6 n) }: T. Y6 w, J. A8 OStandard:7 }6 a7 R, B7 p3 |. E! Z. z4 N, _
IEC 60950 Ed. 37 J6 ?8 ?, q" \/ H- f8 B+ Q# S1 e
Sub clause: --- Sheet n. 376 : O! P0 q: h- U+ W1 x- o. cPage 1(1) / a8 U1 Q; N* q: y4 r, SSubject: 1 f7 \6 \5 N4 R. _; J% ]& TBackfeed protection! X$ [3 [3 |( H1 z' w5 {0 D; S
Key words: ! s' T2 ^ U' X U- Backfeed Protection' W: R5 y8 t0 u* J
Decision taken at' w7 c7 p4 _( U; ?
37th CTL Q U7 _' Y1 h# G8 A2 lmeeting/2000 further 8 ]' m8 |3 P* {9 G* ~. K" H8 ]to the trial period# O Q5 Z6 C) @. C* v. V# C3 z% K
consultation+ j5 e5 P J4 @7 G( f
Question: * }- t8 `: ~! ?4 [" @; g/ m- O3 V" cThe product in question is a power supply to IEC/EN 60950 with an internal Battery Back Up " K& w$ x8 {& }Unit (BBU). The BBU provides 170Vdc, which is connected to the “output” of the mainsrectifier 5 b( ~2 k' |5 Z0 L7 J% T: ^1 M- Obridge. The question is related to the backfeed protection to prevent potential risk of 2 g6 P3 k+ A0 b6 s+ _/ k" {1 eelectric shock from being present on the mains supply terminals during interruption of the % l$ e# ~1 v) {4 }; ~primary power or removal of the plugs from its socket.: B0 q1 X8 |1 F9 ~4 D6 O; _
Different interpretations have been experienced, and we are looking forward to a consensus.9 a: v" `6 N, ^& @8 M# `6 f
Three alternatives may be listed as follows: : k# I8 M. g+ w% a) F5 ?9 N1. 7 w6 p" |' J" m* S0 y0 f: ]. eThe BBU can be considered similar to parts of an Uninterruptible Power Systems (UPS), for( }+ _1 H5 q$ a0 x
which there is the European standard EN 50091-1. However this standard is not an IEC - u; g' [/ K7 m6 r' d! ^standard and may not be applicable outside CENELEC. EN 50091-1 specifies requirements for 6 `& N- |8 Q/ obackfeed protection. Sub clause 2.1.5 states that such switching device shall be of an all pole ' L4 b m. b. _& Stype having an air gap in each pole in accordance with EN 60950 Table 5 – Minimum! W$ v5 K4 r% t# R9 _& C1 ^- \
clearances in secondary circuits and using the column headed “circuits not subjected to 9 B* c1 Y5 n* S! d% {* Dtransient overvoltage” – for reinforced insulation at the working voltage that is backfed (in this' S# |. s1 Y+ X
case 170Vdc). , C n* Y* m7 X" V2. # k8 k. b' g. j: u5 k4 v& R) g6 ^- UAccording to sub clause 2.2.6 (IEC 60950), it is likely to apply reinforced insulation between* }5 j: }6 r$ p7 |" R. b' z' V. f
unearthed hazardous voltage secondary circuit and unearthed accessible parts. 1 }* L6 D( {1 D* U8 f3.* w$ o* @& }8 A* B# e" l
If the voltage on the mains plug does not exceed 60Vdc (or 42.2V peak) in the event of a ( \/ [; q) ^' r6 fsingle failure, when the mains plug is disconnected and the battery provides 170Vdc to the: r7 b9 t, n0 T: Y/ S, z- z
“output” of the mains rectifier bridge, no additional requirements for “backfeed protection” ! I! a; K" f; U/ H7 }) qshould be applicable. : a2 w. Z( q/ l; E2 ]" d& i( I* }Decision: ( X, W: M* B8 M4 p8 ]" O. j1 WInterpretation Item 3 is the correct one 0 Q: D8 X# _, W6 w 2 ]: P1 o, Z, a) {[attach]77180[/attach] 1 n$ b7 \2 E S2 M7 j7 C 9 y) K' U1 f" W b$ T" y2 }; K8 \$ V1 G& |" X8 X2 R5 z2 m
DSH 376" ?7 s% ~# Z/ T- ^