Fault Finding on Light Circuit With old Fuse box Ask questions and find answers to many subjects relating to electrics and electrical work

[Venom Bottle]

Hi,

I have an old fashioned fuse box at home. Recently my upstairs lights have started flickering. Initially its fine, after the lights are on for a couple of hours the flickering starts. This has gotten worse and worse and then a the fuse wire went (with a bang/pop). I’ve checked light switches and terminals/junction boxes in the loft for any scorching/water ingress/loose connections and everything looks sound.
Could it just be that the fuse wire was worn out? Would the time delay from turning on the lights to flickering starting be consistent with a basic fuse wire wear or does it seem more likely there is a separate issue?

There is a slight smell on one bedroom light fitting after removing the bulb but no witness marks of any kind.

Thanks

[Mr White]

Fuse wire if not stressed will last for many years, it doesn't wear out.
Without testing it is not possible to say what the actual cause is.

But I would ask is it all the lights, or just upstairs? (You said loft) The other thing you may want to look at is the cable runs, is there any damage?

[Venom Bottle]

Hi,

Thanks for the reply.

Its just one of the two light circuits. It covers the whole of the upstairs and for some reason, part of the downstairs extension.

Because its the upstairs - the majority of the wiring it in the loft. Recently i changed two traditional ceiling fixtures to ones that came with globe lights. (Metal ones). These don’t have terminals built in so i moved all the connections into two junction boxes in the loft. I’ve been back and checked my work and I’m happy with it - there’s no witness marks either.

I only bought the house 2-3 years ago so no idea how long the fuse wire had been in. There is a little bit of scorching on the fuse box behind where that fuse goes. - but it could have been there since a long time ago.

Tempted to switch the fuse wire out and see how it goes. Just a little worried that if there is some issue, a fire might ensue before the fuse goes. A little bit of a conundrum. Might have to get somebody out - not ideal in the current climate.

[Mr White]

Changing the fuse wire will not solve your problem, but if you want to change it go ahead, BUT change it to fuse wire that is rated at 5 Amp NO BIGGER.
Fuse wire is designed to be the weakest link and if there is a problem it will fail first. If you do use a bigger rated fuse wire, then that is when you are more than likely to get a fire, since the fuse will not fail but the cables will.

I see you did not mention if you have examined the cables in the loft, its an easy thing to do, and costs nothing, you should look for integrity of the cable, and more to the point any damage / worn or gnawed parts. I would also advise you do not lift the cables just in case you find one with severe damage.

Did you not get an EICR done before you purchased the house?

[ericmark]

Count the watts, in the main ceiling roses which act as junction boxes are rated 5 amp so in real terms that is max fuse size, so that is 1150 watts, fluorescent often takes more than bulb rating, so think of it at 1000 watt.

With LED hard to exceed, but with tungsten specially with down lights at 50W each it is very easy to exceed 20 bulbs.

Wilex have stopped making consumer units, but you can still buy the Wilex converter block to plug in MCB at £11 each, however I swapped the old fuse box for a consumer unit, think it cost around £260 for all RCBO version plus fitting of course.

[Venom Bottle]

Hi Thanks for the feedback, I'll try and answer your questions and give a little update from my side too.

I've only inspected the cables that I have messed with when I installed the two new light fittings/junction boxes. Half of the loft has raised boards which conceal some of the wires. I can dismantle it to check if nothing else shows up.

I never had an EICR done / had never heard of one at the time.

There are only 6 bulbs and one tube on the circuit. This morning I replaced the fuse wire (with 5W of course), switched on just one light - that I haven't altered in any way - and was not switched on when the flickering/fuse break happened before.
Nothing happened for one hour and then the light started flickering. I switched off the electricity and removed the fuse block. The fuse wire is intact (probably because I was quick).

So the issue seems to be unrelated to any specific bulb, bulb fitting or switch. I suppose that narrows it down to loose or damaged wiring somewhere.

The only other thing I can say is that the flickering never starts straight away, it's always after an hour or two. Does that indicate anything for anyone?

Thanks

[Mr White]

Don't want to say "I told you so" but I did say it was pointless changing the fuse wire.
I had a call a few days ago, similar to your thinking, they wanted to change the lighting MCB (Modern resettable fuse) as it would not switch on. I told them to take ALL the lamps out and switch on the MCB and put the lamps back in one at a time, when the MCB trips, that is the duff lamp. They found the duff lamp.

You need to check all the lighting cables, even if that means lifting boards, there is no "magic wand" that electricians have that instantly says where the problem is.
Logically it has to be something to do with what you have done OR disturbed since it was ok before you touched it. It may be, in doing what you did you may have pulled a cable from a JB and made it loose.

Since we can't be there, if you want, post clear in focus pictures of your JBs (with the lids off) it may help.

E.I.C.R. stands for "electrical installation condition report"

[Venom Bottle]

Hi,

I haven’t checked all the wires yet (job for this weekend) but had one more revelation.

One of the upstairs lights (the landing) isn’t on the upstairs circuit. That is to say that the fuse is currently removed but the landing light still comes on. Can only assume its on the downstairs circuit (which seems odd!). The strange thing is that it flickers. Would suggest that it’s something common to both circuits in the loft. Perhaps a common return line. I did have a bird in the loft a couple of months ago...

[ericmark]

Sounds like a borrowed neutral, in real terms it is a borrowed line (both line and neutral are called live) but this is likely a second fault and nothing to do with the flickering and tripping, although it may give some pointers.

Today two way lights use three wires between the two switches, on is always line, years ago to be able to use twin and earth instead of three core and earth, it was common practice to borrow the line from another light switch, but since it could effect hearing aids giving a mains hum on them, the practice was discontinued.

There was nothing wrong with the practice, in that it did not break the regulations, when both light switches were supplied from the same fuse, however in the 80's there was a move to using two inch spot lights so room was part heated by the lights, this lifted the power used to over 5 amp, so there was a practice to split up and down stairs lighting circuits, this should have included renewing the two way switching cable, but often it was not done, so there are 4 combinations of two way switching, two result in lights off, one lights powered from down stairs and other powered from upstairs, this has never been permitted, but often happens.

The delay has me scratching my head, would seem some thing is heating up, in the old days I would look at the fluorescent lamp, as being the only light with components in likely to be affected by heat, but with compact fluorescent lamps (CFL) and LED lamps there are components in every bulb, seems likely one bulb is causing the problem.

However putting in fuse wire is a time consuming job, I fitted these https://www.tlc-direct.co.uk/Products/WYB6.html another advert here https://www.screwfix.com/p/wylex-6a-sp- ... -mcb/48030 they are double the price of the normal MCB but if they will fit in your fuse box, it would make life a little easier, personally I swapped whole box. I also have RCD protection now, but I realise these are unusual times.

As an electrician I do have meters able to help, but they are no magic cure, and if a wire has been damaged by a rodent touching the cable could result in a nasty shock, this is where I would be wearing gloves, but it is so easy when cables are damaged to end up touching a live wire, and with no RCD protection that could be fatal, so I am not happy at the thoughts of you hunting for the fault.

Intermittent faults are the worse you can get, but I would want to eliminate bulbs, so if you remove all bulbs, does the fuse still blow, if not, can you use those bulbs in another circuit and do they work OK in another circuit.

Normally I would say don't play, get an electrician in, but I understand not normal times.

[Venom Bottle]

Hi,

I’ve been in the loft today with the main breaker off. Took my tiny torch with me.

Bit of a nightmare battling with stacks of insulation but i managed to check most cables and didn’t find any damage. In one corner though, I found some convoluted wiring that would seem to be the start of the common return line from both circuits. It looks pretty messy but no reason to doubt that it isn’t electrically sound.

I did discover a hole in the loft though which explains a bird in the loft situation I had a few months ago. Its in the same corner where the wiring is. No dampness or anything but possible the bird might have disturbed something.

Its over the bathroom and that has a clad ceiling. It looks like the wires go down between the original ceiling and cladding before probably going down a box section in the corner of the house all the way down to the ground floor. I can’t access the inside of the box section. I had a leaky toilet situation a couple of months back and its possible and likely that water might have seeped into that box section.


Obviously not keen to start destroying things if there is a more simple fix elsewhere. Regretfully, despite the pandemic - i’ll probably need to get someone in at this point.