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发表于 2011-8-21 22:36
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RATIONALE:7 t s7 r/ h4 ~) {/ j# K
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Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds- J0 f3 _% J: V. n$ Z$ Y k
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.39 {% w; e, ~* A
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary+ n" T+ f& C0 M, |9 V1 ]8 b
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
1 M6 C; o0 B' f8 d8 |0 vexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
X, p, U4 |! ?% n: I. q! qCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
2 v+ S# F9 m3 E# |/ Ra similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and6 I5 z! {7 Q _/ z/ E
the note is attempting to document that the breaking capacity of a fuse is a valid consideration
7 F8 [( h* r; b% E9 {' I9 gduring performance testing.9 L. b9 i/ ?' P1 m0 ]
$ j, L7 O/ k2 s. T- z8 `. ?9 K' vFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially0 f. w& K% a# q% S
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal6 g. v2 j! r! b: s6 f' F+ ]! R) G5 _9 d
pieces can be expelled through nearby enclosure openings.
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: t: U7 v7 J2 p* h+ h; n- Y2 _) CAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one8 s4 `: ^9 C; C! Y8 b
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or1 h/ F; A$ X6 h( Z
bridge live parts or insulation. With another application, it may not burst since the short circuit
1 |, O+ Q6 z5 ]' [across a component was initiated during the zero crossing of the mains supply voltage. With5 p3 S& T, m- [+ k8 V ?) c
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical2 i9 M# r- Z3 p
insulation.
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A bursting fuse also is not an acceptable test result in either IEC or UL component fuse5 U9 z X) C% @6 M4 r* t/ {
standards. |
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发表于 2011-8-21 22:58