DIY Doctor

Voltage present on light switch pattress

Postby Simon_365 » Sat May 02, 2020 4:30 pm

After some help please. I replaced a timer switch that controlled my hall lights with a conventional light switch. That all works fine. As I was finishing the job I checked the screws that secure the switch to the metal pattress back box with a digital multimeter, and found there was 50v between it and my front door handle. This disappears when the circuit is isolated, and comes back when it is powered up. I undid the screws to make sure I'd not crushed any wires, I hadn't. The pattress has 50v on it when the switch is pulled out away from the metal. The earth cable appears to be screwed securely to the terminal on the pattress. I've checked the screws on the other light switches in the house. Most are zero, some a few volts, but a couple are up to 30v. Any ideas what is going on? Bit worried it's the insulation degrading. Would you expect the MCB to trip it at 30v - 50v? Thanks.
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Postby Simon_365 » Sun May 03, 2020 5:20 pm

Had a look inside the consumer unit. All the earth wires in there appear to be terminated correctly. 243v between the live and neutral, and live and earth. Nothing between the earth and neutral. Measuring circa 50v between the busbar and earth. I also get a reasonable spark when I short the light switch pattress screws to a radiator, so I don't think the multimeter is imagining it.
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Postby Mr White » Sun May 03, 2020 9:53 pm

Either induction or a bad earth.
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