DSH 376 Backfeed protection

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DSH 376 9 `" y; i& i! K8 ?. c

Backfeed protection

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60950(ed.3)

Standard:
$ `, }+ n0 E2 U, ]/ KIEC 60950 Ed. 3
% t. P% U1 _* B/ P, D+ e* {. |Sub clause: --- Sheet n. 376
Page 1(1)
2 a( X1 w- U0 n: M" SSubject:
Backfeed protection
Key words:
- Backfeed Protection
Decision taken at
37th CTL
meeting/2000 further
5 }% s+ L, q+ w, p3 lto the trial period
consultation
Question:
" C1 N/ d% @' a7 k3 k8 Q0 tThe product in question is a power supply to IEC/EN 60950 with an internal Battery Back Up
9 a) h* H7 B5 aUnit (BBU). The BBU provides 170Vdc, which is connected to the “output” of the mainsrectifier
+ c! }1 A1 i, ]! \0 `bridge. The question is related to the backfeed protection to prevent potential risk of
% w6 E! }4 `2 ^. d/ G" h+ `" selectric shock from being present on the mains supply terminals during interruption of the
) ]- ^; _: W# k* o' Uprimary power or removal of the plugs from its socket.
  p' U) D& e2 r1 R) }Different interpretations have been experienced, and we are looking forward to a consensus.
) l% Q* Y5 H* s$ _Three alternatives may be listed as follows:
1.
- t3 f! c, F; \2 p7 IThe BBU can be considered similar to parts of an Uninterruptible Power Systems (UPS), for
" S8 Q) d# Z- |; f, N& J3 uwhich 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
  y* T. ^* R& U% V& ^8 {backfeed 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
  O! [% G  T6 `, K! t& fclearances in secondary circuits and using the column headed “circuits not subjected to
transient overvoltage” – for reinforced insulation at the working voltage that is backfed (in this
1 O+ ?0 l3 P: t# }case 170Vdc).
2.
According to sub clause 2.2.6 (IEC 60950), it is likely to apply reinforced insulation between
  e9 T8 y9 b1 ?- v$ k/ D" c/ Yunearthed hazardous voltage secondary circuit and unearthed accessible parts.
7 e7 M# W  G* u5 L7 a3.
If the voltage on the mains plug does not exceed 60Vdc (or 42.2V peak) in the event of a
. i) a& _7 k) v+ f; dsingle failure, when the mains plug is disconnected and the battery provides 170Vdc to the
5 A/ h  d% K4 Z' N“output” of the mains rectifier bridge, no additional requirements for “backfeed protection”
should be applicable.
Decision:
% ^. A/ ~: T; E; y, e/ VInterpretation Item 3 is the correct one
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