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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
" j! {4 h$ Z) b+ J6 T) K9 wthe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3% n6 y# H7 M+ Q) N) z. j' Z
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
/ b( J% e6 Z6 @" z+ yCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current7 i9 A9 d' k: s$ m4 B, m3 J# f2 W
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and6 M4 d1 [7 O8 ^6 ~
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus* _5 S( ]: S( h3 A! c& @7 O
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and( R' Y' z4 H# C; L! B
the note is attempting to document that the breaking capacity of a fuse is a valid consideration5 z# P7 E6 t6 @& k$ ~
during performance testing.
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. W$ U7 A6 x' }0 S! NFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially1 \+ L: R3 F1 q8 A
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
5 Q1 W& z. m2 \+ S5 ]) l. `pieces can be expelled through nearby enclosure openings.
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( Y* A$ _) [/ q$ f7 G1 _Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one
; K3 ^: I# ]2 N4 n. m0 Z' E/ Mapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or
' W* R( u. o5 x( Nbridge live parts or insulation. With another application, it may not burst since the short circuit4 z0 c! |4 A9 _& M# E/ {
across a component was initiated during the zero crossing of the mains supply voltage. With
1 [5 |7 Y0 y j |3 ganother application, the fuse may burst, expelling metal pieces that bridge live parts or critical
" R0 `6 U7 W2 V: A% q- V$ ginsulation.
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F8 O7 ^6 @/ Q7 @% U+ xA bursting fuse also is not an acceptable test result in either IEC or UL component fuse
# n: N) v: w9 n& v& q Tstandards. |
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发表于 2011-8-21 22:58