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发表于 2011-8-21 22:36
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RATIONALE:3 Z7 [% L% m/ F b) [( L m% Z
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Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
( u( B% Y+ j4 H' sthe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
2 k$ `$ A' c+ o0 w3 R4 G" ?(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary- O6 Z6 f- c# J
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
- X* |" q, N+ j8 G4 ^+ s x* ^exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and6 K1 @, @6 Q* x( r
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
, k7 B- m0 ^/ C d" U9 J8 d+ Ya similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
# e9 ]- e' j& L9 e# z4 Y1 O+ Fthe note is attempting to document that the breaking capacity of a fuse is a valid consideration
( x/ l- F( @! O; Z. Yduring performance testing.
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" N+ L2 ?9 K4 n! H4 O/ |Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
. f, `5 m# i% `bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
. E, X0 x8 ^ P# U2 @pieces can be expelled through nearby enclosure openings.7 k, t: f! k7 b: ~6 D% K
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Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one
& C) [4 p! F0 a& [/ V: _# [application of a fault, the fuse may burst but none of the glass or metal may exit the unit or
0 C" R8 w" x" T( n; b Ebridge live parts or insulation. With another application, it may not burst since the short circuit
7 k% @& o* d* P+ e- ^across a component was initiated during the zero crossing of the mains supply voltage. With/ Z2 U. i9 u) E
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical. W6 f* {- b2 L4 H" [6 [1 {, Y
insulation.
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- n; s! l. E5 E Q1 jA bursting fuse also is not an acceptable test result in either IEC or UL component fuse0 ^3 W/ X5 Q# L) y) G/ X" F; E+ U
standards. |
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发表于 2011-8-21 22:58
