Immersion triping RCD

[GTL]

Hi,

I have just upgraded my consumer unit to a new 12 way MCB slit load. On commissioning everything is working fine with the exception of the immersion heater.
Initial examination of immersion heater showed that the neutral wire sleeve had burned about 25mm along its length and that the thermostat showed signs of overheating. I have replaced immersion heater for a new one and I still the RCD trips.

The scenario:-

The line is from 16A MCB on non RCD side of unit.
From MCB to timer to 20A switch to immersion.
When powered up, immersion through timer works fine with TV or hover on. However, when I switch on the kettle or cooker independently the RCD trips.

I have bypassed the line under the floor and have gone from timer to 20A switch with 2.5 T&E which works until I switch either the kettle or the cooker on. The RCD trips.

I have bypassed timer and have gone from MCB to 20A switch with 2.5 T&E which works until I switch either the kettle or the cooker on. The RCD trips.

Both the cooker and the kettle on together or independently no RCD trip. (Immersion disconnected).

The Cooker on 40A MCB.
The kettle is on 32A MCB kitchen ring.
Immersion is on 16A MCB .

My brain is frying on this one. Can someone please help.

 

[JohnD]

they are all devices which are prone to slight earth leakage (heating elements and especially watery appliances are prone to this)


It is likely that there is a background leakage which is below the tripping current, but when you add an extra appliance, it adds up to enough to cause a trip.

Which circuits are on the RCD side, and which on the non-RCD side?

Did you install it yourself?

 

[GTL]

Hi John,

Yes I have fitted it myself.

RCD side = :-

40A Cooker.
32A Kitchen Ring.
32A Bottom & Top Floor Ring. One bedroom house 5 sockets.

Non RCD Side =:-

16A Immersion.
6A Bottom Floor & Kitchen Lights.
6A Top Floor Lights.
6A Bell Transformer & Front Door Light. (Transformer has no earth).

Unused MCB's not in unit.

 

[JohnD]

I am not familiar with Scotland's rules on changing CUs. In England it is notifiable work and has to be tested and certificated. Which side is Berwickshire?

cooker would be better on the non-RCD size (and the cooker switch would be better with no socket)

I am guessing you do not have an insulation tester? Maybe you have some damaged cable somewhere.

p.s. what brand is the CU, and how many spare ways do you have on each side?

p.p.s have you got supplementary bonding in the bathroom, bonding together the lighting and immersion CPCs to all the metal pipes? Which circuit is the boiler on?

 

[Spark123]

What sort of supply is it? I'm thinking a N-E short somewhere on an RCD protected circuit, TN-CS supply. What are the IR test results like for it?

 

[GTL]

Hi John,
Your right, Scottish rules are either the council comes and test and certify or spark comes and test and certify. I've arranged the latter.

There are 3 spares on the RCD side and 1 spare on the non-RCD side.

No insulation tester.

Volex CU

Bonding:

I recall some years ago the water board notifying the neighbourhood of the replacement of old lead mains water pipes to alkathene and it would be the householders responsibility to make sure their earthing was connected to electric boards earthing. This wasn't a problem as my bonding was already connected to the boards earthing block. I vaguely remember there is bonding to pipes under the bath the central and heating pipes and there is a bond from the boards earthing block to the mains though I think that may be useless now with the new mains water pipe.

I don't have a boiler. I run storage heating on its own supply. It comes on at midnight and no problem there.

 

[daytona]

you said the flex was damaged on the immersion heater.

have you tried replacing this rather than repairing it.

just trying to start at the beginning

 

[daytona]

have you mistakenly dumped the neutral feed for immersion at the board on the rcd neutral bar?or accidentaly swopped a ring main neutral for immersion neutral.

might be something wonky there mate

 

[GTL]

What sort of supply is it? I'm thinking a N-E short somewhere on an RCD protected circuit, TN-CS supply. What are the IR test results like for it?

Hi Spark123,

A wee bit too techi for me at this time O' night. I think you may be right with the N-E short on the RCD side. Meter test showed N-E leakage prior to the new immersion being fitted but haven't tested with heater in thinking that was the problem. Having said that I remember many years ago talking to Ross the electrician and saying that I had noticed my meter turning ever so slowly when everything in the house was off. He thought I was daft but then thought there may be leakage. I did notice a year or so ago that after the board installed a new meter my bills went down and assumed the old meter was faulty. I worked out I was out of pocket by £1000.00 over many years but couldn't prove it.
Sorry for the digression.

 

[GTL]

have you mistakenly dumped the neutral feed for immersion at the board on the rcd neutral bar?or accidentaly swopped a ring main neutral for immersion neutral.

might be something wonky there mate

Hi daytona,

I understand were you are coming from and had thought this and checked it out. I'll recheck in the morning.

 

[GTL]

you said the flex was damaged on the immersion heater.

have you tried replacing this rather than repairing it.

just trying to start at the beginning

Hi daytona,

Yes, I have replaced this cable to the new immersion with 2.5 T&E Heat Resistant to the 20A switch.

 

[Taylortwocities]

And, has that solved the problem??

 

[GTL]

And, has that solved the problem??

Hi Taylortwocities,

No!

 

[JohnD]

move the cooker to the non-RCD side. this might not be the only fix you need, but it will reduce backround earth leakage through the RCD and reduce nuisance tripping.

it does sound like there is another fault somewhere, might be on old wiring, will be difficult to trace without the right gear. With your multimeter you could start by testing resistance on each circuit between conductors (disconnected from the CU) even without an IR tester you may spot where the prob is coming from

If you can lay your hands on a blank Electrical Installation Certificate you can work through the Tests section and see what you find.

 

[GTL]

move the cooker to the non-RCD side. this might not be the only fix you need, but it will reduce backround earth leakage through the RCD and reduce nuisance tripping.

it does sound like there is another fault somewhere, might be on old wiring, will be difficult to trace without the right gear. With your multimeter you could start by testing resistance on each circuit between conductors (disconnected from the CU) even without an IR tester you may spot where the prob is coming from

If you can lay your hands on a blank Electrical Installation Certificate you can work through the Tests section and see what you find.

Hi John,

Followed your advise and moved cooker to non RCD side.
Disconnected everything else except immersion. Both working fine.
Connected up and down lights and transformer. All working fine.
Connected up and down sockets. All working fine
Connected kitchen sockets. TRIP.
On examining kitchen sockets I found one socket had a wall screw that was not fixed properly and fixed it.
Another single socket has ring and spur for washing machine and seemed to be crammed tight against the back of the switch. I have left this slightly off from the wall and will sort that lot out shortly.
After leaving this switch off the wall everything is up and running with no trips.

Right boys' Pub!

Many thanks to all who gave leads to resolve the problem especially JohnD
Thanks again guys.