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发表于 2011-8-21 22:36
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RATIONALE:
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; Q# O5 U6 j% V$ E3 j* s* u" bGenerally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds6 c. i% q& ]- g1 T
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3$ r0 T9 y* |* B, T3 D( b, R
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
: x; i2 E" i; I$ |2 ]- NCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current% z( h0 o/ ]' f
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and5 M4 e- i- j# b7 f2 X1 t
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
$ U4 f4 y; [5 C9 O9 xa similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and: G ^. z3 R, N( j8 x; V
the note is attempting to document that the breaking capacity of a fuse is a valid consideration2 d+ M6 H# v3 S0 G$ u# @: Q
during performance testing.6 v+ ]1 w1 C4 E! \; A6 C1 m# _
. J! q0 O( k* z0 O3 m$ ^% xFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially7 R; S, X$ S) K3 s
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal5 y! Z4 x T& J" }3 }( B
pieces can be expelled through nearby enclosure openings.
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& a* l: P1 e* ` E+ z$ ~% E, NAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one
`6 Z8 c) j6 ~0 {application of a fault, the fuse may burst but none of the glass or metal may exit the unit or
& ]" |! N' E( o+ V5 j# X6 K- Kbridge live parts or insulation. With another application, it may not burst since the short circuit
8 W+ _8 W, e/ S% uacross a component was initiated during the zero crossing of the mains supply voltage. With
: z |+ X: F( `! q- N Banother application, the fuse may burst, expelling metal pieces that bridge live parts or critical
& t+ {: O) R& X$ [insulation.
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. j* q/ [: k' I. c* Z2 rA bursting fuse also is not an acceptable test result in either IEC or UL component fuse
4 g9 ^! W) U! ]" \standards. |
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发表于 2011-8-21 22:58