Help - RCD intermittent tripping

Worth actually unplugging it in case there is a faulty filter cap.

I am having trouble with the terminology here and can't quite work out what you have. Too many "breakers" for starters! :-)

So it sounds like this is a split load CU with two "sides", each side protected by a standard RCD. Each side equipped with a number of Miniature Circuit Breakers (MCB), supplying a number of circuits.

So far so good, or do you actually have some RCBOs (Residual Current with Overload (circuit) Breakers - i.e. combined MCBs and RCDs in the same device?

First we need to know what tripped... was it just a RCD or was it a MCB or both?

I have never found any shortcuts to this.

Things that have been found to be responsible here are:

- a toad nesting in the outflow from a sewage digester, raising the water level and shorting the motor

- a failed CH pressure vessel leading to overpressure when hot causing glands to leak water onto electrical kit.

- a neutral earth short in some wiring where I omitted to grommet a back box.

- a mouse chewing through a cable, and becoming a carbonised leak

- a washing machine motor developing a winding to ground short in te armature.

The good news is you already have a split RCD setup

But I am confused..."the RCD1 on the main circuit breaker, an MK by Honeywell, tripped" and "only the Sockets breaker refused to go back on."

Does this mean that only turning the sockets back on, trips the RCD?

If so that is already better news .

The thing to do is to wait till it trips again, and then remove every single thing from the sockets, and I mean remove - not just switch off.

IF then the sockets MCB does not cause tripping, you have an appliance fault. Otherwise you may have a short or fault in the house wiring

You can check for *that* by switching off the MCB for the sockets and disconnecting the two ring neutrals from the MCB. And then test neutral to earth resistance on the ring. If that turns out to be the issue you will have to take every socket off the wall and test every bit of cable

- but chances are with metal backboxes the fault will be in the back box.

If you have an appliance fault then you can try neutral to earth shorts on every appliance with it switched on - but not plugged in !

Once you have eliminated earth neutral faults, you can look at live earth faults.

A distinct possibility is a failure of a mains filter capacitor.

Switch off each unit one by one and record when the trip happens again.

Then investigate that unit in more detail

The worst possibility is that you have a lot of mains filtered stuff on that ring, and none of it by itself causes the trip, but the combination of two or more units does.

It is as per:

RCD1 trips.

Can't now recall what the MCBs did, I think they remained in the on position.

If I left the Sockets MCB off I can put the RCD1 back on. If I then turned on the Sockets MCB the RCD1 trips (fairly sure the MCBs remained on).

Sockets MCB can be turned back on after about 1hr.

Sockets MCB on RCD1 covers most sockets around the house except kitchen area and that includes second freezer in the utility room, and gas boiler (same location), plus bedrooms, my study with the computer gear which at the time were all off as I was in the midst of repositioning my new computer to replace my laptop.

Lots of 4-way adapters some ganged up together, ie a couple of 4 way adapters connected to a 4-way adapter. But been like that for a year of three.

I have an

Mmm.

Freezer and boiler are both possibilities. In particular if there are any leaks in the boiler

Correct.

See my reply to John Rumm for more detailed explanation.

Why though is it all ok after ~1hr?

Left off, or specifically turned off (I thought you said the MCB did not trip?)

Generally if a RCD trips when you energise a circuit it indicates that wither there is high earth leakage on that circuit, or the whole system has high earth leakage anyway, and hence is "sensitised", and any significant transient might cause a trip.

If turning that MCB on can reliably cause a trip (even if only in that first hour) then that may help. Since you can go through the process of turn on, trip. Unplug one device from the circuit. Reset RCD etc, and try again. If it still trips, reconnect previously disconnected device, and disconnect the next one. This may let you isolate one device that is causing a problem.

So the boiler etc are *not* on that RCD then?

something needs to cool down - or warm up.

Well the RCD can cool down. In my case it means that the water that had leaked out of the CH circuit had dried off :-)

I prefer the unplug absolutely everything on the circuit that can be unplugged and then add things back one at a time until it goes pop. It could be a combination of two units that takes it over the limit (but usually it is a single bad unit toaster or kettle are obvious suspects).

I have known kettles and boilers cause a trip the first time they are used after a longish idle gap but then are OK for a while.

A 1 hour delay before it will reset properly seems a bit odd to me.

It is definitely worth unplugging everything. If the fault remains then it has to be in the wiring somewhere. Ingress of rodent or slug into a junction box somewhere (although the latter tend to create a permanent fault since they are highly conductive).

Rephrasing:

Sockets MCB on RCD1 covers most sockets around the house including second freezer in the utility room, and gas boiler (same location), plus bedrooms, my study with the computer gear which at the time were all off as I was in the midst of repositioning my new computer to replace my laptop.

RCD2 covers kitchen sockets, house lights and kitchen cabinet lighting.

But everything is now plugged in and working as it was this morning and most of Wed.

Spiders?

In that case you have a fault either in the ring main itself or in something that is plugged into it. You need to determine which it is.

Pray that it is a fault in an appliance somewhere because finding a ring main fault is pretty tedious even if you can binary chop the circuit (ie making it temporarily not a complete ring main for testing purposes).

Probably the time it takes for whatever is allowing the leakage current to dry sufficiently that it no longer causes the problem any more. Early morning dew or condensation might be implicated as a mechanism.

The other useful question is who was doing what at the time the mains tripped out - that may be a good clue as to the guilty party.

You will find the definitive answer only after you have found the root cause of the fault which with an intermittent fault may be some way off.

On outside fittings, yup. Small insects can migrate damp soil into fittings as well.

You would normally get more tripping after a spell of heavy rain or damp weather.

Does your Utility Room have a washing machine? or tumble drier?

Indeed but has not been used in this period of woes.

What can be a real sod is an Earth to Neutral short, this will remain un tripped until the current draw of the appliances exceeds a certain level.

Best bet is to remove or disconnect everything then put say a decent single load like a two bar electric fire or kettle, if just that one thing trips out then thats the likely problem!!

Things that plug in like the computer can be moved to another circuit and tested that way. Brian

Yes I do often wonder if these protection systems are rather overkill. What would you expect to be doing much better with all those different breakers than simplifying it with the same types everywhere? In this case one also assumes that each appliance can either be turned of with a switch or unplugged from a socket so going around them one at a time would seem to be called for here. Brian