RCD tripping

18 Sep 2007
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United Kingdom
Hi all

Hope you can help me. I have a split load MEM CU, a few months ago, I started getting occasional RCD trips. Recently these have become more frequent and I'm now trying to get to the bottom of what's causing it.

I have 2 MCBs on the RCD protected side of the board and 4 on the non RCD protected side. The protected ones are: socket ring main, cooker. The non-RCD protected are: 2 x lighting ring, smoke detector, water heater.

I've tried the following to trace the issue:

Isolating appliances - fridge freezer powered from cooker socket, cooker turned off at cooker socket and socket ring main isolated at MCB. Trip still occured.

Swapped cooker circuit with water heater circuit so that I could power fridge freezer from non-RCD protected circuit and avoid defrosting. Note my cooker is only a single over so the 16A water heater MCB should be sufficient. Can now isolate both RCD protected circuits at MCBs. Trip still occurs even with both RCD protected circuits isolated by their MCBs.

Removed many (not every) socket and fused spur faceplate to check for water ingress. No evidence of water in any of them.

Measured neutral to earth resistance (with CU off at main switch) of each circuit with the neutrals removed from the bus bar. All circuits reported open loop so no clue as to potential earth leakage from here.

Electrician popped round and tested the RCD which is working fine and not tripping early.

I'm waiting for him to get back to me with a date as to when he can do a full investigation.

In the mean time are there any more things you guys can suggest I could try. The trips are getting more frequent and driving me mad although at least the fridge freezer is no longer defrosting.

Could it be electrical noise from the fridge freezer triggering the RCD even though it is not on an RCD protected circuit?


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A few points

isolating with the MCB will not isolate Neutral, so any neutral/earth leakage will still occur

You say you have the cooker on the RCD side. Cookers tend to leak especially as they get older so it is better to have cookers on non-RCD. Do you mean you have been isolating the cooker with the big DP switch?

It might be an appliance. Watery appliances like kettles, washing machines, dishwashers, immersion heaters are most prone to earth leakage. it may not be a single appliance with a big leakage, it may be several adding together.

Remember that boilers and their pumps contain water and electricity. A small drip can cause intermittent tripping. It may be worse after turning off, as it will no longer be hot enough to evaporate the drip. I hope you have a DP switched FCU for the boiler, try turning it off when heat is not required, and look for traces of leaks round the pump. Leaks inside the boiler case will need a boiler engineer.

It can also be cable damage, e.g. cooker standing on its flex, dog gnawed flex, mice under floor gnawed cable. I hope it is not a nail e.g. into a wall penetrating a cable.

If you have any out-door circuits, such as shed, garden socket, fountain, fishpond, lighting on a spur, they are very suspect as prone to rain penetrating the enclosure or damaged cables.

Can be caused by a leaking pipe dripping onto a connection e.g. under the bathroom floor, but this is more common onto downstrairs ceiling light fittings. There should not be any connections under the floor, such as junction boxes.

How many spare ways do you have in the CU?

You are very lucky (or wise) to have a MEM. RCBOs are readily available to fit these, the advantage is that you can RCD-protect a single circuit, so an earth leakage fault will cut that circuit but not affect other circuits in the house. this is a useful option if you can't easily trace the fault, or if it is cumulative from several circuits. It will also identify which circuit has the problem.

Is yours a Memora 2000 (rectangular case), or a 2000AD (slight curves on front door)?
Thanks for the reply John

I've been isolating the cooker on the faceplate switch which I believe is DP on cooker faceplates. Originally, the cooker was RCD protected, I have moved this to the non-protected side of the CU.

The ring main to which all the watery things connect including boiler, washing machine, kettle are connected has been isolated by way of the MCB. I understand the 2 neutral bus bars are connected to each other (via the RCD), should I also unplug the watery things + unwire in the case of the boiler to totally remove them from the system? I don't think the boiler switch is DP, looks like a normal fused spur to me.

Will check the kitchen ceiling rose (below bathroom) as this sounds possible.

1 spare way on CU on non-protected side.

It's a Memora 2000


you need to unplug them

the boiler FCU might be DP if fairly new; if not, buy a new one it will save you having wires hanging out. Get a good brand like MK, Crabtree or MEM.

If you can lay your hands on one of these RCD adaptors that people use with lawnmowers, you can try using that with your suspect appliances when you have to plug them back in. In my experience they will trip at the same time as the RCD if the fault is on the appliance plugged in (not otherwise) and are ever so slightly faster than the bigger RCD in the CU.

Leave all appliance unplugged when not in use. If that cures your tripping, plug half of them in for a few days, and the other half plugged in for different days. This may help you establish a pattern. If not, and you have you electrician back, ask him to fit an RCBO on the non-RCD side, and put one of your Ring circuits in there. The part will be about £35. Make sure you keep the removed MCB (and, later, the RCBO) for future use. Tel him before his visit so he can order the part (or ask if he minds you buying it yourself)

The Neutral bars are not connected together, one of them is fed from the load side of the RCD, and isolated when the RCD trips. the other is fed direct from the load side of the main switch.

p.s. computers and their ancillaries usually have earth leakages.

p.p.s when removing your sockets, did you find all earth wires were covered with G&Y sleeving, and all back-boxes earthed to the switchplate? May not help, but sometimes it makes a difference esp if the wall is damp.
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I tried one of the plug in RCDs on the fridge (before I moved it to the cooker socket and then to the non-RCD protected side), I also tried it on the 4-way adapter I use for my computer, monitor etc. In both cases, it seemed to trip whenever the CU RCD tripped. I assumed it just went as a result of the CU RCD tripping. Will try again with other appliances and using the test switch on the CU RCD.

The earth wires are all sleeved but the back boxes aren't earthed to the switch plate.

What's the most obvious way to tell if a FCU is DP?

Thanks for the advice, at least a few more things for me to try

how to tell if DP switched?

loosely speaking, old ones aren't and new ones are :oops:

I think budget brands might still be made with SP switches, but stick to MK, Crabtree or MEM.

when you take it off, the DP ones have two "supply" terminals and two "load" teminals.

If no switch, can only be SP through the fuse cartridge.

You could check with a multimeter if not sure.
Checked for signs of water in the kitchen ceiling rose (below bathroom) but all seems fine.

Isolated some appliances by unplugging or switching off with DP switch (turns out all except my immersion heater have DP switches)

Isolated: toaster, kettle, washing machine, microwave, plinth heater, paper shredder, immersion heater. Trip still occurred after a few hours

Additional to the above items, I isolated the boiler: - no trip so far :). Just been turning it on when I need it then back off to isolate.

I will leave this configuration a few days and then add things back in gradually but it's looking at the moment as though it could be the boiler which kind of makes sense.

Tested the plug-in circuit breaker (type used with lawnmowers). The one I have trips whenever the CU RCD trips so it's of no use for detecting the cause of the trips.

Cheers Russ
that's good! :)

Is the CH pump accessible, in the airing cupboard or somewhere (some are inside the boiler casing). If you can get to it, have a good look round for green stains, limescale, signs of drips. Route the cable so it has a drip-loop before entering the connection box (which should be at the top of the pump so that water does not run into it). Look for supplementary bonding of the pipes in the airing cupboard which will reduce the risk of electric shock.

If not the pump, the next likely source is a small drip inside the casing going onto an electrical part (it could also be a failed part on the circuit board which an engineer familiar with your type of boiler should be able to find)

(edited out a comment here, i was confusing you with someone else)
:( came home this evening and RCD had tripped. The only things plugged into the ring main are:

TV, digibox, AV receiver, DVD player, router, phone, alarm clock, mobile phone charger.

Everything else was isolated. Will unplug the above when not in use and see if it's one of them. Any favourites?

Also, any other tests to reccommend?


mouse gnawed cable under floor?

sometimes there is a general background leakage, which fluctuates, and can cause a trip without having a single apparent point of failure.

Occasional trips can be very difficult to trace, and sometimes they are caused by a problem on the non-RCD side; even from a neighbour's house(!)

What sort of main earth have you got?
First - have a look at the Service Head, where the suppliers (thick) incoming cable passes into a black or grey plastic or iron box. There will be a large fuseholder here, it should all be sealed, and two thick "tails" come out and go to the meter. There may be a metal connector block on the side of it, and there may be a green and yellow wire terminated here. there may also be a label beside the meter saying "protective multiple earth"

Or, there might be a G&Y clamped to the cable sheath

Or, there might be a G&Y cable going to an earth spike driven into the ground (there should be an inspection cove over it saying "electrical earth"

In all cases the G&Y should go into the consumer unit(s) either directly, or via a substantial terminal block. The incoming metallic main services (water, gas, oil) should be bonded to it (if inside the CU you nay not be able to see the connection, but you should be able to see the G&Y cable. There are correct sizes for these G&Y, older installations may have them missing or undersized, but this is easy to correct.

If you can post some pics round the service head and meter and CU, and the various cables connecting and around them, we can probably recognise it for you.

Usually, if you do not have a good earth, the supplier will install a better one for you, sometimes free, sometimes for about £60 to £100. This can be very helpful. Very occasionally, if you have an old supply, and/or overhead cables in a remote area, they might say they can't. You are responsible for connecting to it, but this is very easy and cheap and can be a DIY job (instructions available)

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