ASCO White Paper: UL1008 Transfer Switch Withstand and Closing Ratings

Jan 17, 2020

“UL1008 – Standard for Safety – Transfer Switch Equipment” is the leading transfer switch testing standard for the North, Central, and South American markets. UL1008 specifies robust testing requirements for verifying manufacturer ratings, including Withstand and Closing Ratings. This document describes the need for Withstand and Closing Ratings and summarizes key UL1008 tests used to verify them.

As their name implies, transfer switches connect electrical loads to one of two sources of power. These sources are typically dissimilar, such as alternating current applications with power feeds from a public utility and an on-site generator. While the purpose of a transfer switch seems clear, it is important to distinguish it from the purpose of an overcurrent protection device (OPD).

The purpose of a transfer switch is to connect electrical load to an alternate power source when the normal source is unacceptable. As a result, a transfer switch connected to a power source must withstand and close on short-circuit currents until they are cleared by an OPD. Thereafter, a transfer switch must remain operable so that it can connect to the alternate source.

In contrast, the purpose of an OPD is to open a circuit to protect equipment and mitigate hazards arising from overcurrent conditions. As a result, an OPD must be capable of disconnecting power sources from loads when faults occur.

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