DSH 376 Backfeed protection

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DSH 376 6 u- B0 w) [9 P) L # T2 l  f/ X) H0 P ( s$ C7 }" A( }' a+ a5 [1 G

Backfeed protection : V9 ?7 b& t- e" M; y

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60950(ed.3)

Standard:
IEC 60950 Ed. 3
Sub clause: --- Sheet n. 376
Page 1(1)
4 p( x1 m, \7 z* C. w: JSubject:
: g1 M' a' I6 ~Backfeed protection
/ d- I4 a, b- B2 xKey words:
' |# Y% ]( B4 a+ W- Backfeed Protection
  ~7 q4 [) Z" ?7 q6 d6 eDecision taken at
37th CTL
; }2 D3 S0 I2 qmeeting/2000 further
to the trial period
- E. i! k: k# s0 x8 U4 S$ kconsultation
Question:
! _* v* X6 z  ^4 I4 h: s  Q/ yThe product in question is a power supply to IEC/EN 60950 with an internal Battery Back Up
1 l) Z  n# R2 YUnit (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.
7 D6 C' @$ ^/ L4 vDifferent interpretations have been experienced, and we are looking forward to a consensus.
  ~, U. f  Z) Q& M6 kThree alternatives may be listed as follows:
1.
+ D1 Z7 p; }) s# EThe 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
0 x1 M2 e- H7 e; l" X1 o, {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
  ]) }. C5 ^. M* o; ftype having an air gap in each pole in accordance with EN 60950 Table 5 – Minimum
2 j6 @1 N1 m, \+ `$ q3 f* @* z3 gclearances in secondary circuits and using the column headed “circuits not subjected to
& q& o& T6 h1 s/ ^+ d7 D3 R$ X) F8 qtransient overvoltage” – for reinforced insulation at the working voltage that is backfed (in this
2 \+ r8 c$ E2 M( z' c: b3 scase 170Vdc).
2.
8 H" t7 T$ V. S) B3 B" rAccording to sub clause 2.2.6 (IEC 60950), it is likely to apply reinforced insulation between
5 z. E* g) U: P5 L, B: a2 K$ kunearthed hazardous voltage secondary circuit and unearthed accessible parts.
1 M! ?3 f0 {& h- q3 L4 K3.
If the voltage on the mains plug does not exceed 60Vdc (or 42.2V peak) in the event of a
+ R* }" s8 X* L2 s4 _single failure, when the mains plug is disconnected and the battery provides 170Vdc to the
& [9 _6 b% E  ]# Z8 }6 ]* H“output” of the mains rectifier bridge, no additional requirements for “backfeed protection”
( o7 H/ \6 r# T: c' fshould be applicable.
Decision:
Interpretation Item 3 is the correct one
' _  L8 v" T$ N8 A



) ~0 A5 I  c" a