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发表于 2011-8-21 22:36
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RATIONALE:, l7 w2 |# A+ T9 \5 ~" M
T' R! p% i& B4 |4 l* HGenerally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
( l, t1 V; w h# O# \* Gthe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
9 c8 ?0 v8 B4 R1 H(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
! h( W; [/ ? rCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current6 W7 B$ D6 v- u4 W5 N9 p7 d- s2 d1 {
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
, Q% l( ~; j! f/ r' b" Y% Z) G9 [! lCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus/ ]. M& E& y/ K4 M. c6 u
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and- |; b" z: ]2 e
the note is attempting to document that the breaking capacity of a fuse is a valid consideration
. I7 |/ R# Y3 _ X0 Tduring performance testing., Z9 h7 x& {. ?1 ^, A
* I% o. K* k8 H0 q) o, G3 aFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially# j6 d/ e' ^- [
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
, o- K3 t& u0 Kpieces can be expelled through nearby enclosure openings.
1 r" S7 H7 u9 b b! q: I+ W- ~2 N }3 R( |- Y' `: k
Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one
1 M: z& ^; C) mapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or
9 }3 `; S" K2 t' Y4 O, ?bridge live parts or insulation. With another application, it may not burst since the short circuit. Y! A8 ?. i$ c5 ~" Z2 A
across a component was initiated during the zero crossing of the mains supply voltage. With
$ D# M S7 R6 q! b: B7 ^6 ~another application, the fuse may burst, expelling metal pieces that bridge live parts or critical: j0 D$ s# u a; S8 G; z
insulation.
: D6 f( `9 b# m3 b0 U+ B& T1 e& D( b
A bursting fuse also is not an acceptable test result in either IEC or UL component fuse
6 p/ o7 Y+ w3 ]standards. |
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发表于 2011-8-21 22:58
