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New light fixture affected existing light fixture on same circuit?

Postby fdh » Mon Dec 30, 2019 8:17 pm


I am puzzled about the lights in my house and wondering if anyone can figure this out.

In our hall we have a light fixture on the upstairs ceiling and one on the wall over the stairs. There is a switch upstairs and one downstairs, both controlling both lights together.

I replaced the light fixture on the ceiling. After I took down the old fixture (ingeniously not noting down the exact wiring, i know i know) I noticed there were two wires coming from the ceiling so for the new fixture I used the wiring shown in Figure 2B on this page:

I don't think this is how it was wired originally but it did used to work as expected, so not sure.

After putting up the new fixture on the ceiling the new fixture (4 LED bulbs) works fine and the switches operate as expected, but the wall light (2 halogen bulbs) only emits a dim glow.

Any ideas why the wall light is so dimmed? I had wondered if it is because there are halogen and LED bulbs on the same circuit, or if the existing wiring conflicts with how I have now connected the new light fixture.

I'm planning to remove the wall light altogether anyway, so will maybe get a better idea once I've opened it up (can't do just now as it's high up and I don't have a ladder).

Any wisdom appreciated!
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Postby Mr White » Mon Dec 30, 2019 10:28 pm

You answered your own question when you said
"(ingeniously not noting down the exact wiring, i know i know)"
You have wired the lights in series.
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Postby fdh » Mon Dec 30, 2019 10:53 pm

Thanks! - funnily enough after posting this I sat down with pen and paper and wondered if that's what had happened.

Before putting up the new fitting I googled it and seemed like the diagram I mentioned was the way to go
Then comes the mistake, they join all the reds together, then all the blacks ).

Is that incorrect, should I instead wire them in parallel?
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Postby ericmark » Tue Dec 31, 2019 1:56 pm

Often one of the blacks is not neutral but is line return from the switch, in theory it should have a red sleeve, in practice often not marked.

So all earth wires together.
All red wires together in most cases unless there is a second light in the room.

Often three black wires, two connect together and to blue on drop flex, and one connects to brown on drop flex.

Typical mistake is all three blacks together and typical report is when I switch off lights the trip opens, what has happened in that case is a short circuit when switched on, so lights are actually working when switched off.

In your case still the blacks mixed up, but neutral it seems only connected to lamp you have been working on.

So you need power off, and test each cable with a meter of some sort, helps to have second person to flick the light switch, one cable the red and black will show connected and disconnected as you flick switch off/on etc.

Any attempt to find by guess work is likely to damage the switch, not short cuts, has to be tested.

I tried to set out how to test on this forum once before, and got banned for my troubles, some one decided my advice was dangerous, and then disappeared. However he is correct in a way, any person who has the intelligence to sort this out safely does not need step be step instructions, you know you have a black in wrong place, you know the odd one out goes to the switch, and if you with power off measure between each black and the reds, the black which shows open and closed circuit as you flick the switch is the odd one out which you should mark with red tape and that is the switched line.

If you can't work it out, then time to call an electrician, there is a time to say stop, and get some one who knows what they are doing, but I think from your replies you can work it out.
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Postby Mr White » Tue Dec 31, 2019 5:48 pm

If your lights do not work as they should then as I said earlier

Mr White wrote:You have wired the lights in series.

I glanced at the diagram, it looks correct.
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