Understanding the Withstand and Close-On Rating for Transfer Switches

Numerous short circuit current ratings and references exist for transfer switches that are often confusing and seemingly contradictory. This paper provides some explanation and clarification to help engineers specify the proper equipment to meet local and national regulations.

For an electrical system to operate safely, the design should consider a variety of scenarios where things do not go as planned. One of those scenarios is when a short circuit occurs in the system and causes extremely high currents. An electrical system needs to be designed to safely react to these extreme conditions and, ideally, to continue to function afterwards. This article looks at automatic transfer switches (ATS) which are integral pieces of the power distribution system that help ensure power for home, office, factory or process, when served by an emergency or standby generator in addition to the local utility.

This paper also aims to help engineers understand what the withstand and close-on ratings (WCR) means and provide background information to allow the proper sizing and selection of the transfer switch. Two key abilities of the transfer switch are tested under the WCR: the quantified ability to withstand fault currents for a specified period of time while maintaining functionality; and the ability to close into a fault current and continue to operate. Both abilities are critical to allow the electrical system to sustain a fault current with minimal impact.

File Type: pdf
Categories: KOHLER® White Papers
Downloads: 1

For more information, talk to your local IPS representative or contact us here.