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.


Figure 1: An ATS connects critical loads with the power source that feeds them, which is primarily the utility unless a power outage occurs, or tests are being conducted. Courtesy: Kohler Power SystemsThe 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. 

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