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发表于 2011-8-21 22:36
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RATIONALE:
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/ U+ C; [0 p* S. D: MGenerally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
9 X5 E& `: ]# m7 f) K3 h- Ythe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3- F6 P: m3 I' R/ M: \- I1 a
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
* q) C6 Q/ B# _" wCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current: m5 f/ E+ u p! b3 Z% R
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and) H* M5 u0 k+ T |# l9 s+ o
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus7 |( b' I/ j e# }0 h# y8 g# ]
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
: n; z3 a0 j' Q4 N; jthe note is attempting to document that the breaking capacity of a fuse is a valid consideration8 W8 i- S- i! Z# g2 o$ K
during performance testing.
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+ l$ Y+ i( `- h" mFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
% ?/ _5 g& m# v8 u+ ~$ _4 zbridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
6 ]) U# u; D. P; upieces can be expelled through nearby enclosure openings.( M7 y. n) x) I, {
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Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one
) W5 b8 F; I/ H% `application of a fault, the fuse may burst but none of the glass or metal may exit the unit or
6 l: s- k% K1 [+ Q5 M) c# b. {bridge live parts or insulation. With another application, it may not burst since the short circuit
0 r/ w- j; k; ?- p, K8 \across a component was initiated during the zero crossing of the mains supply voltage. With
8 B( r# K) S8 manother application, the fuse may burst, expelling metal pieces that bridge live parts or critical
3 k& S- ?% F: c% V7 w- G7 einsulation.* P. Y! h- n: L V) M- u) Z% u& d7 b
7 _0 g9 A1 v6 z! m$ e* QA bursting fuse also is not an acceptable test result in either IEC or UL component fuse
' Z9 k# V# r& h) tstandards. |
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发表于 2011-8-21 22:58
