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short circuit and let-through current

short circuit and let-through current

short circuit and let-through current

(OP)
does the NEC recognized let-through current from a fuse when calculating short circuit? if not, what section in the NEC does is this stated? thanks!

RE: short circuit and let-through current

The basic concept is that the equipment in question must have an interrupting RATING that is adequate for the short circuit current - See 110.9.

Then see 240.86 for the requirements for using a "series rating". If you as the engineer are willing to "ensure" that the downstream device will remain "passive" while the fuse does its thing, then 240.86(A) is for you. On the other hand, using a tested combination is a much more accepted approach. The old "up-over-down" method has been discredited for about 15 years, at least.

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