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发表于 2011-8-21 22:36
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RATIONALE:
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: A* _1 i; B) j! F: e4 _Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds2 [- r" J* A4 Y( |4 t9 a2 w3 H
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3- e, O t% I, i& b$ F
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
* h' e* |' a5 ~+ z8 hCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current/ ]5 P1 Y7 H2 Y! j
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and1 l; G/ t$ d+ W0 O( a
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus3 `2 g. I- L! F4 U3 b0 j6 s$ x# U
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
; U8 k3 P3 L: ?& v/ ythe note is attempting to document that the breaking capacity of a fuse is a valid consideration
3 T5 q8 V" }5 P5 pduring performance testing., @0 s/ _& ^& c( u7 ]
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Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
# c- L1 |5 Q! kbridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
5 x: D5 K5 k9 o5 a+ Q; |pieces can be expelled through nearby enclosure openings.% }; f& M, y; \
1 ^2 c7 B1 s* S6 H2 eAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one2 C9 W7 ^3 e! Z+ {# D
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or
. s) q+ x& J, {bridge live parts or insulation. With another application, it may not burst since the short circuit
7 X; C% m0 f+ J$ ~" eacross a component was initiated during the zero crossing of the mains supply voltage. With$ J0 {6 z5 ]% \; Y6 P: _
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical, y U8 I4 a* i9 u# W1 W0 [+ _
insulation.
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A bursting fuse also is not an acceptable test result in either IEC or UL component fuse
7 N8 i4 N6 l8 bstandards. |
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发表于 2011-8-21 22:58
