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发表于 2011-8-21 22:36
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RATIONALE:
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Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
. _ a8 g4 l5 n+ r2 p" n4 B" j) d' Tthe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3/ @# o1 L2 E( R( w
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary* ` A0 H$ {" o5 X$ c9 t
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current; I0 k8 ^$ D/ A$ Y4 V
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and/ R- s) l( D5 N2 a3 y
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus% [# _0 u( ]9 m' L/ R/ l
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
2 k& u% z$ Z9 Q1 m; Tthe note is attempting to document that the breaking capacity of a fuse is a valid consideration
" D1 {: o; T. |# _4 A! @during performance testing.% Y& \2 p- s0 `5 e- }
) u. e6 H3 |' l+ M1 P7 D( CFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
) x3 M4 l5 g. w& l k2 v$ obridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
$ M$ H }; H& W! L8 Hpieces can be expelled through nearby enclosure openings.
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0 b X- R( \1 g: Z) S9 gAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one
* M1 K; t* q7 x. a1 gapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or0 q8 }( Q1 i( |9 `
bridge live parts or insulation. With another application, it may not burst since the short circuit/ U, Z5 ~6 ^7 M% x" g y
across a component was initiated during the zero crossing of the mains supply voltage. With1 x- n' \4 d) R. i& q- o
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical
2 S! x" N; R, E! |0 |- {' x7 ainsulation.
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0 S# J9 P. t9 d9 E8 V. u6 ]A bursting fuse also is not an acceptable test result in either IEC or UL component fuse7 |; K: @7 ]8 [6 D* G) P
standards. |
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发表于 2011-8-21 22:58
