DSH 376 Backfeed protection

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DSH 376 7 B" j5 v4 V) V& A, n; D, }" Z 8 j$ y! y( \1 I

Backfeed protection - U7 K. }! O* z

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60950(ed.3)

Standard:
; w$ t& ]' J. n! ?6 [# uIEC 60950 Ed. 3
Sub clause: --- Sheet n. 376
$ i. G' W; M% K* x. `Page 1(1)
9 ]( M- T3 Z, `! P  ]3 L2 bSubject:
  _5 m3 e, @0 N" GBackfeed protection
Key words:
& G5 k. {) L( K- Backfeed Protection
Decision taken at
& W# ^. f# j& ]37th CTL
meeting/2000 further
* R* P" M: D$ p# Kto the trial period
: R7 z  X; }! ]9 U. k9 aconsultation
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
bridge. 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
primary power or removal of the plugs from its socket.
, b/ B- q/ M' H0 }% g% iDifferent interpretations have been experienced, and we are looking forward to a consensus.
Three alternatives may be listed as follows:
1.
9 Z$ ~+ w- T1 Z; w& aThe BBU can be considered similar to parts of an Uninterruptible Power Systems (UPS), for
6 n# w$ p  l. kwhich 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
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
clearances in secondary circuits and using the column headed “circuits not subjected to
. O/ z, y. Y/ o+ Btransient overvoltage” – for reinforced insulation at the working voltage that is backfed (in this
case 170Vdc).
2.
9 J4 f- Q6 P4 h! J' C" q9 YAccording to sub clause 2.2.6 (IEC 60950), it is likely to apply reinforced insulation between
7 o! M5 F7 O& o/ a( a1 x. t6 ^/ Yunearthed hazardous voltage secondary circuit and unearthed accessible parts.
3.
' B1 P" x* ]) M- z4 g2 pIf the voltage on the mains plug does not exceed 60Vdc (or 42.2V peak) in the event of a
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.
Decision:
. _  W& [- `+ Q& @  i& t' c! w5 pInterpretation Item 3 is the correct one

) D6 U- \3 q/ y: N


* j* y- k* q9 y. U& H3 [8 c; B5 U