What is the middle switch doing?

johnny2007 · 14 Oct 2019

Triffle said:
Thanks all mind duly put at rest, and I shall be sure to teach myself continuity testing.

And I've learned about MK grid switches too, so that's a bonus.

You now know more than most electricians working for the big expensive firms

securespark · 14 Oct 2019

Earth wires are clamped by nut and bolt.

Triffle · 20 Oct 2019

One PIR successfully installed this afternoon

fully operational.

One down, one to go (as soon as we get more dry weather).

EFLImpudence · 20 Oct 2019

Triffle · 2 Nov 2019

Aaaand second one installed this morning, also fully operational.

Thanks again all for your help and advice.

Sureitsoff? · 2 Nov 2019

EFLImpudence said:
What it used to do is obviously impossible to say.

To find out you will need to determine where the two reds in the wire nut and and the cut black go.

Having just seen this thread, what cut black?

EFLImpudence · 2 Nov 2019

OP

Triffle said:
There is also a black wire just cut short, visible (just) between the two red wires at the green wire, further back where it comes in from the wall.

EFLImpudence · 2 Nov 2019

Having had another look, there is also a second unaccompanied red wire so there might be a second cut black.

ericmark · 2 Nov 2019

The thing I looked at was two holes in a ferrous back box, should be a saw cut between the two holes to stop transformer effect, likely so little current it will not matter specially if LED lights, but I would want to join the two knock outs with a saw cut.

JohnW2 · 3 Nov 2019

ericmark said:
The thing I looked at was two holes in a ferrous back box, should be a saw cut between the two holes to stop transformer effect, likely so little current it will not matter specially if LED lights, but I would want to join the two knock outs with a saw cut.

Are you really concerned about that in relation to the backbox of a light switch?

Kind Regards, John

Triffle · 3 Nov 2019

EFLImpudence said:
Having had another look, there is also a second unaccompanied red wire so there might be a second cut black.

Ohh, do you mean one of the two reds which go into the yellow twist on connector? Because there's only one black there?

ericmark said:
The thing I looked at was two holes in a ferrous back box, should be a saw cut between the two holes to stop transformer effect, likely so little current it will not matter specially if LED lights, but I would want to join the two knock outs with a saw cut.

I don't quite understand this, do you mean the two holes should be joined by sawing a gap between them? What is the transformer effect and why would cutting a gap help?

scott1980 · 3 Nov 2019

Triffle said:
I don't quite understand this, do you mean the two holes should be joined by sawing a gap between them? What is the transformer effect and why would cutting a gap help?

You don't have to do that. Google "eddy currents". It's something associated with much higher currents than you would ever see in your average lighting circuit. The fact that your installation has been in place for 40+ years without showing any effects of eddy currents, I really wouldn't give it a second thought...

SpecialK · 3 Nov 2019

Almost looks like a shadow of the red wire. Has the black wire been used as the earth? Perhaps this is why the middle switch is no longer usable?

Triffle · 3 Nov 2019

SpecialK said:
Almost looks like a shadow of the red wire. Has the black wire been used as the earth? Perhaps this is why the middle switch is no longer usable?

I don't think the black has been used as the earth, as there are separate earth wires in there. I think I'm going to have to have another look inside and see if there is indeed another cut black (when there's enough daylight that is

)

It occurred to me yesterday, actually, that there was once another light in this room... when we first moved in there was a ceiling rose and bulb right above the window (as well as the one in the middle of the room).. when the place was rewired we asked them to remove that ceiling rose because we didn't need it.

Now I'm wondering if that was once powered by the middle switch... which then begs the question what did the electricians do to the switch when they removed it..

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What is the middle switch doing?

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