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All of my ceiling, wall, under cabinet lights, have sopped working downstairs. The trip switch has tripped and won't reset. When the trip switch is flicked the lamps come on briefly. By this method, I have checked that all the lamps are now working. Before all of this happened, one of the ceiling lights in the kitchen had stopped working and another flickered briefly before lighting up. These have been replaced.

Any ideas of what to try next before I try to track down an electrician?

An MCB (You said trip switch) will not reset if the fault is still present, which it appears to be.
It is probable that when a lamp failed it has caused a light fitting that has a power supply to fail short circuit.
Unless you feel confident to check all of your light fittings, you are going to have to call a competent person to check for you.
There is also the possibility that a failing lamp could have caused a dimmer switch to fail closed circuit, (Only slight possibility) with that in mind, you could switch any light that has a dimmer switch off, and try the MCB again.

Thanks for the post. Sorry, yes it's an MCB. I date from the era when "trip switch" was used commonly!lamp

I'm moderately competent at home electrics so I could check the lamp holders, presumably with a multimeter set to the correct AC voltage? Before all of the circuit failed, one lamps (in the set of ceiling downlighters) needed failing and another flickered before lighting so these could be the culprits? I have put new lamps in each of these but this did not result in fixing the problem. The lamp holders look fine so what sort of fault am I looking for?

I did also test the MCB for resistance and this showed that there was none so it looks like that was OK.

There are 8 wall lights on the circuit and the same number of ceiling downlighters. None have dimmers.

Any further ideas would be great,

The MCB was never in question.

In short you need to break your lights down.

Can you split the wall lights from the ceiling lights?
If not, take down the light that flickered.
Are all the lamps the same? What are the lamps you have?

Thanks again for your post

Just hopefully, to answer your questions:

The lights are in four groups, each being switchable. All of the lamps and fittings are the same in each group.

Wall lights in the lounge: (4) and diner(4) are E14 9W fluorescent lamps.
Ceiling downlighters: (8) are 2.4W LED spotlights.
Under-cabinet lights (3): I can't get the casing off the fitting! I don't want to guess at 2.5W LED.
Outside lighting (3) E27 42W fluorescent lamps.

However, I called in a qualified electrician, and he says that having tested the lights, the problem is a neutral-to-earth short in the cable that connects the last two fittings on the external lights. A new cable is to be fitted later this week. He has ruled out a fault in the connections between the cable and the fittings. Extracting the old cable would mean removing masonry so this is not going to happen.

Neutral to earth fault will not cause an MCB to trip..........unless it is not and MCB, but an RCBO.
However you do not get cable faults unless something was done recently?

Thanks for the post.

I've Googled the part number on what I thought was an MCB (Cp B6 RT060630B) and it turns out to be an RCBO,

What are the implications of this? It could be a neutral-to-earth fault? What might that be? A cable fault seemed unlikely to me, but what do I know! The cable was installed in 2018. There have been no works around it such as drilling holes or fixing nails in two years.

Any further information or explanation would be great.

A RCBO is three trips combined, the MCB is two trips combined.
So there is a thermal trip, amps shown by a number.
A magnetic trip between 3 and 20 times the valve of the thermal trip, shown by a letter so for example a B6 with trip at under 5 x 6 = 30 amp with the magnetic trip, and around 6 amp given enough time with the thermal trip.

Now the RCBO also has an imbalance trip, it measures amps out and amps in and if these are not within a set amount it assumes some must be leaking to earth.

Most are rated 30 mA which means they must not trip at 15 mA, but must trip at 30 mA and must trip within 40 mS at 150 mA. In the main they trip around 25 mA.

The problem is although the line (live includes both line and neutral) goes through a switch, often the neutral does not, and neutral is around the same voltage to earth, the voltage between neutral and earth depends on the load, so the more load the more likely to trip.

So it is normal to use double pole switches for outside lights, as they are likely to get water in them, although this house they are single pole switches.

Idea is with double pole you can isolate the lights.

Note to isolate both line and neutral must be switched.

This house was wired before the advent of RCD protection, but after 2008 one would hope electricians would use a double pole switch. 2008 was when we started to use RCD protection for all circuits.

Thank you for your post. I think that I understand it a bit better now.

I noted my RCBOs are rated B6, B16, and B32, depending on the load they carry. My troublesome circuit is a B6 as it is a lighting circuit, I assume? More research using the part number shows it to be a single-pole switch.

As my lighting, RCBO is tripping (It has outdoor lights on the circuit) then the fault could well be neutral to earth fault setting off the imbalance trip? I have replaced the dud and flickering lamps in the kitchen ceiling lighting and it appears that these were not the cause of the trip as the trip still occurred after replacement.

However, I am still bemused as to how the cable can be at fault just because it would need a drill bit to go through it or a nail or have the cable's outside insulation damaged (rodent damage?)

There has been no work around the cable since it was installed in 2018 so this leaves me with rodent damage? It seems very unlikely as there have been no sightings of them and no sounds.

Unfortunately, I will never know what the damage is as it is too much hassle to get the cable out. If replacing the cable sorts the problem then the fault must be cable damage? The electrician says that he has checked the three exterior light fittings and they are all fine.

If the replacement cable faults in the future then, I can be more confident that it was rodent damage!

Have I understood this now?

FYI:
If you were to add all the numbers on your RCBO's that you have in your consumers unit it will be well over 100, most houses only have an 80Amp fuse, the numbers 6, 16 & 32 are the MAXIMUM the device can safely carry, not how much they are carrying.


Your "understanding" of the cause of the fault/cable damage is correct, damage to cables does happen, but is rare, I would suggest that it is the outside lights that are the problem, it may not be the actual light, but the connections to the light, it is not unknown for rain to get into external lights. (I did note that the electrician said they were OK, but as I am not there, and you are...............)

Thanks again for your post and advice.

Yes, down to me but I understand a lot better what is going on. I'll see what my electrician has to say and if there is anything interesting to report then I'll get back on the forum.

I have attempted to upload a picture of my consumer Unit for background information.