DSH 376 Backfeed protection

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DSH 376   U8 R0 ^+ j( n, D ; H% g% J7 K8 m  d* v6 D

Backfeed protection   O6 j( ~4 b2 q; z$ ~4 N# v5 g

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60950(ed.3)

Standard:
" {1 l2 w9 Q8 p$ {9 f! ^7 xIEC 60950 Ed. 3
Sub clause: --- Sheet n. 376
Page 1(1)
. F5 {& l1 f6 {' R  f5 w, o" OSubject:
Backfeed protection
6 e& n3 k9 J$ ]2 }+ L' mKey words:
- Backfeed Protection
( y& ^; j3 K  i0 z$ o2 i- eDecision taken at
37th CTL
meeting/2000 further
8 n; b& J& z/ b8 y. [4 Qto the trial period
consultation
# h6 |+ d0 s) ^! d6 B2 AQuestion:
9 j  Z; P6 Q! `$ M3 u1 CThe product in question is a power supply to IEC/EN 60950 with an internal Battery Back Up
  J% v3 U; Q7 ?2 f4 ]" C7 U5 PUnit (BBU). The BBU provides 170Vdc, which is connected to the “output” of the mainsrectifier
( Z3 e+ l, X8 ]- r0 D: d& d9 sbridge. 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
. g5 u$ C) t7 [+ T' N$ j  ~0 Lprimary power or removal of the plugs from its socket.
0 ]5 s; q# u2 l. z8 k, O4 e" kDifferent interpretations have been experienced, and we are looking forward to a consensus.
: a6 \1 R, ]" M. z* Z5 SThree alternatives may be listed as follows:
1.
/ ^" J& m% k8 V4 R' ?! {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
% `7 x* T7 p; N% }standard 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
. p% y- \6 v+ Ytype 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
) `5 |/ w4 E  Z: v+ l7 g% ?6 Vtransient overvoltage” – for reinforced insulation at the working voltage that is backfed (in this
case 170Vdc).
2.
+ N$ m6 O, P8 P, g0 s4 \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.
8 L& k4 d7 P: l3 }3.
If the voltage on the mains plug does not exceed 60Vdc (or 42.2V peak) in the event of a
1 j& q& }/ \( ]  ~& p$ wsingle failure, when the mains plug is disconnected and the battery provides 170Vdc to the
2 {+ x9 _. g: T3 B& e: a/ x/ j“output” of the mains rectifier bridge, no additional requirements for “backfeed protection”
% }6 F9 I; h+ H& U  o) W5 D! Z) @" V+ dshould be applicable.
6 }4 z0 A" L1 U( o8 K, b2 _% KDecision:
3 V3 C! n& e0 S0 B1 ?( ]Interpretation Item 3 is the correct one
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