Overcurrent fault on socket ring

loply · 2 Mar 2017

Hi folks,

I'm experiencing an intermittent fault in the 13a socket ring in my garage.

From time to time, when you're using any appliance, it will trip the B30 breaker in the garage's consumer unit. Occasionally, say 1 time in 3, it will also trip the upstream B40 breaker in the house's consumer unit. It never trips the RCDs.

If you reset the circuit breaker it will be fine for another hour or more... sometimes several hours... but it never trips again straight away.

I've tried unplugging every single item in the garage, and only using one thing at a time, and the problem still occurs so that has eliminated appliances as being the issue.

The problem only occurs when you're using something - so if I leave the garage for days on end it will not trip by itself, but if I go in and plug in a 750w heater or a 500w power tool it will likely trip once every hour or two.

Could do with a steer on how to approach finding the fault.

My best guess is it must be damaged wire/insulation somewhere which is allowing a short circuit when the wire heats up? But the wiring is only 3 years old and fully trunked/protected all round. I've checked for any obvious signs of damage but can't find any.

I've inspected inside the CU and no signs of problems in there, and I've checked a few of the sockets which I thought may have got dust in them, but they were totally clean and no signs of problems.

Would appreciate any advice.

Cheers,
Rich

bernardgreen · 2 Mar 2017

loply said:
Could do with a steer on how to approach finding the fault.

Keep a record on what you are doing at the moment when the MCB trips.

Are they definately MCB and not RCBO which are combined MCB and RCD ( do they have a "test" button ? )

Power tools can take an inrush current ( as much as ten times normal running current )when the motor starts which can trip an MCB

loply said:
which is allowing a short circuit when the wire heats up?

Very unlikely. More likely a cable is trapped somewhere and is being squashed when something happens. Like a cable under a floor board that only faulted when someone heavy was standing on that board

loply · 2 Mar 2017

They're definitely not RCBOs, they are RCD protected but the RCD is separate and doesn't trip.

The problem never occurs when a tool first fires up... two recent examples, I was just using the vacuum cleaner (nothing else plugged in) - a few minutes in it trips. Then last night I was just standing doing something, a small electric heater (750w I think) had been running all night, then suddenly it trips.

Concrete floors, all the cables are in round tubes clipped to the walls, with metal wall mounted socket boxes.

As I say, it's weird because it ONLY trips if you're drawing current... it never trips by itself when the garage isn't in use...

ban-all-sheds · 2 Mar 2017

loply said:
if I leave the garage for days on end it will not trip by itself, but if I go in and plug in a 750w heater or a 500w power tool it will likely trip once every hour or two.

It's not clear what the breaker is in your garage. You say it's a B30, but the preferred value for a BS EN 60898 MCB would be 32A.

The non-tripping current for a B breaker is 1.13In, so a B32 will pass 36.16A indefinitely. If there were a fault which was so close to the limit that another 2-3A would cause a trip then somewhere there's over 8kW being dissipated. That's one hell of a lot of heat. It also implies that every 24 hours you're clocking up nearly 200kWh of use, £8,000 - £9,000 per year. Which I'm sure you would have noticed by now.

What happens if you have an appliance in use in the garage but nobody in there?

Naz1977 · 2 Mar 2017

The MCB and RCD do 2 different things.

MCB trips on current overload whereas RCD is to do with your earth leakage. If you had a trapped cable and lets say the cable was damaged you wouldn't be tripping the MCB but the RCD more than likely.

There are 3 types of MCBs, type B,C and D and they are all to do with how much overload they take before they trip.

Type B is the smallest,whereas it takes 3-5 times , whereas type D is the biggest with 10-20 times the rated current on startup. Ofcourse these are startup currents and the MCB would still trip if you for example had a failure.

From what I read, it seems that the problem happens when you use tools/power tools in your garage. The problem with tools/power tools is the inrush current from the device that causes the spike in current drawn from your MCB. Hence why sometimes you need to use Type C instead of Type B.

What sort of power tools do you have in your garage? Do you have a compressor, drill and ... ?

If you do have some proper power tools, then I recommend changing your MCB to a type 'C' as I assume it is a type 'B' at the moment. It will give you the extra capacity for that moment when you turn on the power tool.
Do you have radial sockets in the garage or ring main?

ban-all-sheds · 2 Mar 2017

Naz1977 said:
I recommend changing your MCB to a type 'C' as I assume it is a type 'B' at the moment.

Oh.

When was it, exactly, that you visited Ioply and took a fault loop impedance measurement?

Ioply - seriously - take no notice of any advice from Naz - he is blundering about trying to do wiring work in his own home, something which he is incompetent to do. He now seems to think that ignorance and incompetence should also not be a bar to giving advice here.

loply · 2 Mar 2017

Hi guys,

Sorry - when I said B30 breaker earlier, I meant B32...

The overcurrent fault is definitely not happening due to having too much power being used by an appliance, as I say it often occurs when I have just one low power device running. It's nothing to do with inrush current because it doesn't happen on startup.

I've checked the electric meter and though I didn't have a way to quantify it, it certainly looked like it was rotating slowly/normally. I don't think there's a big load constantly being drawn therefore.

I could only imagine there's some kind of intermittent short circuit therefore? Just no idea where or how to find it...

ban-all-sheds · 2 Mar 2017

What happens if you have an appliance in use in the garage but nobody in there?

loply · 2 Mar 2017

Well the other day I was stood outside the garage (albeit with the door open), and a fan heater was running inside, and it tripped...

Pretty positive it's nothing to do with standing on something etc, as I say it's concrete floors and ALL the wires are surface mounted in plastic tubing. There's no way you could interfere with anything.

Naz1977 · 2 Mar 2017

Ok, so you need to think about when it started to do it and what was changed around the time?

How many devices do you have plugged in your garage? What are they? What is normally on in the garage and what do you turn on.

You say the cabling was done 3 yrs ago? What make MCB units do you have? It could be that the MCB is faulty or has started to fail. If a good brand, it shouldn't really happen. But if a cheap brand, then it could be a fault.

endecotp · 2 Mar 2017

I recall another thread where the adjacent MCB was getting hot and causing its neighbour to trip at below its normal rated current. That specific issue doesn't sound very likely in this case as you are seeing two different MCBs in two different enclosures trip, but it illustrates the sort of obscure problems that can occur.

If I were you, I'd disconnect as much of the wiring as you can, just leaving maybe one socket. Use extension leads to connect appliances if necessary (with obvious safety precautions). That will help narrow down where the problem is.

You might like to post some photos; maybe someone will spot something unusual.

DO NOT swap them for type C breakers. If you have multiple MCBs of the same rating you could try swapping them around.

loply · 2 Mar 2017

There is an adjacent 32 amp MCB which powers my air compressor, but it is never running so I doubt that's hot. I will check it with a temperature gun out of interest though...

Okay, so I guess I will disconnect the ring and essentially turn it into a radial, initially with just one socket, then gradually reconnect the sockets one by one until the fault starts occurring?

If it is a damaged wire or suchlike that should presumably narrow it down...

Presumably I will have halved the current capacity of the circuit's wiring whilst it's operating as a radial in the meantime though...

I don't know if this tells us anything, but if I have a device with a heat coil - like a fan heater or a paint stripper gun - plugged in at the time the fault occurs, it makes a bit of a loud pop and a brief flash from the fan/gun. Gives me a fright. Don't know if that's just normal because the power was interrupted or if it indicates something...

Naz1977 · 2 Mar 2017

Sounds like you have some proper gear in your garage. At least you use your garage the way it should be, rather than for storing bikes and washing machine and ...

To me, it sounds like you have too many power hungry devices in the garage and are overloading the circuit.

loply · 2 Mar 2017

As I said above, it trips even if you have just one 750w heater plugged in. Or just a single low power tool. It's not overloaded in that sense.

ban-all-sheds · 2 Mar 2017

Naz1977 said:
To me, it sounds like you have too many power hungry devices in the garage and are overloading the circuit.

Naz - Rich has been consistently clear on this:

loply said:
I've tried unplugging every single item in the garage, and only using one thing at a time, and the problem still occurs so that has eliminated appliances as being the issue.

loply said:
if I go in and plug in a 750w heater or a 500w power tool it will likely trip once every hour or two.

loply said:
The problem never occurs when a tool first fires up... two recent examples, I was just using the vacuum cleaner (nothing else plugged in) - a few minutes in it trips. Then last night I was just standing doing something, a small electric heater (750w I think) had been running all night, then suddenly it trips.

Why do you doubt him?

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Overcurrent fault on socket ring

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