Consumer unit main switch trip

[elltim]

Hi all

My old man is having problems with a tripping main switch in his consumer unit. It trips periodically and it is difficult to locate the fault because it isn't an individual rcd that trips.
It will trip at odd times, early in the morning, last thing at night or middle of the night.
At the moment he's switching off one circuit at a time for a couple of days to try and locate the fault.

Any ideas?

Thanks

 

[mikeyd]

Isolating the circuit using the MCB's might not help as it will not isolate the neutral so a neutral/earth fault on the circuit could still trip the RCD.

Unless you are up to disconnecting neutrals in the consumer unit I would strongly advise getting a sparky in as they will have the proper test equipment to find the fault.

 

[elltim]

Thanks for your help.

 

[DCC]

An outside light maybe?

 

[mikeyd]

Thanks for your help.

I don't think I was much help.

I should have added that unplugging appliances, turning off fused connection units and cooker switches could help track the fault if is an appliance causing the fault. But if the fault is in the fixed wiring there isn't a lot you can do to narrow it down without the right test gear and/or disconnecting neutrals.

He hasn't been putting any shelves up recently or done anything else that might have damaged any wiring? Or heard any mice in the loft?

 

[elltim]

they've had this problem on and off since moving into the property a year ago.
2 underground connections (to outbuildings) have been replaced using proper 2-pack resin joints after they were found to be wet. This seemed to have stopped the problem for about 2 months until now.
They have installed an additional immersion boiler system recently.
Could the trip be caused by a combination of faults or is it likely to be just one issue?
cheers

 

[ccrowe]

When you say there's no individual RCD that trips, do you really mean individual RCD or do you an MCB or RCBO? There's a very big difference between them. RCDs will usually protect several circuits whereas an MCB or RCBO protects only an individual circuit, often, in the case of MCBs, with additional RCD protection above them. Sorry if this is teaching you to suck eggs but clarification is important. Do you have a picture you could post of the consumer unit and identify exactly which devices are tripping?

Which appliances are running at the times of the trip? Fridge and/or freezer are good candidates as they're always running. Heating? Immersion heaters? Does he have timed appliances running, say a time switch on a washing machine or dishwasher?

 

[AMEdinburgh]

From a DIYer not an electrician

When we had this irritating problem in our surgery, it was our fridge freezer and the trip occurred at random on the occasions when the motor started. That explained the randomness of our tripping.

The first electrician condemned the RCD in the consumer unit but the manufacturer tested it and disagreed and the manufacturer was right. The second electrician was suspicious of the kitchen stuff and quickly found the fridge freezer was the fault conducting a PAT test on the item.

 

[elltim]

Hi
As requested, i've uploaded an image of the fusebox. It is the far left main switch that trips (not the mcb currently off). During the trip there are no timed appliances running.

 

[plugwash]

Untitled

• elltim
• 25 Oct 2008
• 1

mmm one RCD covering everything. Not ideal especially as it looks like you have significant outside wiring (a barn and a caravan?).

As others have said if it's a neutral to earth fault switching off the MCB won't help.

Outside wiring, white goods and anything involving water are often prime suspects.

Unfortunately even electricians often have trouble tracking down the source of neutral to earth faults.

 

[elltim]

This consumer unit provides supplies to :-
(1) A recently built barn which then has it own fusebox. Sockets/Lights. Nothing switched on here during trip. This is the supply with replaced underground connections.
(2) A recently built shed containing a fridge/freezer. This would have been on during the trip.
(3) An outside light with PIR.

The caravan has been scrapped.

Since switching off the 1st MCB (1) a few days ago the main switch hasn't tripped.

cheers

 

[bongos]

2 underground connections (to outbuildings) have been replaced using proper 2-pack resin joints after they were found to be wet. This seemed to have stopped the problem for about 2 months until now.

As you have isolated the barn circuit and the problem seems to have gone away it could be that there is a problem with the wiring out to the barn. When the underground connections were replaced was the rest of the cable in a good condition? Did the electricians who repaired it provide any figures such as insulation resistance for this cable? It sounds like there may be a neutral/earth fault.

Another posibility is that with this circuit switched on there is enough earth leakge combined between the whole installation to trip the RCD but with it off it falls just enough below 30mA to trip.