How do I diagnose a fault that keeps tripping RCD?

[gutt3d]

We get random, intermittent, occasional faults that cause one of the main breakers on our consumer unit to trip. When it happens, it's not an individual circuit that trips, but instead it's a breaker which controls multiple circuits. Given that none of the individual circuits is tripping, we don't know how to troubleshoot it - nor even how a professional might troubleshoot it.

There appears to be no pattern to when it happens. For example we've had no faults for months - we'd hoped that some rewiring that had been done in one of the bedrooms had cured some problem. However the problem has happened twice in the last 10 days. We might not see the problem again for another few months, but it might happen this evening. We can't see any pattern.

In both most recent occurrences it's happened during the morning (once at about quarter to 8 while wife and daughter were getting ready to leave for school and the most recent time it happened after I'd left for work - it happened some time between about 8:45 and 10:00 when no-one was in the house).

It#s the breaker I'm pointing at in the picture. When it goes, it takes out the hob supply, microwave/combi oven supply, downstairs lights, sockets upstairs, sockets downstairs, and another lot of sockets in the kitchen. Note that we recently moved in (about 2 years ago) and had a new kitchen installed - including all new electrics, with a certificate of major works confirming no problems found. Also note: our house is quite old, and has/had some weird electrics - the kitchen fitters wanted us to have a PME installed (which we did) before they were prepared to start work, though there's still an earth spike. There are multiple sockets circuits - when we were trying to ID them, we used sticky labels (blue dot, blue triangle, blue square, blue rectangle, etc.) to find out which sockets were on which fuse - that's what all the coloured labels are.

In the image above, fuse #6 is for "sockets ground floor + kitchen". If/when the main breaker goes, it takes out the kitchen sockets and boiler. This wouldn't be a disaster, except that the fridge is out. If it happens while we're at work, then no harm done and we won't lose much. Sooner or later it'll happen while we're away, and we'll come home to a fridge full of spoiled food

In the past when this has happened, it has been (but wasn't always) accompanied by a local power cut - this happened at least twice. We thought maybe this is/was being caused by a "micro power surge": is that even a thing, and if so could it cause our consumer unit to trip? And if so, how do we go about proving it in order to get it addressed?

About the best we can imagine for now is to have a professional move the sockets circuit over to the other bank... If the other bank starts tripping out, then maybe we could say that there's some fault on the sockets circuit where the fridge is. Otherwise we'll still have the intermittent fault, but at least if/when it happens again we won't lose power to the fridge!

Any suggestions on how to diagnose/troubleshoot would be very welcome!

 

[terryplumb]

You need to engage an electrician to do some testing. The RCD that's tripping supplies all the circuits on the left of it . When earth leakage, at a certain level ,occurs it trips. The level of earth leakage can be on one of the circuits it protects ,or it could be a total of a small amount across a number of them.

 

[gutt3d]

You need to engage an electrician to do some testing. The RCD that's tripping supplies all the circuits on the left of it . When earth leakage, at a certain level ,occurs it trips. The level of earth leakage can be on one of the circuits it protects ,or it could be a total of a small amount across a number of them.

Fair enough, and thanks.

Out of curiosity, what would be the cause of earth leakage? I'm going to go away and google it right now But it would good to hear it in layman's terms that I might be able to understand

Cool - first Google hit is THIS which is helpful. Thanks again!

 

[Harry Bloomfield]

Fair enough, and thanks.

Out of curiosity, what would be the cause of earth leakage? I'm going to go away and google it right now But it would good to hear it in layman's terms that I might be able to understand

Cool - first Google hit is THIS which is helpful. Thanks again!

It's leakage from the live or neutral to the earth. Leakage from either pole to earth can cause it to trip, and it could be as little at 30mA of current leaking.

You could try isolating those MCB's/switches to the left of the RCD, one at a time if you can, for a period of time, to see if you still get the tripping. That might help identify the circuit with the fault - but be aware that the MCB's only isolate the live, the neutral might have the fault - so it's 50/50.

Also make sure you know what you are turning off, fridges, freezers alarms etc. may need to be kept on.

 

[Harry Bloomfield]

It's leakage from the live or neutral to the earth. Leakage from either pole to earth can cause it to trip, and it could be as little at 30mA of current leaking.

You could try isolating those MCB's/switches to the left of the RCD, one at a time if you can, for a period of time, to see if you still get the tripping. That might help identify the circuit with the fault - but be aware that the MCB's only isolate the live, the neutral might have the fault - so it's 50/50.

Also make sure you know what you are turning off, fridges, freezers alarms etc. may need to be kept on.

I've just seen your link and that is a good explanation.

 

[gutt3d]

It's leakage from the live or neutral to the earth. Leakage from either pole to earth can cause it to trip, and it could be as little at 30mA of current leaking.

You could try isolating those MCB's/switches to the left of the RCD, one at a time if you can, for a period of time, to see if you still get the tripping. That might help identify the circuit with the fault - but be aware that the MCB's only isolate the live, the neutral might have the fault - so it's 50/50.

Also make sure you know what you are turning off, fridges, freezers alarms etc. may need to be kept on.

The problem is that I've no way of knowing how long (or otherwise) it might be before it trips again. On one of the circuits I've got the hob, then on another I've got the oven/microwave, and another I've got the downstairs lights., etc. I don't reckon i'd get away with turning one off for any length of time.

One thing that may be an indicator though: the microwave part of our combi oven died the other week (someone is due to fix it tomorrow, in fact). Shortly before it died, it tripped the circuit (not the whole RCD - just the circuit for the oven/microwave). We reset the circuit, and the microwave continued working that same evening - but next time we want to use it a few days later, it no longer worked. It didn't trip the fuse, but it just refused to work. We brought that combi oven with us from our old place to fit into the new kitchen because it's still a decent combi oven and we were trying to keep our costs down. It's probably 10+ years old now, but it's been with us as long as we've been here - perhaps it's slowly dying, and it's causing our fault. If I were to ask the engineer to check for earth leakage on the combi oven while he's fixing it, would he have the tools to do so?

 

[Harry Bloomfield]

One thing that may be an indicator though: the microwave part of our combi oven died the other week (someone is due to fix it tomorrow, in fact). Shortly before it died, it tripped the circuit (not the whole RCD - just the circuit for the oven/microwave). We reset the circuit, and the microwave continued working that same evening - but next time we want to use it a few days later, it no longer worked. It didn't trip the fuse, but it just refused to work. We brought that combi oven with us from our old place to fit into the new kitchen because it's still a decent combi oven and we were trying to keep our costs down. It's probably 10+ years old now, but it's been with us as long as we've been here - perhaps it's slowly dying, and it's causing our fault. If I were to ask the engineer to check for earth leakage on the combi oven while he's fixing it, would he have the tools to do so?

Another way would be to simply unplug suspect items, like your microwave for a while, but be aware the cause might not be anything you can unplug, such as your ceiling lights etc.. Without test equipment, it is a long process of elimination.

Assuming your engineer is there to diagnose the fault, they should have the equipment to be able to test it - pointless them turning up without the test equipment. Do make it clear that you have an RCD tripping which serves several circuits.

 

[terryplumb]

Do you have any exterior lights that are connected to one of the circuits that the RCD protects ?
Moisture / water ingress, coming into contact with electric cables/ fittings is a common cause of intermittent RCD tripping.

 

[jj4091]

You say you have a combo microwave oven so I guess that means the oven compartment & it's contents can be heated by a heating element as well the microwave side of things. Heating elements for many appliances often give the symptoms you describe. So even it is repaired tomorrow if the fault persists then as Harry says leave it switched off & unplugged for several days two see it the tripping stops.

 

[JohnD]

IME earth leakage is usually from watery appliances such as kettles, fishtanks, immersion heaters, boilers, pumps, washing machines, and outdoor circuits on rainy days. Especially during their heating cycle. Isolate all of those by pulling the plug or with a DP switch for a start. Reconnect individually only when needed for use.

 

[ericmark]

We have three tools, the multi-meter, the one shown can show 0.001 amps, the installation tester often 250, 500, or 1000 volts, and the RCD tester, first two not too expensive at around £35 each the last one likely more than £200 but to be frank I would not suspect the RCD is faulty, so one of the other two, but a type AC RCBO costs £15 and type A £18.60 and you at moment have type AC, plus an isolator to replace the existing RCD so the big question is if it would be worth finding the fault, or moving to RCBO's?

The RCD you have serves 5 circuits, the fault could be a built up over the 5 circuits, once between 15 and 30 mA the RCD will trip, but spread it out you could leak 75 to 150 mA before it trips, and when it does you only loose on circuit not five.

I looked at it when I upgraded my consumer unit, and thought lose 3 freezers full of food and it will cost a lot more than using RCBO's instead of RCD and MCB's.

I had it in my last house, but in 1992 when I fitted the RCD's after my 14 year old son became a radio ham, RCBO's were not really available at a reasonable price for domestic, and like you every so often they would trip, likely due to spikes on the supply, as it could happen 4 times in a week, then not again for 2 years. I had the meters and did test, with no fault found.

Unplugging items not used does help, as you can have neutral to earth faults as well as line to earth, and most on/off switches only switch the line.

I think electricians should take the time to explain the risks of having just 2 RCD's, but some people never seem to have a problem, my mother never had a RCD trip, we know surge protection device (SPD) can remove spikes, but we really don't know if they really work, we read reports which point out the amount of devices which can allow DC on the supply, and we are told we should now use type A not type AC but the type A is only good for 6 mA, which is nothing really.

I tested my own whole house

and am reading 19 mA, which could trip a RCD, but I have 14 RCBO's so that leakage is spread between 14 devices, so unlikely to trip any one device.