DSH 376 Backfeed protection

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DSH 376 9 f$ d+ l6 ^% S: F- i  F ' @. W4 M( ]( }

Backfeed protection , A, l3 Y* _3 m# K) q* S! ?

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60950(ed.3)

0 Z6 ~# I' H4 LStandard:
0 g/ f8 C2 ~, ^; o3 M; _: ]IEC 60950 Ed. 3
9 \+ y0 N7 N6 w0 p0 vSub clause: --- Sheet n. 376
, ]0 A! G. S# v. a  UPage 1(1)
Subject:
( F5 t9 y5 U1 r+ }( [Backfeed protection
5 G0 {" `( F8 `$ o$ cKey words:
- Backfeed Protection
- M; m/ u8 L8 o& i. |; P5 _3 IDecision taken at
37th CTL
3 y  l, h8 B% ], |2 W9 B0 V- s3 ]8 I4 Ameeting/2000 further
, U0 g$ x  o4 j8 G* o, D. qto the trial period
% {3 v2 V( K4 \2 c0 K! mconsultation
9 f7 e. D, ?  W+ V5 g/ HQuestion:
+ ]: _9 g3 m5 W6 ~. B0 RThe product in question is a power supply to IEC/EN 60950 with an internal Battery Back Up
Unit (BBU). The BBU provides 170Vdc, which is connected to the “output” of the mainsrectifier
% Q, N2 ^) I9 P& s! bbridge. The question is related to the backfeed protection to prevent potential risk of
  Z; x$ p1 O! s; [7 n, welectric shock from being present on the mains supply terminals during interruption of the
primary power or removal of the plugs from its socket.
Different interpretations have been experienced, and we are looking forward to a consensus.
Three alternatives may be listed as follows:
( C  a7 x- k6 D! [3 Z3 c1.
3 F5 z. U' L4 i+ L. c- FThe BBU can be considered similar to parts of an Uninterruptible Power Systems (UPS), for
0 l: ]6 g1 [% u) m: zwhich there is the European standard EN 50091-1. However this standard is not an IEC
standard and may not be applicable outside CENELEC. EN 50091-1 specifies requirements for
5 t2 f  N# w. w, o" n# J  Wbackfeed protection. Sub clause 2.1.5 states that such switching device shall be of an all pole
% C& I; [$ J- h; itype having an air gap in each pole in accordance with EN 60950 Table 5 – Minimum
4 C8 b4 T4 e# m" J4 c) aclearances in secondary circuits and using the column headed “circuits not subjected to
8 D: R4 h: |$ }  D* Z1 e/ Mtransient overvoltage” – for reinforced insulation at the working voltage that is backfed (in this
case 170Vdc).
2.
; n- u. f6 O6 b& G- ~2 a9 EAccording to sub clause 2.2.6 (IEC 60950), it is likely to apply reinforced insulation between
unearthed hazardous voltage secondary circuit and unearthed accessible parts.
6 u5 _' C! m4 E. y' Y3.
, D4 M. W2 t5 j7 JIf the voltage on the mains plug does not exceed 60Vdc (or 42.2V peak) in the event of a
% B3 z: u& o/ o: R; o& Qsingle failure, when the mains plug is disconnected and the battery provides 170Vdc to the
“output” of the mains rectifier bridge, no additional requirements for “backfeed protection”
# [; @: V" `  ?# @  qshould be applicable.
( \: y2 k# Y% U/ o' b( XDecision:
9 p8 @2 L8 p- a1 h  gInterpretation Item 3 is the correct one

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