RES Technical Corner by Brett Eliasz, PE

Technical Corner

For the Electrical article this month we will discuss circuit breaker protection options for Transfer Switches and how it relates to the short circuit current ratings for transfer switches.

Transfer switches must surpass 2 short circuit withstand tests. 1. A short circuit when the switch is already in a closed position 2. The transfer switch must transfer, close and hold on to a short circuit

When using circuit breakers to protect a transfer switch there are 3 options: 1. “Specific Breaker” a. Use a specific breaker from a mfg. to get desired short circuit ratings i. Cons1. Different circuit breaker vendors providing different breakers on the same project-gets confusing 2. In future, breaker may be replaced without paying attention to transfer switch it is protecting

2. “Any Breaker” a. Use breaker with instantaneous trip to withstand a fault for 3-cycles (or .050 seconds) i. Was created to alleviate the concerns of the specific breaker option ii. Gives engineer more flexibility when specifying circuit breakers iii. Is considered the “norm” if coordination is not required iv. No short time delay on the breakers

3. Short time ratings a. Came into fruition when selective coordination has become more of a requirement especially in emergency applications. b. “Any Breaker” option could not provide the needed coordination in emergency applications. c. Must use circuit breakers with short time delay adjustment and no instantaneous trip function to achieve the coordination. i. These circuit breakers will increase the time that a transfer switch must withstand a short circuit current. 1. If greater than 3-cycles-can’t use “Any Breaker” option

During design, all of these separate ratings can be found within the mfg. literature. Transfer switches should coincide with one of the 3 options above and depends on the application. The decided-upon method should have a corresponding transfer switch rating. The engineer will need to ensure that the short circuit current rating of the transfer switch (based on method chosen) is greater than the anticipated calculated values at the actual transfer switch.

res - technical corner Hopefully this article finds you well and can be used as a reference for your project needs. If anyone would like to contribute to the Rochester Engineer and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Brett Eliasz, P.E., LEED AP BD+C , RES Director