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I'm in the UK in an old house with fairly old wiring. In one room, I have a 5-bulb light fitting with LED bulbs. I've been battling for months a problem that they are not constant brightness - infrequently they dim for anywhere between .1s to 2s. It's like fingernails on a chalkboard to me and it's our main living room but I've run out of things to try.

Some further information:

  • Similar bulbs and fittings in other rooms on the same lighting circuit without any issues
  • I have changed the bulbs
  • I have changed the switch (now it's a simple on/off not a dimmer)
  • I have tested the fitting in a different location (wired to a plug) and it works fine

Initially I thought it was a problem with bulbs or with a new dimmer switch but swapping things around it seems to be this room specifically. I might be able to lessen the effect using incandescent bulbs as they have better smoothing, but we use this room a lot and I want to save on energy costs... and damnit I want it to work properly! That other things on the same circuit work fine is confusing.

What is likely to be causing this problem and how could I go about confirming this? What can I test?

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It's possible there's a bad connection between the fixture and the supply wiring, I've had situations where I screwed down a chocolate block (nickname for a screw-down terminal block in the UK) as much as I could but there was still a bad connection in it. I've since started using Wago splice connectors as they are much more reliable, and very quick to use.

However, this sounds like bad wiring to me, either the wiring supplying the switch or from the switch to the fixture has issues. This could be the wires themselves being faulty, perhaps the insulation is breaking down, or the gauge of the wire can't deliver the power needed for the fixture. This is potentially dangerous as it could cause a fire, so I would not mess around. Try replacing choc-blocks with better splice connectors, if that doesn't work it's probably best to get a pro in to track down the source of the issue and replace bad wiring.

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  • I hadn't considered a loose connection could cause fluctuations like this so I'll double-check whatever I can get access to. It seems unlikely to be a power limitation - it was previously running 300W of filament bulbs and now we're LED it's more like 40W. Is there any way I can confirm any of this e.g. with a multi-meter, or is it a case of trial and error?
    – Mr. Boy
    Commented Oct 2, 2020 at 12:25
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    You may be able to detect something with a multimeter, if your power is sagging long enough @Mr.Boy . Specialist equipment can detect exactly where the problem is in a wire, and give a lot of detail. Expensive to buy though, cheaper to get an electrician.
    – GdD
    Commented Oct 2, 2020 at 12:42

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