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发表于 2011-8-21 22:36
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RATIONALE:0 R1 ~2 }! {. c
* ^0 I9 e5 s8 z* u: P+ @( j# RGenerally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
# Y+ Y9 s8 y% a8 @* O1 v5 `the breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
7 s* I% C6 u( H C(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary0 z, v* G7 q* O: L) B
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current' z7 f& e0 q# s) i6 v& Y% @! k
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
1 v6 S* W5 C# }7 }$ X, r$ WCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
9 Q! v& I1 t, i% U v: R) qa similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
( e! |. C# I7 Z5 O. ^+ z4 rthe note is attempting to document that the breaking capacity of a fuse is a valid consideration7 H) t( O7 x8 ^1 L
during performance testing.2 Q% q) x- f; R; R
- W3 c; y( E) p4 \Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially8 T* m" b$ v. \/ B* `3 f: W3 y6 i
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
, [; w6 J' V! m/ L4 V' \pieces can be expelled through nearby enclosure openings.
5 H' e* x) I4 ^, f) x
" }6 a. Y+ C+ j! UAlso, a bursting fuse terminating a test also is not a consistently repeatable result - during one
5 j9 e' n- C; @& A' Y- C. gapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or+ M T- X% a7 ^: R' e5 T
bridge live parts or insulation. With another application, it may not burst since the short circuit9 X' O6 b9 N: W3 I
across a component was initiated during the zero crossing of the mains supply voltage. With
/ o4 x- i# O# o% ganother application, the fuse may burst, expelling metal pieces that bridge live parts or critical
. p2 W6 y% E+ d: J/ w2 O& C' ^6 Iinsulation.2 B) ?8 u8 J5 D, N8 |( T @' ~
& _9 w9 } T# _" r, ?' Q d; dA bursting fuse also is not an acceptable test result in either IEC or UL component fuse, V* n' Q g3 s- D* h- Y0 Y
standards. |
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发表于 2011-8-21 22:58