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

 

[brianmoooore]

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.

How certain are you that this happened?

 

[Lewis Curle]

How certain are you that this happened?

It 100% happened! I was in the kitchen when it happened

 

[brianmoooore]

In that case, there was either a coincidental short power cut, affecting your local area, or there is a poor connection in your electricity supply between where it connects to the trunk main and the meter in your house.

 

[Lewis Curle]

In that case, there was either a coincidental short power cut, affecting your local area, or there is a poor connection in your electricity supply between where it connects to the trunk main and the meter in your house.

Trunk main?

 

[brianmoooore]

The main underground cable along the road, or the pole outside where other house supplies branch off, if overhead wires.

 

[yorkieone]

When you say passed an IR test do you mean they had a perfect >999megohm or a just scraped through at 1megohm?



A 40A MCB and a 16A MCB will not discriminate on a fault, it is pretty much pot luck which one will trip first.

1 meg ohm at 500v is ok we would like infinity but it's not an insulation fault

 

[Lewis Curle]

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

Long time no speak everyone, so after a busy few months with the new baby I’m back to trying to figure this fault out. I’ve not been standing idle, I have tried multiple things to try and figure out this bizarre issue.

Short story of it all is I have changed the whole consumer unit in my house for a brand new Wylex one. It’s a split RCD board. First thing I did after changing it was of course test the thing. The issue is still exactly the same after about half an hour of being in the sun house gaming away the sun house MCB tripped along with the upstairs ring and the smoke alarms. Smoke alarms have never tripped before but the upstairs ring has. This was the only time since the new board that those other MCBs have tripped.

So naturally I started to break down the sun house in stages again, I disconnected everything out of the garage DB and the MCB still tripped sometime when I was at work the next day. So i then disconnected the cable feeding that garage DB and taped the ends up and put them safely out of the way of anything. The MCB still tripped. So then I disconnected the the cable at the house db end. And some how the 40A MCB with nothing in it has tripped three times in the past three days. So the sun house has nothing to do with this strange fault in my eyes.

Now I can’t workout how that MCB is still tripping with nothing connected into the thing. The only thing that hasn’t been changed now is the cable that is feeding my DB from the meter box. Nothing has showed up on testing that cable, it has been properly tested and all the results came back perfectly. So now I don’t really have a clue on what to try, other than changing that cable which would be a nightmare to change.

As an electrician I can’t make any sense of it.

I’ve has the grid people out and they have checked the cable into my house and inside the meter box and found nothing wrong.

Now thinking back to the times that it trips and the times that it doesn’t, things inside the house are being used. It doesn’t seem to matter what is being used but it trips a lot more often when my partner is cooking or watching tv downstairs.

I’ve not managed to keep it on for more than an hour when she is kicking about in the house using things. I also would like to say that it is not being intentionally turned off, someone asked that once and I just want to make it clear now!

So thanks for reading, I will attach some photos of the DB and the meter box.

Lewis

 

[brianmoooore]

What is tripping? MCB or RCD?
It's a pity you didn't fit a RCBO board.

 

[Lewis Curle]

I can’t upload the photos they are to big

 

[Lewis Curle]

What is tripping? MCB or RCD?
It's a pity you didn't fit a RCBO board.

The 40A MCB that was feeding the sun house is the main on that’s tripping. Although it’s tripping without the cable in it right now.

 

[DPG]

The 40A MCB that was feeding the sun house is the main on that’s tripping. Although it’s tripping without the cable in it right now.

The 40A MCB is tripping without anything connected to it??

 

[Lewis Curle]

The 40A MCB is tripping without anything connected to it??

Yeah consistently for the past three days

 

[DPG]

Yeah consistently for the past three days

Literally no cable in the output terminal?

 

[Lewis Curle]

Literally no cable in the output terminal?

Literally it’s connected to the busbar and they is nothing coming out of the top side. It’s tripped three times in the past three days. I can’t make any sense of it. Bear in mind it’s a brand new MCB as well.

 

[DPG]

Literally it’s connected to the busbar and they is nothing coming out of the top side. It’s tripped three times in the past three days. I can’t make any sense of it. Bear in mind it’s a brand new MCB as well.

Could you post a photo of the consumer unit with the front off