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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 exceeds9 h: D. L6 C6 C% D C: Y) R
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3: O) I6 m1 i) q% Z- o: l, J5 y4 R2 [
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary1 E# }' X: F; X& X- p6 x' |
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current- e( Z$ J2 z* V U$ }' ?1 L6 r
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and7 b# P2 J& I# {# J
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
1 `0 q, X# A. T0 ?" t1 t' m6 ]a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and( |2 l5 E8 a2 L5 S9 T
the note is attempting to document that the breaking capacity of a fuse is a valid consideration
9 p$ t3 M0 [/ w5 y0 W0 Vduring performance testing.
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Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially) F. f" a6 N) |' k4 i* X! S. z) D! f
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal$ T/ b" }; _2 c7 A
pieces can be expelled through nearby enclosure openings.% |% u: n/ C' _0 `9 |
{- k2 P g! fAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one) s' J+ a7 \( B0 B Z
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or
I/ p- i/ q- C3 Qbridge live parts or insulation. With another application, it may not burst since the short circuit
+ \' ?) ]9 p# l" j Q( C) T' n7 `across a component was initiated during the zero crossing of the mains supply voltage. With
. Y f2 ]& w. T2 N" \; x0 R' vanother application, the fuse may burst, expelling metal pieces that bridge live parts or critical8 m$ R$ h2 g5 e9 a* v; i( e7 f7 h
insulation.1 q' ]+ Z) p6 q6 d* K/ ^* f
+ V9 H! _) y. |
A bursting fuse also is not an acceptable test result in either IEC or UL component fuse# V( z h8 [# y; `3 V0 E8 ^
standards. |
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发表于 2011-8-21 22:58
