RCD Nuisance Trips Conundrum

[Hzatph]

We have a newly installed RCD that trips in a very puzzling way. Let me explain.

We have two separate consumer units. One has a split load RCD and supplies the back of the house - it has never tripped. The other is an older "fuse wire" consumer unit with 16 circuits supplying the front of the house. It is protected by an RCD on its incoming feed and it is this RCD that trips several times per day.

There is no single cause that trips it - sometimes it is a light, sometimes the computer, etc. What is really puzzling is that the TV can trip it. The TV is in the back of the house fed from the other consumer unit and therefore is not even supplied through the troublesome RCD.

The RCD has been checked with an RCD tester and it passed OK.

Any ideas? Our electricians are baffled.

Thanks

[James Salisbury]

Check that the RCD is not too sensitive, check the insualtion been the tv ariel cable and N as well as E. Check there is no N / E reversal

[John Rumm]

16 circuits is quite alot to have on one RCD. It would be interesting to carry out RCD test while on load to see how much extra leakage is required to trip it.

That does not mean that it is not pre sensitied by cumulative leakage from all the circuits it is feeding to the point where it right at the point of tripping with little provocation. You could try to isoate if there is a circuit that is doing the bulk of the leaking by disconnecting each in turn for a day and seeing if that reduces the trip rate.

You did not mention what sort of supply you had, or the trip current of the RCDs. Also what sort of circuits are fed from the re-wireable consumer unit?

[Dave Plowman (News)]

Is there any equipment which could cause the earth from one CU at some point to to be shared with the earth for the other somewhere else? I realise both CUs will share an earthing point.

Perhaps an aerial lead from a DA? Hi-Fi equipment interconnection?

[Hzatph]

To answer some of the questions.

Both RCDs are 30mA.

The 16 circuits have:

4 ring mains 2 power spurs (cooker, immersion heater) The rest are lighting circuits as far as I know.

The only connection we can think between the consumer units is the aerial on the TV - we have TVs on both parts of the house. We tried disconnecting this but we still got a trip. The aerial has a masthead amplifier powered by a transformer on a normal socket fed by the main fused consumer unit.

As Dave says both consumer units connect to an earth which I think is connected to the neutral in the incoming supply. There is a cable from there to ground on the Manweb side of our supply about 30m from the house.

Hope that helps.

Thanks for all the suggestions.

[John Rumm]

^^^^^^^^^^^^^^^^ Prone to high leakage!

Try turning off / pulling the fuse for this and see if it helps.

If that does not find it then you are going to need to start testing the wiring for each circuit with a megger.

[tony sayer]

Also look for Nutral/Earth shorts or leakage, these are often overlooked and don't usually show up until some current is flowing through the system.....

[Hzatph]

I would suspect that, but both these items are switched off. We have kept a log of things that trip the RCD and it is a long list - there is no regular pattern.

[Pete C]

Hi,

I wonder if the neutrals on the the two consumer units are connected somewhere, maybe on a lighting circuit.

Does the TV trip the RCD on switch on? If so it's probably the large current draw from the degauss circuit.

A good way to track down the fault would be isolate each L-N circuit in turn by disconnecting the neutral as well as just pulling the fuse.

Be very careful though, no responsibility accepted if you fry yourself! :^)

cheers, Pete.

[The Natural Philosopher]

Neutral earth leakage somewhere.

[Hzatph]

Pete - you are correct in your statement that it is switching the TV on. I wonder about a neutral connection and also about the degauss type effect. Thank you. We are going to change the 30mA RCD for a 100mA one and replace the fused consumer unit with a modern one with 30mA RCDs on the ring mains and power spurs.This should ease the problem we hope, and it if continues to happen then we will have a better idea of where the fault lies.

[tony sayer]

In article , Hzatph writes

That shouldn't make any difference.

Yes you can do that..

You really shouldn't have to do that. You have either a fault in an appliance somewhere, or you have some leakage somewhere either live to earth or neutral to earth.

We have 6 properties and some commercial premises where a single 30 ma RCD protects the lot in each case, and around one trip every year or two is about the norm for the lot!, and thats usually a cooker element or immersion heater giving up the ghost, and one of the "premises" which is on a remote hilltop, the odd lightning strike!......

[Pete C]

Hi,

I'd expect there is a neutral-earth short somewhere as others have said. When you switch the TV on the current surge probably puts a small voltage on the local neutral which causes more fault current to flow through the short to earth and trip the RCD, even if it's in the other CU as the neutrals are commoned.

I'd get another better electrician to look at it, describe the symptoms to them and see if they mention the above. It should be possible to find the fault by disconnecting each neutral in turn and checking for a short between it and earth. While doing so any connected equipment should be plugged in and any isolators switched on initially in case the fault lies there.

Changing the CUs is just treating the symptom of the problem instead of the cause, a good electricial might find the fault within an hour or less. If your current guy can't diagnose or find a N-E fault then getting him to change the CU might lead to other problems...

cheers, Pete.