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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 exceeds4 M+ ]1 ?, g6 x; W6 x. o$ F
the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
0 l/ a# }; U5 A7 _1 O# |( j(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
7 w' ?' V1 H x6 }0 n* c4 N9 iCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
5 F1 Z( u8 H- Y0 Iexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and1 r/ A& y, S5 ^9 T4 F* u
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus6 b# y& B8 ~5 U
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
, |; ^1 Q' U( z, X, Uthe note is attempting to document that the breaking capacity of a fuse is a valid consideration, G3 k; w( ~! K5 {9 S
during performance testing.' A- d3 x4 d3 q* o
3 h: e, E* Q6 ^% u+ ~' x) g1 zFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially( x! \4 H# }, N" N. R# X
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
5 n! d) k/ l/ H2 n! |( @. Q8 r) Bpieces can be expelled through nearby enclosure openings.
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" H; a8 C, e rAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one2 }4 ^. [4 z. i) X$ |; q4 |
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or! e, [7 ~+ i$ V" r, O: D
bridge live parts or insulation. With another application, it may not burst since the short circuit
, x2 x& Q! f: G' w" Zacross a component was initiated during the zero crossing of the mains supply voltage. With5 @$ y- `: I) N- d$ g' f
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical1 P* W9 v$ _8 i
insulation.( L G0 {6 D1 o5 Q" y
1 v) z/ j& d9 y1 o0 z9 Q4 u; HA bursting fuse also is not an acceptable test result in either IEC or UL component fuse
# K' G4 m1 P; L! j2 d8 qstandards. |
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发表于 2011-8-21 22:58