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.
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
Strategic outsourcing delivers efficiency; Sleeve bearing clearance; Causes of water hammer; Improve air quality; Maintenance safety; GAMS preview
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me