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发表于 2011-8-21 22:36
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RATIONALE:8 U5 }) A# T/ u* }2 R
# l$ T# P" `4 y: P) g0 V2 L2 \Generally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
" V( o2 d3 ?1 |$ T) Uthe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
! ?: B" A8 M7 F2 p, T4 Y(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary0 c$ J" I8 C/ d+ _
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current7 L9 U: x) i) z3 S
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and
8 J- p: X8 Z9 k, m5 oCanada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus+ }5 n( T, E3 C& ?
a similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and
- W- h b1 J6 _7 k# d! a$ u0 |1 zthe note is attempting to document that the breaking capacity of a fuse is a valid consideration
4 j/ A: I+ G" J) S N! pduring performance testing.
9 }& M z" \- b8 W/ ^7 L" @7 z+ U) r( r7 I, R2 D3 E: q7 _. S
Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially! H. `! I' m9 L: p6 p! q7 D
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal
9 f5 \- V" o( Q; spieces can be expelled through nearby enclosure openings.! d W W0 T1 U. |4 o: I
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Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one! T0 i$ R* G5 }/ X
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or
0 W( r) q+ t' O: A( wbridge live parts or insulation. With another application, it may not burst since the short circuit9 \3 D/ O/ G- a$ P3 [/ m
across a component was initiated during the zero crossing of the mains supply voltage. With! Q" s1 _, U2 L5 e8 [, a
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical9 G' B- x; Z" V. v
insulation.) ` Q) Y" x" x) [" l( ^8 u
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A bursting fuse also is not an acceptable test result in either IEC or UL component fuse( p) `; ?# e- K$ U/ o: h X- r
standards. |
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发表于 2011-8-21 22:58
