Why is RCD tripping after replacing light fitting?

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user 37656

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Oct 25, 2023
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I've replaced a 28w CFL kitchen light with a fitting that holds 4 x GU10 (LEDs).

When I turn the light off, the MCB trips. If the light switch is on when I reset the MCB, it remains on, but if I turn it off, the MCB trips. Sometimes the RCD also trips.

To rule out a faulty bulb I removed all bulbs, then I removed the light fitting itself - so no I just have wires - the issue remains the same.

I also didn't take a picture of the previous light wired up - though I do remember that the Neutral and Live wires were not twisted together as the fitting is different.

I've attached pictures of the original circular fitting, the new fitting, and the wires as they are now. Any suggestions of how to move forwards would be welcome - I've replaced plenty of lights before but never had this happen.

Thanks for your help. :)

- though I do remember that the Neutral and Live wires were not twisted together as the fitting is different.

Sounds to me as though you have put the switch permanent live and switched live directly across the 230v supply live and neutral...

So could have damaged the switch contact and/or cable conductors by putting a direct short circuits across them! [ Hence MCB also tripping!!! ]

The physical switch contacts may also no longer be working correctly as they may have welded together or some of the internal plastic rocker parts got hot and distorted..??

i.e. one of those blues should have a brown sleeve on it as it is live NOT neutral
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did you wire the new light EXACTLY the same as the old one?

It sounds to me like you had loop at light, and you have connected all the reds together and all the blacks together (or brown and blue depending on age) so now you have the light switch in parallel with the light causing a short circuit when you turn it off, hence the mcb trips.

If so it is a wonder the switch contacts have not welded yet.

Picture on light wiring will confirm or deny that theory.

EDIT S.L beat me to it with the same theory.
Separate the two blues, leave the two brown's twisted together and/or into a terminal strip to insulate it. See if that stops the trips blowing, if it does put one blue into the blue of the light fitting and the other blue into the brown of the light fitting and connect the earth. If the light comes on but cant be switched off replace the light switch.