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4 e+ o( x) D! r6 l | Backfeed protection
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$ h: Y6 b: M$ l" D7 N6 c+ L. G5 C _ | 60950(ed.3)
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Standard:4 O3 u1 o2 G2 {: C4 a8 d
IEC 60950 Ed. 3% k- C5 u# B; K' T
Sub clause: --- Sheet n. 376
3 h; N4 N: Y& P2 o APage 1(1)
: c& F4 N" W( u$ }9 ^" VSubject:
# U6 G2 J5 N5 M5 F7 \1 Y3 HBackfeed protection
8 x/ R1 d( F. Q8 ~/ j! G% Y( ZKey words:1 l2 j4 U& b# J9 J
- Backfeed Protection
3 t" D! y3 t4 w8 C: S0 ZDecision taken at
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meeting/2000 further% T4 q$ ?* I4 P2 j
to the trial period
^8 p+ }! j8 ^" H( Rconsultation; b( W& X( H6 m- u3 Y' t# F6 R9 l) _
Question:
! Z1 h1 g$ t6 [The product in question is a power supply to IEC/EN 60950 with an internal Battery Back Up
% G5 e4 W5 Q# x wUnit (BBU). The BBU provides 170Vdc, which is connected to the “output” of the mainsrectifier; U/ F! R0 R: ~
bridge. The question is related to the backfeed protection to prevent potential risk of% J e. U3 z+ P1 c, Q, E6 Q& b
electric shock from being present on the mains supply terminals during interruption of the
3 w" X5 ?2 L0 Qprimary power or removal of the plugs from its socket.4 k" T# u: K9 _4 ~, D
Different interpretations have been experienced, and we are looking forward to a consensus.3 M3 a1 H' F @* E
Three alternatives may be listed as follows:
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, n0 X0 F. W' ]$ O: B9 E5 z# cThe BBU can be considered similar to parts of an Uninterruptible Power Systems (UPS), for! ~3 E! c; M: p# V
which there is the European standard EN 50091-1. However this standard is not an IEC; m3 \9 q8 ^! e
standard and may not be applicable outside CENELEC. EN 50091-1 specifies requirements for
- f1 a0 p% R# P- b( Q$ _; ibackfeed protection. Sub clause 2.1.5 states that such switching device shall be of an all pole2 h1 k8 w- L/ r6 H
type having an air gap in each pole in accordance with EN 60950 Table 5 – Minimum2 M1 N5 ^6 M* @1 z+ \6 ~+ M8 A; b
clearances in secondary circuits and using the column headed “circuits not subjected to
$ o! _/ S; n" d! b; Itransient overvoltage” – for reinforced insulation at the working voltage that is backfed (in this( v8 g4 O4 n& B* F9 g# e6 ~' X
case 170Vdc).8 J' J" v% Q9 }/ _, p; p s
2.
* p/ ~, H0 q0 L) dAccording to sub clause 2.2.6 (IEC 60950), it is likely to apply reinforced insulation between3 H. x9 e$ Z3 c+ ^2 T
unearthed hazardous voltage secondary circuit and unearthed accessible parts.
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If the voltage on the mains plug does not exceed 60Vdc (or 42.2V peak) in the event of a2 b6 [. B5 i2 X0 T
single failure, when the mains plug is disconnected and the battery provides 170Vdc to the" k5 b) A9 z/ O) w5 i
“output” of the mains rectifier bridge, no additional requirements for “backfeed protection”" L4 Y% b/ y6 E U, g/ F
should be applicable. e4 X* ]2 Q0 A' P9 T+ M1 a
Decision:: C9 w3 M: C% v7 W% h' |
Interpretation Item 3 is the correct one
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