help fault finding a tripped fuse

aeromech3 · Post by **aeromech3** » Mon Jan 16, 2023 2:03 pm

If an FCU has no switch only a fuse it is single pole protection.
If an FCU also has a switch it might be single or double pole.
Yes please read up; a single pole when off only isolates the positive supply (if wired correctly) and with a fault in the unit it is protecting the RCD still might trip.
A dual pole FCU must have a switch and when off isolates the positive and negative to the downstream units.
But in all this you first stated that the C.U. fuse (MCB) tripped and when reset the RCD trips, now the boiler FCU fuse is 3 amp (or is it an internal fuse) and only looking at the positive load side of your boiler which if not blown (is it?) and should protect the remainder of the house wiring without relying on the C,U. MCB which is likely rated much much higher.
In all this are we to understand that you cannot reset the C.U. MCB without the RCD tripping and therefore you are without the use of your boiler??

Mr Red · Post by **Mr Red** » Mon Jan 16, 2023 3:26 pm

aeromech3 wrote: ↑Mon Jan 16, 2023 2:03 pm If an FCU has no switch only a fuse it is single pole protection.
If an FCU also has a switch it might be single or double pole.
Yes please read up; a single pole when off only isolates the positive supply (if wired correctly) and with a fault in the unit it is protecting the RCD still might trip.
A dual pole FCU must have a switch and when off isolates the positive and negative to the downstream units.
But in all this you first stated that the C.U. fuse (MCB) tripped and when reset the RCD trips, now the boiler FCU fuse is 3 amp (or is it an internal fuse) and only looking at the positive load side of your boiler which if not blown (is it?) and should protect the remainder of the house wiring without relying on the C,U. MCB which is likely rated much much higher.
In all this are we to understand that you cannot reset the C.U. MCB without the RCD tripping and therefore you are without the use of your boiler??

It's a FCU with a switch and a 3A fuse (not blown). Whether it is dual or single pole is unknown as I'm not there to check at the moment but I can see from a photo that there are no markings on the FCU.
Correct, I cannot reset the CU MCB without the RCD tripping and am without operational sockets in the kitchen (including boiler), office and hallway

ericmark · Post by **ericmark** » Mon Jan 16, 2023 9:19 pm

The RCD measures current in and current out, if within set amount all is OK.

Main reason to be diffrent is leakage line to earth, but neutral to earth can also happen, and it is hard to find what is leaking as the more current that is used the greater the volt diffrence between earth and neutral.

We have three tools, the RCD tester, the insulation tester which uses DC and the clamp on ammeter which clearly does not work once tripped.

Because the neutral is often not switched best to unplug, but because not switched testing neutral to earth can find faults otherwise missed.

Remember neutral and earth are connected by the DNO so testing neutral to earth needs doing with main isolator off.

Neelix · Post by **Neelix** » Tue Jan 17, 2023 7:22 am

aeromech3 wrote: ↑Mon Jan 16, 2023 2:03 pm If an FCU has no switch only a fuse it is single pole protection.
If an FCU also has a switch it might be single or double pole.
Yes please read up; a single pole when off only isolates the positive supply (if wired correctly) and with a fault in the unit it is protecting the RCD still might trip.
A dual pole FCU must have a switch and when off isolates the positive and negative to the downstream units.
But in all this you first stated that the C.U. fuse (MCB) tripped and when reset the RCD trips, now the boiler FCU fuse is 3 amp (or is it an internal fuse) and only looking at the positive load side of your boiler which if not blown (is it?) and should protect the remainder of the house wiring without relying on the C,U. MCB which is likely rated much much higher.
In all this are we to understand that you cannot reset the C.U. MCB without the RCD tripping and therefore you are without the use of your boiler??

Simply not accurate. There are versions of fcus with switches that a are still single pole

aeromech3 · Post by **aeromech3** » Tue Jan 17, 2023 3:08 pm

Sorry Neelix, I thought that was covered by my statement:-

aeromech3 wrote: ↑Mon Jan 16, 2023 2:03 pm If an FCU also has a switch it might be single or double pole.

OnlyMe · Post by **OnlyMe** » Tue Jan 17, 2023 7:17 pm

Neelix wrote: ↑Sun Jan 15, 2023 4:20 pm My money is on a boiler problem.

Did you isolate the boiler? And beware lots of fused spurs are single pole so that type won’t isolate the neutral and the live

If the FCU is switched then BS 1363-4 says it must switch both the live and the neutral.

Neelix · Post by **Neelix** » Wed Jan 18, 2023 9:35 am

OnlyMe wrote: ↑Tue Jan 17, 2023 7:17 pm If the FCU is switched then BS 1363-4 says it must switch both the live and the neutral.

Then ask yourself when did BS 1363 - 4 come into affect? and then think about all the installations that predate this.

OnlyMe · Post by **OnlyMe** » Wed Jan 18, 2023 7:27 pm

Neelix wrote: ↑Wed Jan 18, 2023 9:35 am
OnlyMe wrote: ↑Tue Jan 17, 2023 7:17 pm If the FCU is switched then BS 1363-4 says it must switch both the live and the neutral.
Then ask yourself when did BS 1363 - 4 come into affect? and then think about all the installations that predate this.

It was late 40's early 50's.

Neelix · Post by **Neelix** » Wed Jan 18, 2023 10:03 pm

OnlyMe wrote: ↑Wed Jan 18, 2023 7:27 pm
It was late 40's early 50's.

Not sure I agree with that

OnlyMe · Post by **OnlyMe** » Thu Jan 19, 2023 7:44 pm

Neelix wrote: ↑Wed Jan 18, 2023 10:03 pm
OnlyMe wrote: ↑Wed Jan 18, 2023 7:27 pm
It was late 40's early 50's.
Not sure I agree with that

Finding actual dates is difficult but of interest to me. I managed to buy a 1970 copy of the regs last year - the first metric copy. I worked on a 1960's house not long ago and the single sockets were obviously supplied with both metric and imperial screws to fasten the socket to the back box. No cpc on the the lighting but the back boxes were metric thread. The imperial screws were still on the back of the socket and metric ones were missing - it was labelled that there were 4 screws attached to the socket). ISTR it was a Volex socket.

Lew · Post by **Lew** » Fri Jan 20, 2023 3:06 pm

Any leaks recently? Roof perhaps in the weather we had this month? Moisture can certainly trip your RCD.

Mr Red · Post by **Mr Red** » Sat Jan 21, 2023 10:37 am

Thank you all for your replies, it has been very educational for me - I've learned a great deal.

I looked at the price of an insulation tester and the very cold Mrs Red then called an electrician!

I heard what he did 2nd hand as I couldn't be there to observe but I believe he tested the dishwasher, washing machine and combi boiler with an insulation tester (all OK) then opened every socket and FCU on the ring (also all OKish ie some DIY bodges, loose wires etc repaired but not the cause of my problem).

So I could have some heating and hot water he wired a plug to the combi boiler and ran it to a working socket through an extension cord.

So my problem appears to be hidden somewhere in the ring. I do wonder what will happen next, i've asked the electrician for a plan and a quote but haven't had a reply yet, lucky it's not urgent. I think I would insulation test between each socket on the ring, would that be correct or is there another way?

It may be relevant but there has been a huge amount of rain recently and my driveway is flooded, I also had a frozen pipe develop a leak. Another house I lived in used to get mice in whenever it rained hard so now my original thought that a mouse has chewed through a wire is again my favourite for the root cause.

Neelix · Post by **Neelix** » Sat Jan 21, 2023 10:43 am

Mr Red wrote: ↑Sat Jan 21, 2023 10:37 am Thank you all for your replies, it has been very educational for me - I've learned a great deal.

I looked at the price of an insulation tester and the very cold Mrs Red then called an electrician!

I heard what he did 2nd hand as I couldn't be there to observe but I believe he tested the dishwasher, washing machine and combi boiler with an insulation tester (all OK) then opened every socket and FCU on the ring (also all OKish ie some DIY bodges, loose wires etc repaired but not the cause of my problem).

So I could have some heating and hot water he wired a plug to the combi boiler and ran it to a working socket through an extension cord.

So my problem appears to be hidden somewhere in the ring. I do wonder what will happen next, i've asked the electrician for a plan and a quote but haven't had a reply yet, lucky it's not urgent. I think I would insulation test between each socket on the ring, would that be correct or is there another way?

It may be relevant but there has been a huge amount of rain recently and my driveway is flooded, I also had a frozen pipe develop a leak. Another house I lived in used to get mice in whenever it rained hard so now my original thought that a mouse has chewed through a wire is again my favourite for the root cause.

I would ask the spark what he would do next.

It probably means splitting the circuit and doing more dead tests to narrow down the fault.

As for how long it would take - how long is a piece of string.