Product standards for paralleled gensets

Consulting engineers in the U.S. consistently specify that paralleling switchgear must be rated to UL 891 or UL 1558.


Consulting engineers in the U.S. consistently specify that paralleling switchgear must be rated to UL 891 (UL Standard for Switchboards) or UL 1558 (UL Standard for Low-Voltage Power Circuit Breaker Switchgear). UL standards are cataloged online. These standards ensure that industry safety guidelines are being followed and that the power transfer and protective devices and bussing have all been appropriately evaluated as a system for safety under fault conditions. 

UL 891 and UL 1558 require that power transfer and overcurrent protection devices are rated appropriately for the paralleling application. UL 891 specifies molded case circuit breakers listed to UL 489 (UL Standard for Molded-Case Circuit Breakers). UL 1558 specifies power circuit breakers listed to UL 1066 (UL standard for power circuit breakers). Circuit breakers in paralleling applications are typically used for power transfer as well as for overcurrent protection. Draw-out breakers are specified in critical applications such as hospitals and data centers, allowing operators to isolate faults and facilitate inspection and testing of breakers, and easily replace them if necessary. 

Contactors that are not listed to UL 489 or UL 1066 may not be suitable for a paralleling application as they may not have been evaluated for safety when subjected to the higher levels of fault currents present in a system with paralleled sources.

Contactors that are components in UL 1008 listed transfer switches may not be suitable as a power transfer device in a paralleling application. UL 1008 recognizes and lists the entire switch mechanism, not the individual contactor. The individual contactor has not been type-tested according to the UL requirements and does not carry the UL listing separate from the transfer switch mechanism. In addition, UL 1008 lists switches that transfer loads between sources but does not test or recognize devices that parallel between two live sources. Because of these two factors, it is not appropriate to apply the UL 1008 standard to contactors that are used to parallel between two live sources. 

Rich Scroggins is a technical specialist at Cummins Power Generation. Scroggins has been with Cummins for 20 years in a variety of engineering and product management roles and is active in IEEE 1547 and NEMA SC 16. He received his bachelor’s degree in electrical engineering from the University of Minnesota and an MBA from the University of St. Thomas.

Curt , , 09/16/13 05:20 PM:


Your assertion that contactors not listed to UL489 or UL1066 may not be suitable for power switching seems to be your own opinion.

Motor contol centers use many contactors specifically for power transfer. When they are used in conjunction with a breaker in-line it uses the devices the way they were intended, one for protection, the other for switching.

When used as part of a generator, the contactor is covered by the UL2200, similar to UL1008 for the ATS. It convers all the subcomponents.

I would like to remind you that paralleleing switchgear is often not UL labeled so using the breaker as a switching device has become commonplace. But switching is not the design intent of the breaker.
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