MCBs tripping randomly?

[Lewis Curle]

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

 

[UNG]

overloading is a common reason .

I doubt you have read all of the posts in this thread with a comment like that

 

[marconi]

Another simple test after the one I mentioned before this post is we could disconnect and isolate all the neutral conductors at both ends and then energise the SCC with only its line and cpc connected at both ends and wait to see if the sunhouse mcb trips. The sunhouse db mains switch is off for this test. It would have to be left this way for a few days to see if we have removed the N path and thereby stopped the fault current operating the sunhouse mcb. Swap the sunhouse 40A mcb for a 3A B preferably or 6A B maximum for this test as we want a sensitive response.

After a few days reconnect Ns at both ends and still leave sunhouse mains switch off. Keep the 3 or 6 A mcb in place and leave for a few more days to see what happens.

A feature of this cable fault - (arcing between L-N?) - is there may indeed be current flowing between L-N intermittently and regularly but a 40A mcb requires a sizeable current to trip it so most of the time the fault is tolerated. The 3 or 6 A mcbs should respond quicker.

 

[Lewis Curle]

Another simple test after the one I mentioned before this post is we could disconnect and isolate all the neutral conductors at both ends and then energise the SCC with only its line and cpc connected at both ends and wait to see if the sunhouse mcb trips. The sunhouse db mains switch is off for this test. It would have to be left this way for a few days to see if we have removed the N path and thereby stopped the fault current operating the sunhouse mcb. Swap the sunhouse 40A mcb for a 3A B preferably or 6A B maximum for this test as we want a sensitive response.

After a few days reconnect Ns at both ends and still leave sunhouse mains switch off. Keep the 3 or 6 A mcb in place and leave for a few more days to see what happens.

A feature of this cable fault - (arcing between L-N?) - is there may indeed be current flowing between L-N intermittently and regularly but a 40A mcb requires a sizeable current to trip it so most of the time the fault is tolerated. The 3 or 6 A mcbs should respond quicker.

Hello mate I will do the neutral resistance test tonight that you described in the other post! I will also look into getting some smaller MCBs to try what you have mentioned in this one.

I’m burnt out with trying to fix it but I will come back with my findings over the next couple of days!

Thanks again for taking the time to try and help

 

[Lewis Curle]

Another simple test after the one I mentioned before this post is we could disconnect and isolate all the neutral conductors at both ends and then energise the SCC with only its line and cpc connected at both ends and wait to see if the sunhouse mcb trips. The sunhouse db mains switch is off for this test. It would have to be left this way for a few days to see if we have removed the N path and thereby stopped the fault current operating the sunhouse mcb. Swap the sunhouse 40A mcb for a 3A B preferably or 6A B maximum for this test as we want a sensitive response.

After a few days reconnect Ns at both ends and still leave sunhouse mains switch off. Keep the 3 or 6 A mcb in place and leave for a few more days to see what happens.

A feature of this cable fault - (arcing between L-N?) - is there may indeed be current flowing between L-N intermittently and regularly but a 40A mcb requires a sizeable current to trip it so most of the time the fault is tolerated. The 3 or 6 A mcbs should respond quicker.

Hello sir, sorry for disappearing for a week I needed the break from trying to solve it. I have been back at it over the past few days and this is what I’ve discovered.

So I changed my PSU in the pc for a 1000w one, I did that because when my brother came round with his pc it worked with no issues for about 2 and a half hours. After the change in my pc it lasted about half an hour before it tripped (again the 40A MCB in the house).

So I sourced some 4mm tri rated SWA and ran it temporarily from my house to the garage DB to see if the cable was causing the issue. I was on my pc for about an hour just watching videos, then I jumped onto total war Warhammer 3 with my brother and before we even got into the game the trip occurred.

So basically the cable can’t be the issue anymore. Which is good and bad because it would have a nightmare to change anyway but now I’m stuck again. The issue definitely happens more when I am gaming at my pc. But the trip has happened when my pc wasn’t even plugged in. So I’m finding it hard to believe that my pc is causing the issue when it happens without it being involved. It just seems that when I’m gaming and using more power in the sun house it trips a lot more.

So from the top I have now changed
1. The house DBs MCB
2. The cable running from the house to the garage
3. My PSU
4. The MCB in the garage DB

5. I have checked the wiring twice in the sun house and I can find no issues when doing IR test and continuity from end to ends. The sockets have all had the connection made off again to check for loose connections.
6. I have tightened up all the Connections inside each DB and again can find no issues.

What I am now considering doing now is changing the garage DB for a brand new one to rule out the main switch in there being the issue. Then if that doesn’t work I’m honestly considering just rewiring the whole of the sun house just to say it’s been done properly. To be honest whoever done it hasn’t done a great job with cable routes and it looks very rushed (when you look at it all above in the rafters).

I am also considering whether the RCD on the house DB can cause any wierd issues like this.

I just can’t get my head around why the 40A MCB is tripping and not the 16A.

Again thanks for taking the time to read and I would be very grateful for any more of your time.

Cheers, Lewis

 

[Lewis Curle]

Hello sir, sorry for disappearing for a week I needed the break from trying to solve it. I have been back at it over the past few days and this is what I’ve discovered.

So I changed my PSU in the pc for a 1000w one, I did that because when my brother came round with his pc it worked with no issues for about 2 and a half hours. After the change in my pc it lasted about half an hour before it tripped (again the 40A MCB in the house).

So I sourced some 4mm tri rated SWA and ran it temporarily from my house to the garage DB to see if the cable was causing the issue. I was on my pc for about an hour just watching videos, then I jumped onto total war Warhammer 3 with my brother and before we even got into the game the trip occurred.

So basically the cable can’t be the issue anymore. Which is good and bad because it would have a nightmare to change anyway but now I’m stuck again. The issue definitely happens more when I am gaming at my pc. But the trip has happened when my pc wasn’t even plugged in. So I’m finding it hard to believe that my pc is causing the issue when it happens without it being involved. It just seems that when I’m gaming and using more power in the sun house it trips a lot more.

So from the top I have now changed
1. The house DBs MCB
2. The cable running from the house to the garage
3. My PSU
4. The MCB in the garage DB

5. I have checked the wiring twice in the sun house and I can find no issues when doing IR test and continuity from end to ends. The sockets have all had the connection made off again to check for loose connections.
6. I have tightened up all the Connections inside each DB and again can find no issues.

What I am now considering doing now is changing the garage DB for a brand new one to rule out the main switch in there being the issue. Then if that doesn’t work I’m honestly considering just rewiring the whole of the sun house just to say it’s been done properly. To be honest whoever done it hasn’t done a great job with cable routes and it looks very rushed (when you look at it all above in the rafters).

I am also considering whether the RCD on the house DB can cause any wierd issues like this.

I just can’t get my head around why the 40A MCB is tripping and not the 16A.

Again thanks for taking the time to read and I would be very grateful for any more of your time.

Cheers, Lewis

Also I gamed with no issues for about 3 hours when my partner was out of the house and nothing inside was being used. But after about 10 minutes of her getting home and turning stuff on in the house. It tripped and continued to do so two more times until I stopped turning it back on.

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

 

[timhoward]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.