If you purchase or specify manual or automatic transfer switches, the subject of UL 1008 is bound to come up at some point. The UL 1008 listing is not just a performance standard, it is a design and construction standard in the industry. In order to receive a UL 1008 listing, a transfer switch requires rigorous testing by an independent certification agency. The certification is aimed at ensuring durability, reliability and safety during operation, and while this is an important listing, it is not truly understood by many.
To add to the confusion, many people don’t know how to check if their unit is UL 1008 Listed. Knowing this, many companies will state in their sales literature that their device “meets UL standards” to imply that their unit is UL Listed, when it is not. When you are evaluating products, simply look for the UL logo and the word “listed” as well as the category code. The category code can be used to verify the listing and the standard to which it was evaluated, on the UL website.
Since its inception, the certification process and standard have been modified and updated on a regular basis. The 7th Edition of UL 1008, which became effective in 2014, resulted in significant changes to the short-circuit ratings shown on all transfer switch products within the industry.
One of those significant changes involves the ability to protect the transfer switch with “any” circuit breaker. The latest UL changes now require the “any” circuit breaker to comply with specific performance characteristics that will be identified on a label and are based on the short circuit testing performed during the qualification testing of the transfer switch.
This puts more responsibility on the specifying engineer to understand the limitations of the selected transfer switch and to properly specify and coordinate the upstream “protecting” breaker. However, if the transfer switch uses circuit breakers (such as ESL’s StormSwitch product line), then there is no need to coordinate any upstream breaker as the circuit breaker within the transfer switch has been fully tested for the marked ratings.