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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 exceeds2 s" F! |' N$ k, h
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3% i' _( \# a% ^" Q# \- V
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary& c) e' S8 }2 T/ o( T& @3 r2 p( @
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
5 t9 x2 |8 } `6 h" x K$ Uexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and2 F/ m+ @. F% R1 F' s. C$ P0 M
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
s' ?0 W1 s- E; |, |, Ma similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
$ T, A4 J: @8 G" Kthe note is attempting to document that the breaking capacity of a fuse is a valid consideration& b+ M' ^2 e% c6 n4 ?
during performance testing.0 i* z- ~1 D4 L) b
1 Z/ t* s: e. o, N4 VFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially) T/ b+ k% J- ^+ @$ q! L; E4 F2 G- _
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
5 O" W4 H' q: F/ ]2 Vpieces can be expelled through nearby enclosure openings.
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. o' d% f' ^5 r( uAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one
4 m8 X& v5 C+ n6 y9 [4 y7 eapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or" V) B8 [2 F1 l& X& h- t" x+ s
bridge live parts or insulation. With another application, it may not burst since the short circuit8 f( z. V0 V7 e; n5 M/ Z* X/ z
across a component was initiated during the zero crossing of the mains supply voltage. With/ l) F, z! Y& ^" `: Y% k1 W; y
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical
7 j' L, ]$ D' u# V5 M4 W6 `insulation.
9 d; R z2 p' `/ [7 B% S: O
/ q3 q' ^5 f" u2 F6 l! oA bursting fuse also is not an acceptable test result in either IEC or UL component fuse
$ U2 B9 ~3 B1 h) K# h/ k% G& hstandards. |
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发表于 2011-8-21 22:58