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发表于 2011-8-21 22:36
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RATIONALE:) K* D/ u) w0 W$ ~& j+ w( k
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Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
' i( x3 \1 ?7 z1 vthe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3! g3 w. i' t$ e/ U! Y6 m
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary9 k- |0 L' E& a+ U) G! N. j
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current2 b3 x3 \: j( n# W( |- F% i4 P
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
9 J* B- f% G8 j/ K8 }2 xCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus, G3 f% o% s' j! ]
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
7 Y% d7 o' a! v9 y0 B" lthe note is attempting to document that the breaking capacity of a fuse is a valid consideration
7 x6 n d% ]# A) k2 @5 Z1 q. I, dduring performance testing.6 k4 H! F5 b6 `! }: T! L
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Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
* D! m( l$ O0 E% H8 h Hbridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal3 z- k! U0 \ u$ E6 X- f
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; g0 y$ d6 g9 {- y, `4 F
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or6 p, n: F* \4 f4 C+ N' S
bridge live parts or insulation. With another application, it may not burst since the short circuit
# V( W/ A4 _( t+ X6 kacross a component was initiated during the zero crossing of the mains supply voltage. With' X$ o2 u/ R7 U7 N) l/ U$ g
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical- \; j8 T. u+ T* G8 X6 ?3 S
insulation.
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. M( {' R0 E0 Z$ ^9 W2 ~, pA bursting fuse also is not an acceptable test result in either IEC or UL component fuse
9 g7 C2 H4 D1 y0 B& ~% U0 j$ c3 vstandards. |
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发表于 2011-8-21 22:58