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RATIONALE:
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' t( c- v# c$ v1 Z/ \" D. ]Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
. {! V$ b3 R. w+ O" u" B" i% _the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.38 a4 c) r' |& m1 u
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
5 ]) Z0 J- @! m1 m" F/ l4 }Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
( e, s" f$ F1 u# k" @5 N, zexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and$ {. W$ _) u4 \" ^4 ?
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus O3 Y$ ]5 L2 Y. c8 J# e
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
) u Q: D. h& U9 F* C! B! ?the note is attempting to document that the breaking capacity of a fuse is a valid consideration/ Z: X, i( h/ S' y* E& c
during performance testing.6 `" }. m7 z# k' `1 E8 g
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Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially3 p4 h5 Q0 Z, g+ e C7 G! F) E
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal" ]. v( G D) Z# \. ^1 j# P2 G
pieces can be expelled through nearby enclosure openings.6 y) f9 _0 t7 A. k& X' J% G
- b/ ]% ~7 Q7 k) K- W7 x6 @$ ]4 ]Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one
9 c( X8 a1 r" I6 Uapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or
6 Y5 E, A" n' a* w( g( c: u$ zbridge live parts or insulation. With another application, it may not burst since the short circuit
$ H& Y4 j8 M0 m5 w" }9 Lacross a component was initiated during the zero crossing of the mains supply voltage. With" @% g" A" A0 g: Z% s& J
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical w$ u' Y5 w. Z: ~
insulation.4 i* a- r% r# p/ K
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A bursting fuse also is not an acceptable test result in either IEC or UL component fuse( ~8 ~# R% u: @+ W- d
standards. |
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发表于 2011-8-21 22:58
