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发表于 2011-8-21 22:36
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RATIONALE:/ E7 H2 A% I+ W# }
% x: Y ~0 {+ ~5 @* G/ S4 yGenerally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
/ H" g3 [0 v. ]( m3 l" x1 ?1 q% V# [; Zthe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3. S: W4 ~7 J% B E0 e$ `
(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary! J& E( b$ O2 E3 }) x1 |9 t
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current
4 S/ o: ]3 N# ^0 Eexceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and' P% `. U( y$ u9 `1 p
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
# M5 t/ q* E! ~" i2 z, Ia similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
' N# \# D' u7 Lthe note is attempting to document that the breaking capacity of a fuse is a valid consideration
( w$ ^% K- O$ H+ ]) yduring performance testing.
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. J3 T1 H* T, zFurthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially6 ^. k9 q. E+ |! Q4 X3 t+ y* z
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal; |' U$ o+ B5 P; a- c; y4 o# X
pieces can be expelled through nearby enclosure openings.
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Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one
% ?8 J9 J6 ?8 _0 l& [ lapplication of a fault, the fuse may burst but none of the glass or metal may exit the unit or% A2 {3 C6 p; D& r6 V6 k
bridge live parts or insulation. With another application, it may not burst since the short circuit; }5 r2 j9 x9 t B/ L
across a component was initiated during the zero crossing of the mains supply voltage. With( u, q" ]1 m5 _7 {
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical
: Y: D2 q" N* n+ F8 x$ M" }# t8 _insulation.
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5 o! Q' ?$ A" K) W$ {0 w/ qA bursting fuse also is not an acceptable test result in either IEC or UL component fuse
- ?/ ^/ Z! @! Q( w! F6 O Wstandards. |
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发表于 2011-8-21 22:58