectric voltage-withstand test is performed to check the unit still functions after the fault current has passed. UL 1008: Standard for Safety: Transfer Switch Equipment (seventh edition, July 6, 2012, unless otherwise noted) specifies two short-circuit tests:

  1. The transfer switch must withstand a short circuit when the switch is closed.
  2. The transfer switch must transfer and remained closed until the short circuit current is removed.

The short circuit test requirements are presented in Section 9.13 of UL 1008; the dielectric voltage-withstand test requirements are covered in Section 9.14. 

Rating transfer switches

Transfer switch WCRs are typically listed in the specification sheets. The four main types of WCR are specific breaker, any breaker, short time, and fuses. 

Specific breaker: Specific breaker ratings are generally popular on larger installations where a higher level of planning is required (see Figure 2). An ATS with a specific breaker rating (in accordance with UL 1008) must pass the short-circuit test while being protected by that specific breaker. There are numerous specific breaker choices for particular transfer switches. When using a specific breaker rating, it is advisable to select the breaker interrupt and I2t ratings to exceed the transfer switch withstand and I2t, especially with molded-case circuit breakers. As molded-case circuit breakers age, the trip characteristics may change, causing the tripping time to slow and exposing the ATS to energy above the WCR. 

For the specific breaker rating, tests are performed with a specific molded-case circuit breaker between the ATS and the test source. The fault current is applied for the time it takes the selected circuit breaker to clear the specified test current. This time is used to determine which specific breakers may be used. Any breaker that meets the fault current rating requirement and can clear the fault in the same time or more quickly than the tested breaker can have this noted on the transfer switch ratings decal. 

Any breaker (or umbrella breaker): An ATS that passes the any breaker test (in accordance with UL 1008) can withstand a fault of a given magnitude for 3 cycles (or 1.5 cycles for transfer switches with a rating smaller than 400 A with an any-breaker-rating of 10,000 A or lower). This allows an ATS to be used with any circuit breaker that has an instantaneous trip function. Using the umbrella of the any breaker rating has simplified ordering and installing ATSs and circuit breakers. This makes them popular on smaller installations where planning is generally not as detailed. 

For transfer switches rated above 400 A or for those used on circuits with fault currents greater than 10 kA, the short circuit test requires the fault current be applied for a minimum of 50 msec (3 electrical cycles) at a specific power factor. Passing this test allows the manufacturer to mark the switch for use with any manufacturer’s circuit breaker within its rating. This is sometimes referred to as an umbrella rating and gives the designer more flexibility in installation. 

Short time: ATS units using these ratings are generally over-protected and could be properly protected with a smaller WCR with one of the other methods. This is because they are protected by upstream breakers and can withstand short-circuit currents for longer periods as they have a short-time—as opposed to instantaneous-trip—capability. This also makes them popular with engineers, as they are easier to specify, although there is generally a higher cost implication. In 2007, UL amended the 1008 standard to account for short time ratings for transfer switches that are protected by upstream breakers and that have a short time delay of 3 to 30 cycles (50 msec to 500 msec, in a 60-Hz environment). 

Fuses: The final type of protection detailed by UL 1008 is also the most simple: current-limiting fuses. Current-limiting fuses limit the current that passes through them during a fault and ensures the protection of downstream system components from catastrophic failure because they typically clear faults within a half cycle. Current-limiting fuses allow the ATS to be assigned a higher WCR because of the extremely short clearing time. 

The bigger picture

It’s important to not lose sight of how the ATS fits into the overall electrical system. The power system’s X/R ratio, in particular, must be considered. UL 1008-rated ATS units are tested on systems with specific X/R ratios and with circuit breakers or current-limiting fuses deployed upstream. Current-limiting fuses deployed upstream can significantly reduce the duration of a short-circuit current compared with systems that use circuit breakers. This is important because the peak available current at the transfer switch must be understood so the correct ATS can be selected to meet continuous and symmetrical current requirements.

For example, a 200 A transfer switch tested at 22,000 A (symmetrical) at an X/R ratio of 4.9 will withstand 48,026 A (instantaneous peak). However, if this switch were applied in a circuit with an X/R ratio of 6.17, it would be subjected to 50,204 A (peak instantaneous). This may seem like a relatively small jump, but the consequences could be severe: the switch may fail and wider damage may ensue. As with planning backup power systems, attention to detail is crucial. While the transfer switch may seem inconsequential, its effect on the overall system cannot be underestimated.

Allen Frederick is a senior staff engineer with Kohler Power Systems, Americas. He has been with the company since 2008 and specializes in switchgear project management, electric utility distribution system engineering and planning, and control system engineering. Frederick has a BS in electrical engineering from the University of Wisconsin, Platteville.

Steven Ennesser is an electrical project engineer with Kohler Power Systems, Americas. He has been with the company since 2012 and specializes in automatic transfer switches. Ennesser has a BS in electrical engineering from the Milwaukee School of Engineering.