Electrics - Main MCCB tripping

[jac-in-a-box]

Got a Wylex consummer unit fitted and the main supply breaker has started to randomly trip.
Had the odd occassion were a lighting breaker will trip when a lamp blows, which is fairly normal - but not the main supply breaker.

It's just the supply breaker going with all the other MCCB's remaining un-tripped.It's not happening when any particular large load goes on. Is it likely to be the incoming main supply causing the problem or something as simple as an ageing supply MCCB? ('tis around 6-7 years old)
100A 30ma RCD for the main breaker, the remainder are non RCD MCCB's serving the various circuits.

Thoughts appreciated

[Raverbaby]

First of all its a bad idea having a 30ma RCD as your main switch (unless your on a TT system) as this takes the whole house out when it trips.
Do you know what earthing system you have?
Ageing heating elements can cause nuisance trips, also check anything external for moisture ingress i.e. lights, outside sockets.
If it happens often, try to eliminate what you have on at the time, does it happen at certain times.
Unplug anything your not using to try and eliminate it being at fault.
These random trips are often the worst to trace, especially without the proper test meters!

[FamilyGuy]

As above - suspect anything that comes into contact with water. Immersion heaters are often the culprit. In my case I've had a washing machine and an electric shower cause this behaviour. I know someone who had similar with a kettle.

[jac-in-a-box]

Thanks for replies fellows...I'm not sure what sort of earthing system I have. The house is from the 60's and there have some alterations to the down stairs circuits with addition of extra sockets and the removal of the old fuse wire type of distribution unit 8 years ago. Up until yesterday there hasn't been the slightest hint of a problem.

After my initial post I've isolated the power shower, the cooker and the rarely used immersion heater and it hasn't happened since - problem maybe there somewhere? But, if it is, why doesn't it trip that particular circuit breaker? As said earlier if a lamp blows it trips one of the lighting circuit breakers not the supply breaker!

I've checked tightness of all the connections in the consummer unit, nothing loose in there.

Without being able to identfy what piece of equipment is causing the issue (it is happening randomly), would it be worthwhile replacing the main breaker?

@ Raverbaby..."unless your on a TT system" Not sure what you mean by this, a split system with a RCD protecting 2 groups of circuits? Mines not!

I'll be off the check the security of the wiring in the isolated circuits soon, any other advice would be welcome

[TooLateForAName]

TT is where your Earth is provided by a rod in the ground rather than coming in with the live and neutral.

[PH5121]

The reason that the mcb protecting the 'faulty' circuit is not tripping is that the 'fault' is not an overload or a short circuit.

If the DB incomer which incorporates a rcd is tripping it indicates that there is an issue with current leakage. ie a breakdown in insulation between the live, neutral and earth conductors, the rcd detects current in the earth conductor or any imbalance.

As said before, this could be due to water ingress into an item of equipment, damaged insulation such as rodent damage / chafing of cables, or the breakdown of an immersion heater element as they suffer corrosion.

[jac-in-a-box]

PH5121 said:

The reason that the mcb protecting the 'faulty' circuit is not tripping is that the 'fault' is not an overload or a short circuit.

If the DB incomer which incorporates a rcd is tripping it indicates that there is an issue with current leakage. ie a breakdown in insulation between the live, neutral and earth conductors, the rcd detects current in the earth conductor or any imbalance.

As said before, this could be due to water ingress into an item of equipment, damaged insulation such as rodent damage / chafing of cables, or the breakdown of an immersion heater element as they suffer corrosion.

Thanks for the explanations chaps - learning as I go along

Still not sure about the earth, if got the main supply cable coming into a large fuse block (before the meter) with a smaller green earth line from consummer unit running to the large fuse block and then disappearing under a inaccessable void space below the floor.

So far I've isolated the immersion heater at consummer unit and switched off the security lights (run off a fused spur from upstairs socket ring main) and been without a main breaker until I used the kettle tonight. Half way through boiling, the whole lot went off again.

Not wanting to sound stupid, but does that indicate the kettle, which is only months old, is causing the issue - I'm thinking if it was that, it would only knock out the kitchen socket MCB? Other trippings haven't been caused by using the kettle.

Any way of doing some simple checks with a multi-meter to help narrow down the problem area and any merit in changing the main breaker RCD just to rule out any "iffyness" there?

A little too stubborn to call in the pro's just now if I can sort the problem myself!

Continued advice will be much appreciated

[wizzbilly]

you could get a socket tester and check all the sockets especialy one where the kettle is plugged in as stupid as it sounds a bad earth in the socket over the years can cause this , kettles drag quite a lot

[hairyben]

jac-in-a-box said:

Thanks for the explanations chaps - learning as I go along

Still not sure about the earth, if got the main supply cable coming into a large fuse block (before the meter) with a smaller green earth line from consummer unit running to the large fuse block and then disappearing under a inaccessable void space below the floor.

So far I've isolated the immersion heater at consummer unit and switched off the security lights (run off a fused spur from upstairs socket ring main) and been without a main breaker until I used the kettle tonight. Half way through boiling, the whole lot went off again.

Not wanting to sound stupid, but does that indicate the kettle, which is only months old, is causing the issue - I'm thinking if it was that, it would only knock out the kitchen socket MCB? Other trippings haven't been caused by using the kettle.

Any way of doing some simple checks with a multi-meter to help narrow down the problem area and any merit in changing the main breaker RCD just to rule out any "iffyness" there?

A little too stubborn to call in the pro's just now if I can sort the problem myself!

Continued advice will be much appreciated

Right, the breakers are giving you two different kinds of protection. Simply:

MCB/overload breaker stops too much current flowing and melting circuit wiring. used for each circuit, sockets lighting, different sizes (5 amp, 20 amp, 32 amp etc) to protect different cable sizes.

RCD/residual breaker is a current "balancing" device that trips if there's a tiny differential in the current flowing through the live and returning neutral conductors, as this is current flowing outside the system and could be an electric shock. Often only 1 or two of these are used to protect several other circuits, many fault conditions, in particular damp, will trip the RCD first. Combination RCD/MCB breakers can be fitted to each circuit individually but these cost a little more.

It could be your house has several small faults that added together trip the RCD, with intermittant faults like this the best option is to keep unplugged/switched off everything you're not using so when it does trip you have a idea of what was in use.

Like said Anywhere liquid and electric come together is a prime suspect, washing machines, dishwashers, tumble dryers, waste disposals, pumps, water heating, exterior lighting/power, big old electric motors, compressors (fridge/freezers),electric irons,

Don't worry too much about anything double insulated- most electronic stuff TV's hifi's without an earth they're very unlikely to cause a bother.

[jac-in-a-box]

Good explanation hairyben - appreciated!

Haven't really "done anything" apart from pull sockets from the wall and check tightness of cables - nothing loose and the breaker remained on all day and night...leaves me slightly miffed but happy!

I'll see how it goes for the next week or so, if it stays fine all well and good. If not, I'll change the main breaker RCD before calling in the experts.

As has been pointed out, having RCD protection on the main breaker only is a PIA - losing everything when a fault develops.
Assumung Wylex makes a RCD type MCCB that'll fit my enclosure, would it be sensible to change the kitchen socker circuit breaker to provide RCD protection as it's possible that any faults are being caused on that circuit?

[PH5121]

If you want individual rcd protection for each seperate circuit, rather than one device protecting all as you have at the moment you will need RCBO's. These are a combined mcb / rcd device.

You say it is a Wylex consumer unit, Screwfix used to sell Wylex Rcbo's for £30 & vat.

[hairyben]

If you plan to stay in the house for a good while I'd give thought to sorting the consumer unit and it's worth going for an RCBO (dedicated combination breakers) based consumer unit- the popular "dual RCD" units fitted today are little better than what you already have in terms of inconvenience and the client being able to assess and trace a fault. RCBO's are shunned as a board will be more expensive, but can pay for themselves in time as they make it all so easy- "oh, it's the downstairs sockets out- well lets look" rather than "oh my whole/half my house, what do I do, panic about the freezer, call electrician, get mugged by sharks." Also with RCBO's several circuits can have small fault currents and the board will stand it whereas all those circuits on one RCD breaker will add up and trip it.

If you're peeking behind sockets the one thing that might give you RCD trouble is damp/mould, not uncommon on old solid walls, replace any with more than very light corrosion.

[jac-in-a-box]

Nothing untoward today, so fingers crossed!

Having gained a better understanding of what protects what and how, I think any reoccurence will see a box full of RCBO's replacing the current set-up.

Thanks for all the advice guys - appreciated