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发表于 2011-8-21 22:36
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RATIONALE:" C3 Z& e9 y/ y& N7 Q7 I
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Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds3 k, @2 c1 T9 r. A$ Q+ S% M. W
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3 P% n) ~% Z# l3 y/ K# f; \4 y
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary- o) [1 R. b6 s4 G0 j! ~. [. p, Y
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
# `# j5 `9 Z. ^' d$ q) kexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
: J) c5 v# z5 d* v% lCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
) D2 X+ @5 ^+ u- f0 Oa similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
' y% t& c' t8 m$ Y" n& Bthe note is attempting to document that the breaking capacity of a fuse is a valid consideration
" ?6 q% y, u5 V9 B q$ oduring performance testing.
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Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially) r. D K5 m; w. j# i
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
1 N# C p3 i5 U b; }0 cpieces can be expelled through nearby enclosure openings.% [5 e* w$ b% R0 G+ T
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Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one7 `$ O$ a" Z6 R& v; y/ Z/ M
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or
: ^$ \( f1 a4 Y3 c) [bridge live parts or insulation. With another application, it may not burst since the short circuit
; P) M2 @6 _* l1 w: }4 N2 bacross a component was initiated during the zero crossing of the mains supply voltage. With- [3 ]5 ]: B: b5 R7 z! d
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical
6 d* o: j+ V/ M; Hinsulation.
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A bursting fuse also is not an acceptable test result in either IEC or UL component fuse
W8 G4 k' d! F! }$ c- Zstandards. |
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发表于 2011-8-21 22:58