Part P Question and Earthing Issues ?

14 Feb 2006
Reaction score
United Kingdom

Got called by my neighbours tonight as their RCD kept tripping when they turn on high power loads. They had a spark round last week who couldn't find the problem (said it was probably something they had plugged in which had the fault)

After trying to find out what they had plugged in and not getting very far I went through each circuit in turn with a simple continuity test, narrowed it down to neutral-earth fault with a bit of cable between 2 sockets.
As I haven't fully read up on part P / what is notifyable - can I replace a bit of 2.5mm t&e between two sockets on a ring, or is this something thats notifyable ?

As a separate thing - while I was there I noticed the Neutral sat at around 40v to Earth, as they were off out for the night I didn't get a chance to do much other testing on this, although the one quick test I did is that its still 40v with the RCD and isolator off (tested supply side, load side the neutral drops to 0v).
Supply is TT, main earth cable from rod comes in from rod to CU earth block, along with bonds from various pipes.
As people don't get a shock off things that are earthed - could this be a supply neutral problem ? or is there something I've missed ?
Sponsored Links
The first thing I'd want to do is carry out an EFLI test on the earth rod.

Are you in an area that an upgrade of the supply to TN-C-S would be available?

It might be that the voltage between neutral and earth is causing the RCD to trip.

What rating is the RCD which is tripping?
The first thing I'd want to do is carry out an EFLI test on the earth rod.
Its between 10 and 100 ohm according to my socket tester, however a spark is due to come in later today and no doubt will have the correct test gear to give a more accurate reading.
Even if it is 10-100 ohm, does the reading actually mean much ? - I mean if the transformer up the pole isnt properly earthed then every house is going to read high no matter how good their own rods are ? I'll go a check ours !

Are you in an area that an upgrade of the supply to TN-C-S would be available?
I dont know. There are only 6 houses connected to the local transformer and I know at least 4 of the 6 are currently TT, one of which was built within the last few years.

It might be that the voltage between neutral and earth is causing the RCD to trip.
I plugged in my socket tester (one of those little yellow ones to give earth loop impedance, and the 3 led's for fault diagnostics) - the results were rather interesting.
The earth loop on it reads at the 10-100ohm reading, which I seem to remember is outside the regs for MCB (type B) trip times.
The fault LED's read Black-Orange-Green, which isnt on the list of faults, however its a kind of combination between the "open earth" and "phase earth reversal". Its probably not able to read the results correctly with 40v neutral-earth and a bad loop impedance so not supprising its giving funny readings really.
What rating is the RCD which is tripping?
Its a 63A/30mA on the whole board - no split for lighting
Electrician came in this morning - didnt believe me that there was 40v between neutral and earth yesterday ! its not there today !
Today the voltage neutral-earth of their house is around 0.5-1v, as is ours.

He tested the earth rod and said it was "fine".

He also tested the bit of cable that I thought had a neutral-earth fault and said that yes the bit of cable did need replacing.

When I plug in my socket tester today I get a good reading saying the earth loop is 5-10 ohm, and that the polarity/voltage is correct.

All is well .... until that 40v returns !
and when it does the spark told me to measure live-neutral and live-earth voltage to see if its just imbalance of phases, or if its actually neutral floating high (ie making live about 280v compared to earth !)

Just for interests sake - I forgot to ask the spark when he was round - what is the earth loop supposed to be ? is it < 8 ohms ?
Sponsored Links
The maximum allowed for a TT installation is 200 ohms, but ideally it should be below 100 ohms.

Did it rain overnight? This will reduce the resistance of the earth rod, and may have allowed the stray voltage to go to ground.
ah - 100 ohm - socket tester reading 5 to 10 ohm is well within then !

Nope - it didnt rain overnight - looks like it may later today though.
Got a call from neighbours earlier today saying things were tripping the RCD again. After looking, it was the same 40v earth-neutral fault.

This time I tested the Earth-Live (280v), and the Neutral-Live (240v).

Called EDF and reported it as a fault - 45 minutes later someone turns up !!

(apparently if you mention anything like 280v they are out like a shot ;) )

He's tested at substation, which is apparently fine, then tested at each house, which also is fine :confused:

The problem has disappeared again !

Oh the joys of an intermittent fault :evil:

By the way - its been raining most of the day.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local