John asks this question, which concerns a local authority community centre, testing and possible induced voltage effects. It has been answered by SELECT (Theme – Regulations and Legislation):
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Question: Whilst carrying out an EICR on a local authority community centre, we have noted the following:
- We removed the fuse from a way within a distribution board, protecting a single-phase ring circuit (3L3) but still recorded a voltage of 27V when testing between line and earth.
- We then tested between line and line on the ring and recorded a voltage of 34V.
- We disconnected the circuit neutral from the neutral bar and recorded a reading of 27V between this disconnected neutral conductor and earth.
We repeated the above process on a second circuit; this time a radial (6L3) and recorded the following:
- Line to earth - 54V
- Neutral to earth - 51V
- Line to neutral - 51V
We again repeated the process on a third circuit (a ring on 8L3) and recorded the following:
- Line to earth - 22V
- Line to line (on ring)
- Neutral to earth - 24V
- Line to neutral - 24V
If any circuit within the board is disconnected, the voltages recorded on the above circuits reduce.
All voltages disappear when all circuit fuses are removed or the distribution board is isolated by opening the main isolation switch for the board.
The circuit wiring is carried out in single core PVC cables in steel conduits with the conduits being used as the circuit CPCs. All conduits have been confirmed as being continuous using a wander lead.
We suspect the voltages recorded may be induced from adjacent live circuits - can you please give me your thoughts?
Answer: The voltage problem encountered was possibly because of induced voltages from cables installed in the metal conduits and the conduits not having separate CPCs.
Some investigation work will be necessary and the client should be notified that some remedial work may be required to adapt or segregate circuits - and perhaps install separate CPCs.
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