DSH 376 Backfeed protection

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DSH 376 9 I( P9 w& U5 T1 z7 [% g4 U

Backfeed protection

-

60950(ed.3)

Standard:
4 `+ {1 L6 q2 b4 PIEC 60950 Ed. 3
5 F2 s/ s5 H0 s* t7 w' K' ESub clause: --- Sheet n. 376
Page 1(1)
Subject:
Backfeed protection
; n7 R/ ]: X) h! Z2 \: s8 UKey words:
- Backfeed Protection
# D% F0 v5 |3 ]/ NDecision taken at
37th CTL
9 g" ^5 A- V# s3 m# x9 B3 ^meeting/2000 further
to the trial period
# a0 o1 i# [4 X. e* Vconsultation
$ m4 U# L+ }$ B0 H% yQuestion:
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
* O, N" v6 M+ @8 T3 O- y6 Ybridge. The question is related to the backfeed protection to prevent potential risk of
4 S- A# \6 v0 e% j( Z1 j) |electric shock from being present on the mains supply terminals during interruption of the
primary power or removal of the plugs from its socket.
3 M, M( I3 e. I. KDifferent interpretations have been experienced, and we are looking forward to a consensus.
Three alternatives may be listed as follows:
. A8 T0 R  Q; D6 ~$ k1.
The BBU can be considered similar to parts of an Uninterruptible Power Systems (UPS), for
: Q9 d( D  T3 m) c+ C3 uwhich there is the European standard EN 50091-1. However this standard is not an IEC
) A* B) F, H) u# a* _2 Y0 Tstandard and may not be applicable outside CENELEC. EN 50091-1 specifies requirements for
backfeed protection. Sub clause 2.1.5 states that such switching device shall be of an all pole
7 w1 H/ G4 g8 h$ Y: wtype 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
% W& ^" V9 I" k4 t7 V) d& a% ]0 O+ _transient overvoltage” – for reinforced insulation at the working voltage that is backfed (in this
% @+ d3 t! A. U: ]! z* Zcase 170Vdc).
% q$ E7 B. M% b8 j' q: E5 R0 R5 B2.
" F' A7 X0 O; T7 P3 w# I! e& I/ LAccording to sub clause 2.2.6 (IEC 60950), it is likely to apply reinforced insulation between
4 P9 B) v, q! b* t- m/ bunearthed hazardous voltage secondary circuit and unearthed accessible parts.
$ F* {* k' g3 r7 D) g$ S3.
If the voltage on the mains plug does not exceed 60Vdc (or 42.2V peak) in the event of a
/ G6 b0 R# }; _- ysingle failure, when the mains plug is disconnected and the battery provides 170Vdc to the
. v% B4 \+ m" z* V/ V“output” of the mains rectifier bridge, no additional requirements for “backfeed protection”
should be applicable.
Decision:
Interpretation Item 3 is the correct one

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