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发表于 2011-8-21 22:36
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RATIONALE:6 E2 Y7 {6 v9 }" t$ H
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Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds7 h/ _2 e1 a% q; P1 Z* j7 M0 h: l& i
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.33 v# x- _! g. a, X
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
4 d. E% d/ A" |( _Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current9 q" |% |6 B: u9 a0 m, I. A
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and: T# H$ o4 v: f# d9 y( G/ b
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
( T0 [5 t3 j$ i5 e* ga similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and! `% b9 w A9 {# f. b
the note is attempting to document that the breaking capacity of a fuse is a valid consideration
^# G1 {* ]/ o; W! bduring performance testing.' A9 v' z6 z4 B8 U2 [9 S2 Z4 q" s1 R
5 p' {8 D h; f4 gFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially% U3 d. Y+ d5 d) Z, _. _) @
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
5 ?& u o4 {& N j( |pieces can be expelled through nearby enclosure openings.
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% l- [1 l. J/ n) v) H/ j# {- wAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one& k9 B0 R/ J# ?* d; o4 B2 u$ L
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or
# C" q- P6 R0 {+ U' K/ I. ybridge live parts or insulation. With another application, it may not burst since the short circuit; a3 B# r; f( n; S5 U2 @* D, p
across a component was initiated during the zero crossing of the mains supply voltage. With! Y: d4 ?% v' l8 t, \
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical$ `0 p4 \. r+ l. j) V. h
insulation.
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A bursting fuse also is not an acceptable test result in either IEC or UL component fuse
1 F1 j# Z4 c- Sstandards. |
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发表于 2011-8-21 22:58