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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
0 Z, a& X8 n+ m) D5 g2 D) ?" w0 Xthe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3' \3 j: ^* M# [6 D; }* C
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
9 n. N) `: ^) i3 zCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current _1 ^# L- N: b7 {
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
# F# ~- P# L1 kCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus! X) Z1 W+ Y! j( f
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and) U, Y. A- Y, {& m
the note is attempting to document that the breaking capacity of a fuse is a valid consideration; \/ Y, ^9 O* X
during performance testing.
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Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
) n: J- q' q# p8 L* n, o4 N; tbridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
- B1 Q+ w2 q- j |# I$ Ypieces 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
# ]0 w1 g1 _7 \ `application of a fault, the fuse may burst but none of the glass or metal may exit the unit or5 m9 `3 }& u8 Q) ~. g8 B' |0 p
bridge live parts or insulation. With another application, it may not burst since the short circuit
' i V9 r) r8 y4 Vacross a component was initiated during the zero crossing of the mains supply voltage. With* m2 s1 R0 p8 W/ S; o& X0 e" j
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical
5 e. F- X6 e9 p1 W& uinsulation., `) i8 J% ]* w% ^
, d: b5 S; E# M' @/ Y; |% n9 RA bursting fuse also is not an acceptable test result in either IEC or UL component fuse; X! }- {. g$ J
standards. |
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发表于 2011-8-21 22:58