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发表于 2011-8-21 22:36
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RATIONALE:4 L0 v8 O+ |# S6 P! H+ _9 V' q
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Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds2 J* l0 {8 L' u5 ~( ~, k
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
+ S% e( P$ G5 z3 f b* |(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary( D5 U" ?) P$ x. q7 H( P
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current$ e! j: T N/ ^( H/ Z" | v
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and: \ J5 a* z/ k8 A* y
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
/ b5 j5 x" |+ l1 G( p' y6 B* aa similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
1 A$ \: X+ f/ w3 o7 k+ n; i3 mthe note is attempting to document that the breaking capacity of a fuse is a valid consideration
% {) C3 i7 p" U. [( v# O5 xduring performance testing.
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Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially' C. p. d& e& T' f- S# {
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
, v$ |1 e- g# ?6 V$ f) |pieces can be expelled through nearby enclosure openings.
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Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one
; |" m4 b. `$ E/ _application of a fault, the fuse may burst but none of the glass or metal may exit the unit or8 ~3 V0 h5 ]9 c( _ W: f
bridge live parts or insulation. With another application, it may not burst since the short circuit: w: S4 c$ q6 j2 ^
across a component was initiated during the zero crossing of the mains supply voltage. With
4 Q; _8 T/ _1 D$ h+ _! lanother application, the fuse may burst, expelling metal pieces that bridge live parts or critical
5 _/ K. c; Q( X1 u/ E1 |5 T% \insulation.% L+ `4 d1 m& `- y0 N" I) a* ^
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A bursting fuse also is not an acceptable test result in either IEC or UL component fuse* f J; I# I1 p3 t2 y9 c
standards. |
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发表于 2011-8-21 22:58
