MCBs tripping randomly?

Lewis Curle · Oct 13, 2022

Hi all,

So I’ve been having a strange fault in my house which has just appeared over the past few days. About a month ago I moved my pc set up into the sun house to make space for my second kid.

Inside the sun house/garage I have a DB that is fed of a 40A type B breaker fed by a 6mm core cable.( it’s not a SWA it’s a wierd cable with loads of little cables as a neutral) but I have insulation resistance tested that feed cable and it’s all good. On the garage DB I have a 32A ring main and a 6a lighting circuit. I have had my pc setup in there for a few weeks now and it’s been totally fine up until a few days ago when the 40A MCB is tripping of at random times. It can go off when I’m there and gaming or off through the day when I’m at work.

The circuits in the garage have all tested out totally fine and the feed cable going to the garage DB has tested out fine as well. But for some reason the 40A MCB in my board keeps tripping off. I have changed the MCB as of yesterday and it has went off again today whilst I was at work. There is nothing on in the sun house when the MCB trips. 40A is huge MCB for a house and I can’t get my head round what is causing it because there isn’t even anything above 10A going through that circuit.

Now this would be fine if it was just this circuit going off. As I could slowly work my way through what is wrong with it. But over the past 4 days as well as my garage MCB tripping I’ve had both upstairs and downstairs rings trip off, and as of today the upstairs lights have also tripped off.

They sometimes trip at the same time as the garage but they have tripped off just by themselves. I have tested all the rings and they appear to have no issues with the cables.

What makes it even more confusing is I have a split RCD DB, not once has the RCDs ever tripped on either side. But I have had the garage MCB trip on the left RCD and lights on the right RCD trip at the same time. This has also happened with the ring mains in the house.

I can’t get my head around how a single circuit on its own like the garage can appear to have an effect on another single circuit that are on separate RCDs.

Could it be an issue with the DB itself? Because as far as I can tell after testing each cable they haven’t got any issues. The MCBs can stay on for hours and just trip off.

Any help into this would be great as I’ve been pulling my hair out for days now.

Thanks, Lewis

timhoward · Oct 27, 2022

Is there anything that you use at same time as your PC that has stayed plugged in throughout? Monitor(s), Speakers, Printer, Fan, desk light.
Had you not already tried a completely different cable I'd have said the most likely cause was a small L-N fault with the split-con cable that tips the balance when under greater load. (It's very difficult to simultaneously IR test every single N conductor and every single CPC conductor, though you may have tested with clips when they are terminated.)

I don't suppose you want to add a clamp meter to your ever growing tool kit do you? They start at around £30 (screwfix)
It would be very interesting to see the reading of the outgoing live when everything is ostensibly "off" in the sun house.
And also the reading when your PC is fired up and running.

Lewis Curle · Oct 27, 2022

timhoward said:
Is there anything that you use at same time as your PC that has stayed plugged in throughout? Monitor(s), Speakers, Printer, Fan, desk light.
Had you not already tried a completely different cable I'd have said the most likely cause was a small L-N fault with the split-con cable that tips the balance when under greater load. (It's very difficult to simultaneously IR test every single N conductor and every single CPC conductor, though you may have tested with clips when they are terminated.)

I don't suppose you want to add a clamp meter to your ever growing tool kit do you? They start at around £30 (screwfix)
It would be very interesting to see the reading of the outgoing live when everything is ostensibly "off" in the sun house.
And also the reading when your PC is fired up and running.

So I’ve tried everything plugged in and off and there’s seems to be nothing in particular causing it to happen, it just seems when there’s current at all being drawn down the circuit it can cause it to happen.

I think I have an amp clamp meter already if this is what you are referring to? I will do the test tommorow sir and come back with results! Thanks for your time

timhoward · Oct 27, 2022

Lewis Curle said:
I think I have an amp clamp meter already if this is what you are referring to? I will do the test tommorow sir and come back with results! Thanks for your time

Yes, that's what I meant. Obviously take great care when doing this.

brianmoooore · Oct 27, 2022

Lewis Curle said:
Also I gamed with no issues for about 3 hours when my partner was out of the house .........

I’m very confident she isn’t turning it off just to make that clear!

Are you absolutely certain of the last statement? If it were my partner disappearing for hours on end to chase pixels around a monitor screen, then it would be highly likely that is the cause.

marconi · Oct 28, 2022

Lewis Curle said:
So I’ve tried everything plugged in and off and there’s seems to be nothing in particular causing it to happen, it just seems when there’s current at all being drawn down the circuit it can cause it to happen.

I think I have an amp clamp meter already if this is what you are referring to? I will do the test tommorow sir and come back with results! Thanks for your time

Thank you for swapping the sub main cable which was very helpful.

Are you using those surge suppressor socket strips and if so have they been left plugged in and switched on but with nothing plugged into them?

How long is the cable run in spilt concentric between house db and sunhouse db? I am wondering if there is a cable charging effect which may be tripping the 40A mcb due to voltage disturbances up stream within your house or even external to it. If this was the case there would not need to be anything connected and powered up for the 40A to trip from this cause. But it would be exacerbated by the current waveform of your IT power supply. And also items turning on and off in the home.

As an experiment, please swap the house 40A B (trips on 3 to 5 x 40 ) curve for a same make 40A C curve mcb (trips on 5 to10 x 40 ).

Lewis Curle · Oct 28, 2022

brianmoooore said:
Are you absolutely certain of the last statement? If it were my partner disappearing for hours on end to chase pixels around a monitor screen, then it would be highly likely that is the cause.

Yes I’m quite certain she isn’t turning it off intentionally

marconi · Oct 31, 2022

I have been studying the images of your home consumer unit. It seems the top left securing screw is missing. Has it fallen out and lodged behind the din rail and causing mischief or damage to busbars/cables there which might explain your problem?

I also wonder if the ends of securing screws (the ones present now and also those in place in earlier times) have damaged cables behind the plaster board. Perhaps there are fault(s) hidden out of sight here.

One would have to be very careful investigating because the split concentric is live even with the main switch off and it may be damaged and unsafe.

Lewis Curle · Oct 31, 2022

marconi said:
I have been studying the images of your home consumer unit. It seems the top left securing screw is missing. Has it fallen out and lodged behind the din rail and causing mischief or damage to busbars/cables there which might explain your problem?

I also wonder if the ends of securing screws (the ones present now and also those in place in earlier times) have damaged cables behind the plaster board. Perhaps there are fault(s) hidden out of sight here.

One would have to be very careful investigating because the split concentric is live even with the main switch off and it may be damaged and unsafe.

Hello mate sorry for the late reply, so in regards to the cable that is feeding the garage. In my mind I have already ruled that out since I ran the temporary cable to test. But I didn’t notice the lose holding screw in the board and will investigate. I will be careful so don’t worry!

I have also picked up a 32A type C MCB to swap out and will do as you’ve said in the other comment! I don’t think I’m going to have the time to do it tonight work has been manic. But I will tommorow and will get back to you with results.

As always thanks a lot for your time!

timhoward · Oct 31, 2022

The main reason I was advocating a clamp meter test is that knowing whether the B40 breaker is hovering 'on the brink' all the time is a key clue.
In the absence of this, switching everything else off and watching the electricity meter for a few minutes could be equally helpful!
Swapping to a B32 may also provide a clue.

I can't see how this isn't either
a) A permanent L-N fault that the PC occasionally pushes over the edge
b) an intermittent L-N fault caused by thermostat / timer / sensor or at a stretch the weather conditions.

If you could borrow a small UPS for your PC then it would rule it out as the inverter is essentially an isolating transformer. Or short term try running your PC via a long extension lead from e.g. the cooker switch socket plate and see what trips then.

marconi · Nov 1, 2022

The OP and I clarified a few points PM in order not to repeat much of what was reported in the open forum. Next the OP is going to check sunhouse main switch by disconnecting the N in and N out wires of the main switch and joining them together so that only the line passes through the main switch and then energise sunhouse db via House B40A.

marconi · Nov 26, 2022

Lewis Curle: have you been able to investigate this problem further to your last message?

Lewis Curle · Nov 26, 2022

marconi said:
Lewis Curle: have you been able to investigate this problem further to your last message?

Hello mate sorry for being MIA again, I’ve just had my second kid so been very busy with her!

I’ve not had much chance to look into it but what I can say which is new is, the couple of times I’ve had to time to give gaming a go a couple of things have happened.

The first was when the sun house MCB tripped a few other 32A sockets in tripped.
Another time my partner turned on the oven in the kitchen and for a split second everything in the house lost power and came back good again. Nothing tripped in the distribution board but my pc turned off and back on again very quickly.

Seems like an overload but I know for definite that there isn’t anywhere near enough amperage getting used down the sun house circuit. (I measured this with my amp meter and at full usage with my pc, heater and lights all on. It measure out at 13ishAmps). It still looks like having other things on in the house is somehow effecting the sun house circuit. Again I’ve tried just having my pc on a temp ring main but the result is still the same. But the pc worked for a week when I moved it back to the upstairs ring.

I’ve spoke to my electrical distributor and they say it sounds like a loose connection somewhere. But I’ve checked every connection in the distribution board twice over and the connections in the meter box that I can access. I’ve got them coming out next week to check the connections that im legally not allowed to touch.

But so far I’ve still can’t figure out what is happening

.

The sun house has continued to trip and what seems like random times for the past 3 weeks to and my pc isn’t even turned on at the plug right now. So yeah still very confused mate!

Thanks for getting back in touch though

marconi · Nov 27, 2022

More and more it seems to me that the fault lies with your electricity supply cabling. The oven switch on event with power outage suggests a poor connection upstream of your main consumer unit. And you have been clear that other mcbs in the house mcb trip occassionally.

I reckon the reason your 40A mcb for the sunhouse trips often is this type of cable has a higher linear capacitance than other types of cables because the N and E are wrapped around the central line conductor which makes a capacitor with large area plates with small separation between them. The surge charging current of this cable is Ic = C x dV/dt which where dV/dt is the rate of change of potential between the 'plates' of the cable capacitance. Thus high C and high dV/dt produces high Ic often of such magnitude to be interpreted by the mcb as a short circuit fault.

This charging current can be even higher than simply C x dV/dt if the cable is charged between L-N say + and - (or - and +)when the power outage happens and it is reconnected to the supply during the part of the mains cycle when L-N is - and + (or + and -) because then the supply emf and capacitor stored potential are added together.

Capacitance of underground cables - https://www.electricaleasy.com/2017/04/capacitance-of-underground-cables.html

Your gaming power supply perhaps stimulates this supply poor connection to fail during high game load because it draws a very peaky current waveform.

brianmoooore · Nov 27, 2022

Lewis Curle said:
Another time my partner turned on the oven in the kitchen and for a split second everything in the house lost power and came back good again.

This is the key phrase. If the power went off, and came back on again with nothing being done, then short of there being a local momentary power cut affecting the whole area, it can ONLY be a 'loose connection' of some kind, from, and including, the main switch, back to the branch off of the local supply network.

marconi · Nov 27, 2022

Forgive me for not saying so earlier - congratulations on your new baby daughter! What good news.

Lewis Curle · Nov 27, 2022

brianmoooore said:
This is the key phrase. If the power went off, and came back on again with nothing being done, then short of there being a local momentary power cut affecting the whole area, it can ONLY be a 'loose connection' of some kind, from, and including, the main switch, back to the branch off of the local supply network.

Thank you to both of you for replying! I am going to get the electrical supply people down this week to check the connections that I can’t check. In the meter box that is. I am very confident that there is no loose connection inside my DB.

I will let you know if anything is found.

Thanks!

LastManOnline · Nov 27, 2022

Another question is it even possible for a single circuit such as the garage one, to have an effect on other circuits in the DB.
[/QUOTE]

No.It is,nt.If the other circuits are connected in parallel rather than in series and if the fault in question is an overload.The electrical issues you have described are very unusual and unlike any I personally have ever experienced.
90% of domestic electrical faults will be become known in 90 seconds or less.The other 10% will take a little longer but generally you are leaving the premises within an hour with your callout fee in your pocket.
The one crucial factor that I find could be the solution to this intriguing problem is the presence of a conventionally trained electrician on site. (Though you are clearly very clued in)

Jamie sparky · Nov 28, 2022

This is a conundrum, I have only quickly browsed through the comments. Do you live in a rural area? And do you have a PME supply?

Jamie sparky · Nov 28, 2022

Issues like these will normally have 2 solutions. Either you have multiple faults occurring simultaneously which is unlikely or you have an unusual fault that you don't often encounter. The only possible fault I can imagine is a dodgy neutral on a PME seystem, this will not necessarily trip RCDs nor will it show on consumer side testing

Lewis Curle · Nov 28, 2022

Jamie sparky said:
This is a conundrum, I have only quickly browsed through the comments. Do you live in a rural area? And do you have a PME supply?

I live in the outskirts of a city, so not really but yes at the same time

I’m pretty sure it’s a TNCS, the earths and neutrals are connected somehow in the DB.

But there are no additional earth points in the house, like a spike for the garage or anything like that.

Jamie sparky · Nov 28, 2022

Just want to confirm some details. The issue occurs with the mcb tripping that feeds a sunhouse with your pc being the main significant load. You also have others circuits trip spontaneously in the dwelling but they are seemingly intermittent and do not occur instantly when any load is present. You have a split rcd consumer unit and the fault can occur on both sides of the DB but do not trip the RCDs

Jamie sparky · Nov 28, 2022

Also would like to congratulate you on your recent arrival and I am also a bit of a PC freak too lol

Lewis Curle · Nov 29, 2022

Jamie sparky said:
Just want to confirm some details. The issue occurs with the mcb tripping that feeds a sunhouse with your pc being the main significant load. You also have others circuits trip spontaneously in the dwelling but they are seemingly intermittent and do not occur instantly when any load is present. You have a split rcd consumer unit and the fault can occur on both sides of the DB but do not trip the RCDs

Yeah so long story short. The sun house MCB trips out when I’m on my pc which is now in the sun house. It only trips when things in the house are being used, I can game for hours when my partner isn’t cooking or watching tv (basically using anything in the house). She’s definitely not doing it purpose I want to make that clear as it’s been mentioned

It sometimes trips in 10 minutes and sometimes it takes an hour but as you can imagine as a gamer that’s basically unplayable. It worked fine in the sun house for 2 weeks then it started happening

seemed to happen with the rain and bad weather coming in.

I’ve tried basically everything you can think of. Even running a temp ring main through to the sunhouse whilst the sunhouse was totally disconnected. My pc setup alone was plugged in to that temp ring main, but the MCB was still tripping. We’ve talked about this in the forum but the sun house alone isn’t the problem.

It’s not my PC because it works totally fine in the house, it can’t stay in the house because we need the room for baby number 2. Thanks for the congratulations!

We’ve now discovered that the house sometimes loses power for a split second, when heavy loads get fired up like the oven. This has only happened a couple of times though. To my memory it’s only happened when my pc has been on in the sun house. I’ve checked all the connections in the DB and the meter box (that I can legally touch). Found nothing and the fault is still happening. It’s a maddening fault but I’m waiting on the electrical suppliers coming out to check the connections I can’t touch to see if any are loose.

But I’ve had a lot of great people trying to help out in the forum and I’m sure we’ll figure it out eventually. But any advice you can offer also would be welcome!

Also nice setup I’m jealous that you can use it with no issues

Lewis Curle · Nov 29, 2022

Jamie sparky said:
Just want to confirm some details. The issue occurs with the mcb tripping that feeds a sunhouse with your pc being the main significant load. You also have others circuits trip spontaneously in the dwelling but they are seemingly intermittent and do not occur instantly when any load is present. You have a split rcd consumer unit and the fault can occur on both sides of the DB but do not trip the RCDs

So other MCBs have tripped but it’s mainly the sun house one. The RCDs have never tripped. MCBs on either RCDs have tripped yes and I’ve tried swapping the sun house one so it’s been on both RCDs. The sun house MCB has tripped when nothing in the sun house has been on. But mainly happens under load

Lewis Curle · Dec 7, 2022

marconi said:
Forgive me for not saying so earlier - congratulations on your new baby daughter! What good news.

So I’ve had chance to get busy on investigating this more.

1. I have been in every connection that is involved with the mains coming into the house. The connection coming out of the 100A fuse has been checked, followed by the meter, then the isolation switch. All of this is in the white cut out on the side of my house. Nothing obvious I got a couple of quarter turns on some but either way the fault happened after checking that.

2. I have fitted a separate RCD one way DB next to my house DB. This is for the sole purpose of the garage being on its own. I double up the mains in the top half of the main isolator in the house DB and used that to feed the new garage DB.

So the garage/sunhouse is a stand alone thing as far as the wiring goes the only thing is has in common with my house is, the cable from the cutout (meter box) to the DB.

I fitted it at around 1pm today and I’ve had my pc on since that time. The 63A RCD main switch has tripped as of about one hour ago. Didn’t notice anything he effected in the house. So this is the first time that an RCD has tripped during the whole of this fault.

I wasn’t gaming in my pc I just left it on watching a YouTube video. We’ve had a busy day in my house with all sorts of appliances being used. Can’t tell when it tripped specifically but it was around the 6-7pm mark.

So it’s ran for 5-6 hours. I didn’t find any loose connections inside the DB, I had to terminate it all again to make this new DB fit. So pretty confident that is no longer the issue.

The only thing I can think of to do now is to run a totally new mains cable from the meter box to my garage which will be Atleast £100 of cable. Just to totally separate it from my house. Would rather not but it’s the only thing I can think of to do now

Hope all of that made sense, I’m still very puzzled and feel like I’m never gonna sort it.

Thanks for reading