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Joined: Mar 2001
Posts: 2,056
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As discussed in the safety forum, arc flash & blast seem to have been overlooked in the past as the serious hazards that they are.
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Joined: Aug 2001
Posts: 7,520
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Things like arc-flash, and fault currents in general are being emphasized a lot more here these days as well.
e.g. There is much greater emphasis on selecting fuses which can adequately break the prospective short-circuit current, and tables/graphs of "let-through" energy for different types are now common-place. It's not long ago that such things were unheard of.
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Joined: Feb 2001
Posts: 308
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I once crossed a hot and neutral connection in a smoke eater in a ceiling. It was dark and I was too lazy and proud of my good eye sight to get a flash light. When I turned on the breaker, immediate sparks flew out of the panel, as I jumped back and started watching the buss melt, I reached in and luckily swiped off the breaker without injury. (I of course don't recommend this!!) Was this just a faulty breaker or could the aic have been exceeded and caused the breaker to malfunction?
Needless to say, I don't try to determine wire colors in the dark anymore.
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Joined: Aug 2001
Posts: 7,520
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I don't know the breaking capacity of a typical U.S. C/B, but a lot of our domestic types are rated at about 6000A.
The utilities point out that in some cases (i.e. very close to a large xfmr) the short-circuit current for a residential supply can be as high as 16kA. Our regs. allow the lower-rated C/B to be used, so long as a suitable HRC (High Rupturing Capacity) fuse is used as backup. In practice, the PoCo main service fuse at the meter serves this purpose.
The old rewireable fuses (still in widespread use) are very bad on dead shorts, having a rated breaking capacity of just 2000A.
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Posts: 1,158
Joined: May 2003
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