DSH 376 Backfeed protection

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DSH 376 - K+ W0 e& c9 l0 L9 }3 D) U

Backfeed protection 1 q5 S0 ^" k& b& g: U  a& u

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60950(ed.3) / p" X0 U( G# w: L' a" ^7 Q( E

  z# {! d' B3 U4 k6 p, r; @Standard:
IEC 60950 Ed. 3
4 i: D2 a' e8 I4 o' r& SSub clause: --- Sheet n. 376
Page 1(1)
Subject:
* ^: a# z% u5 UBackfeed protection
, m4 s0 k5 ?9 @: b; i# D6 A/ dKey words:
* p4 O7 R0 r- G' l- Backfeed Protection
Decision taken at
37th CTL
0 I+ l3 ?$ c; {meeting/2000 further
to the trial period
consultation
5 ~% F7 `4 `4 f7 N- oQuestion:
  G- R$ S  e1 j1 j  \* i+ q0 ]+ uThe 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
bridge. The question is related to the backfeed protection to prevent potential risk of
" F$ T+ v! c# |% k0 Zelectric 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:
1.
* T2 x0 N2 G$ q- m' J9 @( c) Z/ TThe BBU can be considered similar to parts of an Uninterruptible Power Systems (UPS), for
, J3 ?  N8 t0 L) e( t* \7 Vwhich there is the European standard EN 50091-1. However this standard is not an IEC
1 T9 ]- K+ }( K) V5 lstandard and may not be applicable outside CENELEC. EN 50091-1 specifies requirements for
$ r+ t0 ?/ Z$ @7 F% Rbackfeed protection. Sub clause 2.1.5 states that such switching device shall be of an all pole
type having an air gap in each pole in accordance with EN 60950 Table 5 – Minimum
5 D4 F6 _% j6 Z/ i% b; E; @clearances in secondary circuits and using the column headed “circuits not subjected to
% ]) G  k9 V9 W$ ]! @" Dtransient overvoltage” – for reinforced insulation at the working voltage that is backfed (in this
' M- a; p7 N! ^* n1 c9 |: S5 C7 G$ Hcase 170Vdc).
2.
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.
6 [' `* {# E4 s7 v3.
If the voltage on the mains plug does not exceed 60Vdc (or 42.2V peak) in the event of a
. @& N; q8 q) z5 J! B0 V* E, _single 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”
should be applicable.
% T8 Y; c$ |4 D% e: T# R- WDecision:
. ~: V# y6 I: H  WInterpretation Item 3 is the correct one
* R6 \4 B  B* R5 ~
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