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

 

[James]

If there is beer involved, count me in!
maybe even some cheesecake?

 

[timhoward]

Tim can bring his self calibrated Megger and I'll bring some southern comfort.

If you start drinking it in Cornwall you'd better get your creative theories down on paper around the Bristol area, leaving it any later could be risky!

If there is beer involved, count me in!
maybe even some cheesecake?

If I'm allowed to take over navigation then I propose a detour at J40 of the M6 - Venison Burgers at the Mill in Mungrisdale, a couple of pints of Old Peculiar at the Dog and Gun in Keswick, en-route back to the coach we pass the Castle and the Oddfellows Arms, then take the scenic route via A595 to Carlisle with a few nice village pubs on the way.
We just need a designated safe-isolation person....

 

[marconi]

Lewis Curle: One of the photos you sent me on my phone has the isolator switch-fuse and above it a grey plastic box. Could you open up the grey box and take a photo please and post it?

 

[marconi]

To save me re-reading everything you have written - once there has been an mcb trip event do you ever have further trips the same day? Or is the earliest next event sometime during the following day? Does it only happen once the street lights start to come on or are on?

What kind of street lights in your road?

Is the consumer unit in a hot room or cupboard which gets hot ie airing cupboard, above a cooker or built in oven

When the mcbs trip could you feel them shortly afterwards to see if they are getting warm/hot through prolonged overcurrent due to overloading of the circuit.

In regard to overloading, could you list what is connected to each final ring circuit and post so I/we can consider the likelihood of overloading them?

Have any plug fuses ruptured during the period of this problem?

What type of space and water heating do you normally use?

How many fridges and freezers are plugged in and switched on?

Do you have a microwave? Is it a combination microwave?

An air fryer?

What type of oven do you have? Radiant rings, IR halogen, induction?

Would anyone in your street be using an electric arc welder, air compressor or car lift powered by the mains?

Are their many EV chargers in use in your road?

A bit left field and please don't be offended but would you suspect anyone in your street has a cannabis farm with its heavy heating, lighting and fan load?

Are their many solar panels to be seen along your road?

How far away in metres is the supply transformer for your overhead lines?

 

[marconi]

The query about cu location and ambient temperature and loading on your final circuits is a line of thinking to do with derating the current handling capacity of mcbs dependent on temperature inside the consumer unit and grouping of mcbs side by side. See:

http://www.hagerelectro.com.au/files/download/0/32587_1/0/TECHINFO_MCBS.PDF

Note the power dissipation per pole table. Power dissipation per pole also applies to RCDs and the Main Switch. I just wonder if the way you consume electricity is such you have episodes of several high amperage and reasonably long duration current flows which are causing your consumer unit interior to warm up so much that derating of mcbs for temp and grouping becomes significant, thence they eventually trip on overcurrent overload.

A simple way to test this would be to run the consumer unit with the cover off for a week if of course this can be done safely.

 

[davesparks]

A simple way to test this would be to run the consumer unit with the cover off for a week if of course this can be done safely.

No that can't be done safely, and I'm surprised to see anyone suggesting that a CU could be left without its cover fitted for a week.

Moving the MCB to the end of the row would be a far safer way to prove this.


However I doubt that the cause of this problem is electrical. The MCB apparently trips with no outgoing conductor connected. I have seen protective devices trip with no outgoing conductor connected a few times and the causes have been faulty/damaged devices, vibration or mechanical impact.


In situations like this it is easy to get distracted by very interesting, vaguely plausible, technical explanations for the problem when in reality an over-tightened terminal screw may have caused the mechanism to twist slightly.

 

[marconi]

No that can't be done safely, and I'm surprised to see anyone suggesting that a CU could be left without its cover fitted for a week.

Moving the MCB to the end of the row would be a far safer way to prove this.


However I doubt that the cause of this problem is electrical. The MCB apparently trips with no outgoing conductor connected. I have seen protective devices trip with no outgoing conductor connected a few times and the causes have been faulty/damaged devices, vibration or mechanical impact.


In situations like this it is easy to get distracted by very interesting, vaguely plausible, technical explanations for the problem when in reality an over-tightened terminal screw may have caused the mechanism to twist slightly.

Sensible advice. Forgive me for I will not say any more in response.

 

[mainline]

No that can't be done safely, and I'm surprised to see anyone suggesting that a CU could be left without its cover fitted for a week.

Marconi did say IF it can be done safely.
That would depend on the circumstances.
The O/p is an electrician, and he maybe the only one with access to it.
It also could be in a lockable cupboard.

In situations like this it is easy to get distracted by very interesting, vaguely plausible, technical explanations for the problem when in reality an over-tightened terminal screw may have caused the mechanism to twist slightly.

The MCB has been replaced with a new one.


Maybe the Op lives in a train station or on a building site.

 

[LastManOnline]

Sensible advice. Forgive me for I will not say any more in response.

I am not inclined to add any more comment to this thread as in my first post (post 67).Having hastily read the thread,I suggested that the issue likely lay with the OP who I assumed (wrongly) was an amateur.I should have read the previous 66 posts more thoroughly.I want to offer my sincere apologies to the OP.
Now that I have read the thread thoroughly ,my initial impression of this being an unconventional and unique problem remains the same.
For that reason I do hope that "Marconi" stays on the case as I believe he has a unique skill set that facillitates "thinking outside the box"
in ways that most conventionally trained sparks dont.It would be marvellous to find out the cause of this problem.

 

[mainline]

Wondering if the O/P has tried it with the lid off, and then put a decent load on it.

 

[OnlQQker]

( it’s not a SWA it’s a wierd cable with loads of little cables as a neutral)

Got a pic of this cable Op?

 

[brianmoooore]

Don't recall the post quoted above, but that will be split concentric cable. The insulation on the individual neutrals in this cable is thin and fragile, making it more susceptible to neutral - earth faults than other cables.

 

[OnlQQker]

Don't recall the post quoted above, but that will be split concentric cable. The insulation on the individual neutrals in this cable is thin and fragile, making it more susceptible to neutral - earth faults than other cables.

Op's first post (page 1)

I will look up spilt concentric cable as never seen it before!

 

[OnlQQker]

I'm guessing it's this stuff but 6mm

25mm² Split Concentric Cable (Cut Length Sold By The Mtr)

Single-phase split concentric cable

www.cef.co.uk

So does that bundle of neutrals connect into the consumer unit? cuz on the pics the OP posted I couldn't see them?

 

[davesparks]

I'm guessing it's this stuff but 6mm

25mm² Split Concentric Cable (Cut Length Sold By The Mtr)

Single-phase split concentric cable

www.cef.co.uk

So does that bundle of neutrals connect into the consumer unit? cuz on the pics the OP posted I couldn't see them?

Yes that's the cable, if you look at the picture in post #94 you can see the bundle of neutrals connected into the Neutral bar and also another one in the top of the main switch.
Presumably this board is fed via a split-con submain.

 

[davesparks]

Don't recall the post quoted above, but that will be split concentric cable. The insulation on the individual neutrals in this cable is thin and fragile, making it more susceptible to neutral - earth faults than other cables.

Interesting, is it a particular brand of cable that you've had this problem with?

 

[brianmoooore]

Interesting, is it a particular brand of cable that you've had this problem with?

No particular brand. It's a characteristic of the cable - the insulation on the neutrals is thin and easily damaged, especially inside the CU.
I normally twist the neutrals tightly together and wrap the bunch with a few layers of PVC tape.