DSH 376 Backfeed protection

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DSH 376 ; X0 U& F8 w2 m2 |& J/ m: A  g $ l4 x1 y# _9 K# O

Backfeed protection

-

60950(ed.3)

Standard:
IEC 60950 Ed. 3
Sub clause: --- Sheet n. 376
Page 1(1)
Subject:
% k' Q$ m: c) {7 Z4 }6 Y% {; JBackfeed protection
/ |; f3 T( d4 j- a. m  p% J- xKey words:
. e2 M  [' b' v6 R3 F2 u- Backfeed Protection
7 X+ {2 k, H; `Decision taken at
/ v/ `9 p' }, R9 r# S/ m37th CTL
meeting/2000 further
# ]+ ]; j' s; I  F/ k! D0 U0 Jto the trial period
consultation
4 d& f- @# ~  z! i/ l# s2 M% G8 \Question:
The 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
4 l# x5 z+ n% e1 ?6 |1 x$ t6 X$ Xbridge. The question is related to the backfeed protection to prevent potential risk of
electric shock from being present on the mains supply terminals during interruption of the
& x5 c/ r, @1 K: q8 Pprimary power or removal of the plugs from its socket.
# Y+ w  m( U. y) A* t* ~9 q! J+ JDifferent interpretations have been experienced, and we are looking forward to a consensus.
  t. b0 V" w  @7 L# UThree alternatives may be listed as follows:
- j- ~6 S, l- {3 f. C' M1.
The BBU can be considered similar to parts of an Uninterruptible Power Systems (UPS), for
which there is the European standard EN 50091-1. However this standard is not an IEC
0 G3 E0 e  Z7 u1 b- L$ ustandard and may not be applicable outside CENELEC. EN 50091-1 specifies requirements for
! `; ~0 B. y, p/ v6 w# I) G- X$ \backfeed protection. Sub clause 2.1.5 states that such switching device shall be of an all pole
$ j2 f( O. J7 o1 @( J6 ~type having an air gap in each pole in accordance with EN 60950 Table 5 – Minimum
clearances in secondary circuits and using the column headed “circuits not subjected to
# o0 c% A5 ?9 ntransient overvoltage” – for reinforced insulation at the working voltage that is backfed (in this
case 170Vdc).
0 {. b4 E7 C8 U: B2 L2.
According 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.
& c( v0 _! q: Q3.
If the voltage on the mains plug does not exceed 60Vdc (or 42.2V peak) in the event of a
3 v  c; H" j) f5 K+ W. H) Usingle failure, when the mains plug is disconnected and the battery provides 170Vdc to the
& R/ d9 e0 z$ z" ^) D4 H  Z7 z  n“output” of the mains rectifier bridge, no additional requirements for “backfeed protection”
should be applicable.
. N$ J! \% i8 F3 C3 V/ ]0 F* R8 pDecision:
% N9 ?( F; S- a& b( M" XInterpretation Item 3 is the correct one
0 e1 ~' @7 n, B* ]9 b% o
) k1 u) `2 E1 b' p2 D
' b5 ]; }5 }( M. d

7 Z; ~/ d7 B& D