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发表于 2011-8-21 22:36
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RATIONALE:3 [/ N; ~ ^- {6 [
' T) ?( T9 c# v" h, l8 [- o5 y5 \/ m7 gGenerally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
* o4 }4 F6 Q4 othe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
. y- E0 u- [. t% S(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary! X( s! L; H& ^$ t+ O K) D3 g
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
$ m4 | h0 b: P) f" Z2 Nexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and6 v$ n O8 x4 e, R* i) c0 {2 j
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus+ c! X- H9 V8 d
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
, q0 o+ `8 { {# Y4 g" _the note is attempting to document that the breaking capacity of a fuse is a valid consideration1 p. h& ]& |# }# B& u8 r5 L y
during performance testing.
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Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
1 e8 F" m( h- J4 Y3 x$ p3 d6 Cbridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal2 Y7 @8 |4 F% O0 Q
pieces can be expelled through nearby enclosure openings.
0 f3 O! p* y. m" o) T0 M# F
- F' N" _0 c. x' I( GAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one
) ~) w8 x. i! |" \7 E* _. [2 Dapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or
v$ p8 S, [7 B4 c! `8 gbridge live parts or insulation. With another application, it may not burst since the short circuit
( W1 v" X: D2 G) e: Z2 }/ [ Eacross a component was initiated during the zero crossing of the mains supply voltage. With
4 q5 S; B- N& T% Q$ J0 k5 ]another application, the fuse may burst, expelling metal pieces that bridge live parts or critical
- P# S! l- y ?; binsulation.
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0 d4 ^; e5 i- q" B% x# B: `A bursting fuse also is not an acceptable test result in either IEC or UL component fuse
( W- } q- @ r$ S7 i; J+ E+ K8 Sstandards. |
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发表于 2011-8-21 22:58
