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发表于 2011-8-21 22:36
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RATIONALE:. q- ^; o# z5 r \
& F0 s0 S$ D/ T& UGenerally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
& ?+ y0 o; R/ K1 Ithe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
" H5 [# f" j' p0 M(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary
: l) i! W' V7 ~& H# LCircuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current; P, l ~9 }6 R# i- E" t) c
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
( O0 L8 r T2 Q0 q6 v& PCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus3 D+ _2 k8 U6 D, N
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
, Z' N9 \1 K* zthe note is attempting to document that the breaking capacity of a fuse is a valid consideration2 `" W- }! l$ E s, Y& a+ C
during performance testing.
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/ k3 H2 X2 H2 @2 B' L AFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially
9 W2 I5 p" B; q5 k2 U9 s. V, Kbridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal- `' ], p7 y. v. N+ U7 c( \
pieces can be expelled through nearby enclosure openings.0 c2 [5 I. C+ m( y" o
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Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one
! S7 ~4 _9 Y x& {6 p+ n' kapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or
' D+ B* }* G/ ]5 ^2 L8 @$ mbridge live parts or insulation. With another application, it may not burst since the short circuit
6 ^: e. t. r* O' b. Y4 D0 P% yacross a component was initiated during the zero crossing of the mains supply voltage. With l- x8 h' }0 _. S7 V8 b
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical
! o& c# i! Y: H( d. W8 [insulation.* f; I8 n! O0 u* z
w: k; ^% n7 F, M6 k6 s) tA bursting fuse also is not an acceptable test result in either IEC or UL component fuse
. q4 y I3 ?! X$ ~0 w" ^standards. |
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发表于 2011-8-21 22:58