|
|
RATIONALE:% o7 N4 q5 |' T5 P
& N! j3 ~' N* r6 z$ pGenerally, when a fuse bursts it is because it is subjected to a short-circuit current that exceeds
' W& c3 H' Q! lthe breaking (rupturing) capacity of the fuse. The informative Note to sub-clause 2.7.3
: u& O" X% h' |9 m5 p(Short-circuit backup protection) states “If fuses complying with IEC 60127 are used in Primary' R: M6 ~% ^3 `. o, ]
Circuits, they should have high breaking capacity (1500 A) if the prospective short-circuit current0 K) F% `8 k' m
exceeds 35 A or 10 times the rated current of the fuse, whichever is greater.” (In the U.S. and' u1 G# F3 A: q" S+ |7 m, X# O/ ?! i
Canada, UL 248-14 fuses are indicated as appropriate alternative fuses per Annex P.2, and thus
, M% w) s' l t, C' O1 t4 Na similar consideration applies.) Typically, high breaking fuses are ceramic, not glass cased, and% X* `+ u9 x+ J) r% ^' U' F
the note is attempting to document that the breaking capacity of a fuse is a valid consideration$ u; c$ G, R8 H) @2 [* G
during performance testing.
9 h' ^' |/ n$ o j* y. ]+ Z
! {! S( X6 ?2 F( ^7 q/ K! p9 `Furthermore, a bursting fuse can expel dead metal parts onto surrounding circuitry, potentially( _, J) r, Z9 `- d) r, q
bridging insulation and contributing to risk of fire or electric shock. Similarly, glass and metal+ X X# q- d) g8 O( |! W: B
pieces can be expelled through nearby enclosure openings.9 w2 y7 O9 g2 _
1 E5 X# {: R I- ]9 ~
Also, a bursting fuse terminating a test also is not a consistently repeatable result - during one8 V5 n) Z0 \! X) c
application of a fault, the fuse may burst but none of the glass or metal may exit the unit or3 k3 Y& h' k, u# O
bridge live parts or insulation. With another application, it may not burst since the short circuit
, A2 U3 n: G. L! c% m; `across a component was initiated during the zero crossing of the mains supply voltage. With. l( t- ^4 _( D
another application, the fuse may burst, expelling metal pieces that bridge live parts or critical, s/ D8 O: }& M9 y2 g: H
insulation. N Y& z" e O- O7 k2 t& n
4 y: v/ h- X0 c, b) @3 P
A bursting fuse also is not an acceptable test result in either IEC or UL component fuse q" b0 J2 @" M2 O y6 L E
standards. |
|
/2 
|关于安规|小黑屋|安规QQ群|Archiver|手机版|安规网
( 粤ICP13023453-10 ) 
窥视卡
雷达卡
楼主
显身卡
发表于 2011-8-21 22:58
