Worrying earth problem

Davisonp · Jan 13, 2012

Hi all,

I have been looking into a suspected earth leakage problem.
I have purchased a Metrel earth leakage clamp, and started by measuring what I expected would be the earth leakage on the consumer unit by clamping the main earth into the board. The clamp metre was fluctuating rapidly and reading between 100mA and 150mA, so that confused me as at least one of the RCD’s should have tripped with that much leakage.
Then I clamped the Gas and Water bonding. One read between 100 and 150mA, the other read between 200mA to 240mA.
Then I clamped the main distributor’s earth lead from the incoming supply to the MET, and this time I read between 1000mA and 1250mA.
I switched off the consumer unit main switch and set the clamp meter to read amps not milliamps, and even with the consumer unit turned off it still read 1.2 amps.
I tried my Megger clamp meter and that confirmed that with the consumer unit main switch turned off, there is about 1.2A flowing through the main earth cable.

Later I tried the Metrel meter at my house and it read a steady 5.5mA current through the main earth between the supply earth and the MET. I then turned off my home consumer unit and the reading dropped to 0.5mA, which I assume is background and nothing to worry about.
I also tried plugging my kettle into my PAT tester and clamped the Metrel meter over an exposed earth wire (in an adapted extension lead) and the PAT tester said the leakage was 0.55mA using a differential calculation, and the Metrel meter said it was 0.47mA, so it only differs by 0.08mA so it looks like the Metrel meter is working correctly and reasonably accurately, and there really is 1.2 amps coming from somewhere in my client's house.

I am not sure what to do now. Does this mean there is a problem with the distributor's earth? Or am I misinterpreting the results?
Is this situation as dangerous as I believe it is?

WDMDL · Jan 17, 2012

For example with a pair of semi-detached houses, it is common to see looping through.
It's very easy to spot... if your service head has two cables, one will be the incomer and the other a loop to next door.
If your service head has only one cable, a quick nosey next door is required to see if theirs has two...

Simon.

Davisonp · Jan 24, 2012

Final update.

I sent an email to the customer services with a photo of 1.2 Amps coming down the earth cable with two direct questions. Is this safe? And who is responsible for making it safe.

[ElectriciansForums.net] Worrying earth problem

The next morning I got a phone call from the distributors top trouble shooter who wanted to investigate himself.

He had some difficulty in identifying where the current was coming from, and tried PME’ing the supply 50 meters from the house, but when that did not change anything he came back again the next day with several new ideas.

What they actually did was:

Pull the distributors fuse, then separate their earth cable, from the MET.
With a multimeter, they measured the Amps between their earth and the supply neutral. It measured 1mA.
They then did the same between the MET and the supply neutral. 850mA this time.
Now, they connected the supply earth to the supply neutral, thus giving a very good return path and measured the current with a clamp meter. If there is current coming down their earth, it will flow directly down the neutral. The current was measured with a clamp meter and it measured 0.0mA.
Lastly, they connected the MET to the supply neutral, and this time they found that there was a current of over 3 Amps (The neutral has a reduced impedance compared to the earth so the current would be greater).
This proves the current is actually coming from outside, through the house system and down the supply earth to the transformer.

Basically, someone in the area has a bad earth connection and is sending current through the ground, and this house has such a good connection to the ground that the current is using the house system as a return path to the transformer, but the voltage drop means the 1.2 amps is at less than one volt.

The Trouble Shooter put another PME connection in at the junction to the main power cable in the road, and was thinking about turning the house into a proper PME system, but eventually he decided the added dangers of PME were not justified in this situation, so the 1.2 Amps measured on the supply earth was actually less than 1 volt and does not pose any danger.

NOTE: The Trouble Shooter is going to retire in June, and has been working on the distribution network for over 50 years. In all that time he has never come across a situation like this. It appears to be a one off.