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发表于 2011-8-21 22:36
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RATIONALE:
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" k% s' L# T; S$ d' d6 EGenerally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds: `, l& _) h6 d( v$ p$ `
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
( ]8 c. m8 W2 p- G; }(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
3 E7 B' P2 r ICircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
+ m- V( N6 f6 q! @* E, Jexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and1 m3 |' m# u4 S d w& f- M) z
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus, O# W5 |1 h( p4 z" I; Z- O, o
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
8 d8 M, C% s; o; ]0 E- xthe note is attempting to document that the breaking capacity of a fuse is a valid consideration: _# M1 h& }$ d+ r( {: a( e& x
during performance testing.
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Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially4 c5 M7 n' F4 t( F! J
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal2 \' t5 o/ n$ Y
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
2 s0 {, ?6 @4 ?application of a fault, the fuse may burst but none of the glass or metal may exit the unit or0 j- E3 s+ R! G4 K
bridge live parts or insulation. With another application, it may not burst since the short circuit. x' S/ J) X7 x& ]( ?
across a component was initiated during the zero crossing of the mains supply voltage. With+ I# j9 d1 E6 U+ S4 j
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical" C1 @# W* H" \( `) J9 N
insulation.
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3 R, F# y# R2 a! e2 }8 [A bursting fuse also is not an acceptable test result in either IEC or UL component fuse
7 ~! u# S) {' {" Istandards. |
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发表于 2011-8-21 22:58