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发表于 2011-8-21 22:36
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RATIONALE:
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1 T( h9 f) X7 F3 W5 JGenerally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds' x/ ~" R p5 ^% ^$ l
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
) n! L" j- b* O9 [4 L3 r(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
4 w V" q Y0 B( H3 {4 O8 sCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
) _9 P# ^1 F7 H2 [# S1 z8 {6 cexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
$ j6 ]% ]6 C+ Q; ]$ w* Z) KCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
& H. i; a ], `, V0 sa similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
. r7 t5 x' H4 z0 `the note is attempting to document that the breaking capacity of a fuse is a valid consideration7 _# n& u! e; i+ ~4 y6 w( A
during performance testing.3 Q" H) V' ?; M& [# a @
& B2 }3 o* s7 z) n8 OFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially$ d9 P! y! N3 q- l
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal2 Y1 G' }6 S2 w
pieces 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
: }7 y' s, M+ Z9 b* s* b napplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or6 K- G+ E5 }7 |# G
bridge live parts or insulation. With another application, it may not burst since the short circuit
& v6 E0 p, ?% z# ]/ S" R* R/ qacross a component was initiated during the zero crossing of the mains supply voltage. With2 }; F {- m+ I, v
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical
+ @8 R3 B2 @! ?+ s, e( x* n2 Finsulation.
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A bursting fuse also is not an acceptable test result in either IEC or UL component fuse2 K6 {; E3 r6 d
standards. |
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发表于 2011-8-21 22:58