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发表于 2011-8-21 22:36
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RATIONALE:) m: J7 f3 x% F! c) j
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Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
! D$ [: ~$ p V' Ethe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
$ t+ t! a( f$ f(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
, Q( D0 Y4 l' H# h* U4 e' w: SCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
3 P R4 f- \+ T/ c6 f; c* _2 gexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
J0 m( x3 l6 J& h" h; Q0 O4 fCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
" \0 T; w G4 D1 w" l& _& j; ua similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and. H ^/ R2 K! b- s
the note is attempting to document that the breaking capacity of a fuse is a valid consideration
7 m) h/ F% H2 P/ z1 U8 ^) I3 {during performance testing.- p p u) C1 G+ Q' K) q9 { B
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Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
0 D0 F6 V" [7 Hbridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
. P; l4 W# `2 B( v) Xpieces can be expelled through nearby enclosure openings.
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Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one
+ ?, L4 o& e; \3 U; sapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or
, b7 i. m: v4 q: I1 Z4 T3 ]bridge live parts or insulation. With another application, it may not burst since the short circuit
h0 n( M( }# L# ^8 ?1 gacross a component was initiated during the zero crossing of the mains supply voltage. With
1 l9 }% U" u+ w2 B+ m( ranother application, the fuse may burst, expelling metal pieces that bridge live parts or critical
5 `4 S4 k4 T: _1 } X# Ninsulation.
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7 ^6 ]8 @- m& j% s/ [8 T$ T$ ?7 OA bursting fuse also is not an acceptable test result in either IEC or UL component fuse
5 Y3 c6 |0 ?% o* f2 j# h5 s& @; |standards. |
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发表于 2011-8-21 22:58