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Induced voltage or dodgy wiring?

Farmboy
A domestic Flat has 1 * 6A lighting cct, 1 * 32 RFC, and 1 * Ckr cct. Supply is TN-S.

A 2G plate switch in the kitchen (which has a flat roof / attic space above other rooms) is fed by a twin and earth - one core feeds the commons of the two switches, while on one switch the other core supplies two downlights. A sheathed single core from the second switch feeds two wall lights. Each downlight is fed by a single T&E. One wall light has a T&E loop in / loop on, the other light just a T&E in. All wiring is concealed in walls.

At the downlights, 244V (L-N) appears with the switch turned on (using a MFT). However, with the switch off, approx. 23V still shows. With the switch off and a lamp installed in either fitting, while testing at the other, V=0. The 23V does not appear at the wall lights.

With the CB’s turned off on the other ccts at the CU, the 23V remained, as when the N’s were disconnected also. V=0 with the lighting CB off.

L/N – E IR test with all switches on is > 300MOhms.

Before hunting about for things like dodgy connections in JB’s in a grotty/dirty loft space, any ideas what may be going on please?


Thanks


F
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  • 0
    It does sound like it's just picking up a harmless stray voltage from an adjacent conductor - more likely capacitive coupling rather than induction - possibly between L and SL in the T&E switch drop. (Presumably the single to the wall lights happens to be spaced further away from the L so is affected far less)


    The same effect has been known to make low energy lamps flicker on when switched off - it's not that unusual.


    Modern high impedance voltmeters are notorious for spotting such things - older "analogue" meters tended to have a lower impedance so just connecting the meter would usually allow the voltage to collapse to next to nothing. A kludge would be to lower the impedance of your meter by shunting it with a moderate resistance (a few k Ohms) - if the voltage then disappears, then it's almost certainly just capacitive coupling and nothing to worry about. (I gather some meters have such a facility built-in these days). Probably worth doing a check just in case it is something nasty giving similar symptoms, but I would have thought the chances of that were low.


       - Andy.
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  • 0
    It does sound like it's just picking up a harmless stray voltage from an adjacent conductor - more likely capacitive coupling rather than induction - possibly between L and SL in the T&E switch drop. (Presumably the single to the wall lights happens to be spaced further away from the L so is affected far less)


    The same effect has been known to make low energy lamps flicker on when switched off - it's not that unusual.


    Modern high impedance voltmeters are notorious for spotting such things - older "analogue" meters tended to have a lower impedance so just connecting the meter would usually allow the voltage to collapse to next to nothing. A kludge would be to lower the impedance of your meter by shunting it with a moderate resistance (a few k Ohms) - if the voltage then disappears, then it's almost certainly just capacitive coupling and nothing to worry about. (I gather some meters have such a facility built-in these days). Probably worth doing a check just in case it is something nasty giving similar symptoms, but I would have thought the chances of that were low.


       - Andy.
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