Lighting MCB keeps tripping after changing the kitchen light

[strangecharm]

I'm not sure what's going on with my kitchen light.

I've replaced an old 28W CFL fitting with a new 4 x LED GU10 fitting (a cheap one from Screwfix). Although the light turned on after flicking on the MCB, when you turn it on or off the MCB trips, and sometimes the RCD, too.

I removed the bulbs to see if they were the cause, but the issue remained. So I removed the fitting, too. The MCB still trips.

In all the light fittings I've previously installed I've always twisted together the live and neutral wires - but although I've done it here to fit the block in the new fitting, in the original fitting (picture B), the only wires twisted were the earths.

I'm not sure whats going on, but by a process of elimination, twisting the neutral and live wires is obviously caused the MCB to trip - nothing else has changed!

I didn't take a picture of the original fitting as I wasn't expecting problems, so I'm not sure how to rewire it. Looking at the available connector - which has two holes labelled N, and 2 labelled '1', I assume these are for the separate neutral and Live wires. Having a plastic body, the earth wasn't needed?

Any help or explanation of what I'm doing wrong would be welcome.

 

[westward10]

Is there another light being switched on or is that the only one.

 

[strangecharm]

Is there another light being switched on or is that the only one.

So the switch for the kitchen is 3 gang, but the MCB only trips if the kitchen light is turned on. I've just untwisted the wires - so the neutral and live wires are now separate - and the MCB no longer trips when you turn on the switch.

 

[timhoward]

In all the light fittings I've previously installed I've always twisted together the live and neutral wires

You have been lucky up until now.
It is frequently the case that one cable 'delivers electricity' and another cable is connected to a switch.
(Sometimes there is a 3rd cable to deliver electricity on to the next room)

It seems almost certain that one of your cables is the supply, and one is connected to the switch. If you just join them together as pictured you are shorting out neutral and live when the switch is on. You should really have paid more attention to what you removed.
You can't make this work with what you have there, another connector block (Wago / choc block) is needed as well as tester to determine which cable is which.

 

[westward10]

So the switch for the kitchen is 3 gang, but the MCB only trips if the kitchen light is turned on. I've just untwisted the wires - so the neutral and live wires are now separate - and the MCB no longer trips when you turn on the switch.

So I assume this switch only turns the one light on, if so you have a scenario are described by @timhoward above.

 

[strangecharm]

You have been lucky up until now.
It is frequently the case that one cable 'delivers electricity' and another cable is connected to a switch.
(Sometimes there is a 3rd cable to deliver electricity on to the next room)

It seems almost certain that one of your cables is the supply, and one is connected to the switch. If you just join them together as pictured you are shorting out neutral and live when the switch is on. You should really have paid more attention to what you removed.
You can't make this work with what you have there, another connector block (Wago / choc block) is needed as well as tester to determine which cable is which.

Thanks, Tim. I've been reading up and thought something like this had happened. It might not be luck - I think most of my wiring experience is using 1960s-era cabling...

So a voltage detector pen will tell me which grey cable is the supply, and which is the switch (I also have a multimeter).

I'm not clear how a block would make the fitting work, though. Do I just need a different fitting?

 

[westward10]

Thanks, Tim. I've been reading up and thought something like this had happened. It might not be luck - I think most of my wiring experience is using 1960s-era cabling...

So a voltage detector pen will tell me which grey cable is the supply, and which is the switch (I also have a multimeter).

I'm not clear how a block would make the fitting work, though. Do I just need a different fitting?

I would suggest you need an electrician.

 

[strangecharm]

I would suggest you need an electrician.

You're probably right, but at this point, I just want to put the original light back up.

 

[goasis]

You're probably right, but at this point, I just want to put the original light back up.

Unfortunately it doesn't matter what light you put up, unless you can safely identify which cable is the supply, which is the cable to the switch, and then how to terminate them successfully and safely, then you're going to have issues.

 

[strangecharm]

I do appreciate that. I think I've worked out what I've done, and how to fix it. Before wiring that, though, I'll draw a diagram and post it here tomorrow to check I have understood things correctly. If someone could confirm it's correct, that would be welcome.

 

[littlespark]

Although I agree that this is a “supply” cable and a “switch” cable…. I am surprised it wasn’t noticed when you took the old fitting down that the two browns were together, not connected to the light…. and the two blues were across L and N.

I am also thinking now that wiring like this, with a dead short across the switch when operated, has possibly damaged the switch, and/or the circuit breaker. Especially if it’s been done a number of times.

 

[pc1966]

Top tip, though too late now, is always photograph what you are about ti disassemble. And again after reassembly.

It helps if something unexpected happens and you need help, and it also serves as a record of when something was fixed should you need to replace a failed accessory, light, etc, and suspect it should still be under warranty.