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发表于 2011-8-21 22:36
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RATIONALE:# Y/ Y$ C$ T7 x9 f2 A
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Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds r; R+ L" G' F
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
. j. U8 b% U+ f( N% t: C7 U(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
3 D4 g5 G% y" c' SCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current F8 K# T+ B6 E8 N5 Y; M% z
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and3 t C: u7 v( s" v
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus6 J. B/ Y6 n/ y& z d7 w! q
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
$ V8 G2 ?* ~5 Othe note is attempting to document that the breaking capacity of a fuse is a valid consideration1 e0 ^0 V' y- W: P
during performance testing.
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# S7 }% n! ~9 Q" p8 M: n" ?Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
L' B' X8 A+ \9 k+ ~2 g) jbridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
# a9 X3 f$ ~% i- O% \: Z& Ypieces can be expelled through nearby enclosure openings.. p, n, x& p% q0 ~( z8 Z
% j4 J/ B# T: C0 ~4 f' YAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one- \7 O& l& @2 R; w
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or
. \4 z2 S% [4 I. ?7 Nbridge live parts or insulation. With another application, it may not burst since the short circuit- h8 r+ D* u* M& a) c7 C6 B2 a
across a component was initiated during the zero crossing of the mains supply voltage. With
Z9 Z4 y+ H5 x# G* I. S7 yanother application, the fuse may burst, expelling metal pieces that bridge live parts or critical7 G! g- C ]1 i v4 _
insulation.
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A bursting fuse also is not an acceptable test result in either IEC or UL component fuse
& E, `( x% i( |standards. |
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发表于 2011-8-21 22:58