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Cummins claims "unprecendented" safety rating for switches

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Minneapolis based Cummins Power Generation's transfer switches received unprecedented "withstand and closing" ratings from a U.S. product safety testing and certification company after being tested for their ability to surive sever levels of short circuit, the company said.

The open transition and closed transition switches demonstrated short-time ratings of 25 kA at 10 cycles for 125- to 260-amp switches, 30 kA at 30 cycles for 300 to 600 amps, and 50 kA at 30 cycles for 800-amp switches in tests designed by Underwriters Laboratories.

After testing, the switches operated at full load safely.

"They were undamaged," Cummins Director of Power System Development Gary Olson said. "They could take the short circuit, conditioned for 30 cycles... (and) still carry rated load."

The product was designed for use in data centers, banks, government office buildings, as well as in hospitals and emergency shelters. It is not however, appropriate for large data centers.

"This would probably suit more mid- to smaller-size data centers," Olson said.

The UL rating validates the product in the eyes of engineers as an "independent assurance" of its high performance level, according to a Cummins statement.

Specifying transfer switches' UL ratings also helps engineers with selective coordination, the process of selecting circuit breakers and transfer switches with a combination of timing and withstand parameters that allows for localization of an overcurrent condition by delaying or preventing current faults from tripping upstream protection devices.

"As he's designing protection for his system, (an engineer) wants to make sure that (in case of a fault), the only breaker that trips is the closest upstream to the fault," Olson explained.

With a transfer switch capable of withstanding fault current for up to 30 cycles, the engineer can adjust time delay on the overcurrent protection device to prevent upstream breakers from tripping unnecessarily.

The U.S. National Electrical Code can require selective coordination in electrical systems of data centers, banks, government offices, hospitals and emergency shelters.

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