ECN Forum Forums Code Related Discussion NEC & other Code issues Limit Fault currents

Yes, there is an article in EC&M Magazine about the very thing Gwz has described? http://www.ecmweb.com/ar/electric_dont_felled_higher/

This is a quote from the article -

"If the downstream circuit breaker starts opening on a high-current fault within the first half cycle, it may "fool" the upstream, fully rated device into not opening, because the fault current steadily decreases as the downstream contacts open. The result is a downstream device clearing the entire fault, even one well beyond the device's rated interrupting current. The outcome: self-destruction."

Apparently this was not a problem with older breakers that took 2 or 3 cycles to open. Some newer breakers can begin to open in the first half cycle, creating the problem you mentioned.

Re the "turf war". The breaker manufacturers are scrambling to develop ways to cause breaker contacts to open in less than a half-cycle, and thus produce a current limiting circuit breaker. They can see that, with ever higher available fault currents, current limiting fuses/breakers will be the OCPD of choice.

Ed

The fuse manufacturers have been using the term "newer" style breakers for over 25 years.

Very few (except large drawout power breakers) will stay closed for 2-3 cycles. When breakers are this slow they are usually listed with a withstand current rating rather than an interuppting rating.

Almost all molded case "loadcenter style" miniature breakers begin opening within the first 1/4-1 cycle. This is why the NEC requires actual testing. It is not unusual to find some small amp breakers that are faster than some large amp fuses.