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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
6 R8 m3 l/ e2 f* [the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3* E2 n( F: ?6 z( ^: b
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary) x6 @5 z+ `) E! L- |4 a
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
) ?7 Y3 n4 p/ ^- L$ v! K5 eexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
9 v! Z6 ]: b" m z+ |3 ~' BCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus: y1 U6 ], d$ B6 l. E8 U' M% P& Z* I
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
! a" g: _! u* Z9 P9 }( bthe note is attempting to document that the breaking capacity of a fuse is a valid consideration" [8 c. X9 j2 l1 b
during performance testing.$ M& ^2 b* o0 p% y, A( T
6 {6 M/ C% U6 l% @& J YFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially& T5 E3 r2 i$ b; ~' W8 j
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal% u9 Y: [1 V3 h& {7 P
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
! v' s+ P" E5 ]- xapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or5 k2 E# s$ s8 _* T4 A: Z* i+ U* w
bridge live parts or insulation. With another application, it may not burst since the short circuit+ F' _+ U# _0 k- D2 O
across a component was initiated during the zero crossing of the mains supply voltage. With
$ `7 @7 U0 b( _& D6 B' janother application, the fuse may burst, expelling metal pieces that bridge live parts or critical
+ o! K, o, j" ^5 {insulation.8 o( I) i1 I1 S, m
7 Y8 A4 r6 |+ {( I( z) _. H9 QA bursting fuse also is not an acceptable test result in either IEC or UL component fuse
7 h3 G A8 L) M) s Vstandards. |
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发表于 2011-8-21 22:58