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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
* L, d# b: [# l$ \the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
" l$ q- e3 q" [+ |7 r$ H(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary& |! b1 Z- ?0 B2 T C0 f! H
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current _+ r/ P5 ] H6 S/ h
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
3 A5 q0 L; T! n% BCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus5 F/ l" y! ~$ e$ N* P5 o( S
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
2 a* i6 q( K3 g' p* E' U6 Fthe note is attempting to document that the breaking capacity of a fuse is a valid consideration
- q# b) m6 r% G( G/ v! K. D$ wduring performance testing.. i- A; s$ F( }
/ {4 p7 V% {0 T! i/ f. |Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially( K2 z( I* n* P# [ n3 m
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal$ A& T8 s g! B$ h% M# F! ^
pieces can be expelled through nearby enclosure openings.: M. a7 U' a; i, o I
8 I1 X& _4 ~1 p% E5 N& b$ X2 f* e) oAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one6 j& M) k( B- {
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or" {4 k) O9 _5 p" M4 n
bridge live parts or insulation. With another application, it may not burst since the short circuit5 O* N) E1 R3 U: R4 R
across a component was initiated during the zero crossing of the mains supply voltage. With
% Q6 Q; x3 z0 m3 W# Q* v6 u. uanother application, the fuse may burst, expelling metal pieces that bridge live parts or critical
, c5 C/ w' H" y4 J: Ainsulation.
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5 C _; w6 {. F RA bursting fuse also is not an acceptable test result in either IEC or UL component fuse9 U g( u H# P3 Y6 O
standards. |
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发表于 2011-8-21 22:58
