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发表于 2011-8-21 22:36
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RATIONALE:
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Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
( w* U6 I& [9 @3 Y: u$ ithe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3% m" u1 g" J; t. H! S
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
- Z+ B9 [) j9 y; k) GCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current# b r9 |# n! y1 x- u1 A. r( {2 k
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
. m) ~0 R+ F: r% C8 h; w* y2 C! |/ fCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus( n4 ^/ g$ @3 f$ c/ H! X* a
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
. A. @" x. A/ v+ f5 i3 gthe note is attempting to document that the breaking capacity of a fuse is a valid consideration' v8 O+ n6 g+ x& \2 S' Q1 V/ o! i
during performance testing.- s! D6 k) i0 d2 d' w4 Z% L
" T4 h+ J/ f- R% W$ z, j1 Y: A8 gFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
/ `; a9 a! I& |8 I: p$ Nbridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal" ~5 @1 y2 w) L0 Y
pieces can be expelled through nearby enclosure openings.
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/ J5 \/ h/ p4 u: g5 eAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one
/ t* p# t, r3 H6 ] Capplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or
. n" U/ `/ Z7 W0 V3 W: mbridge live parts or insulation. With another application, it may not burst since the short circuit7 u! C5 H5 s! }* M
across a component was initiated during the zero crossing of the mains supply voltage. With& h7 _+ I* [0 p
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical
: x% d! Y6 X' r% ^7 ?insulation.# u* ]: j' @1 B4 g0 P
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A bursting fuse also is not an acceptable test result in either IEC or UL component fuse; e, W! \" s: _3 s9 u# `
standards. |
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发表于 2011-8-21 22:58