Earth leakage

[Dairyspark]

Hi everyone,

So today I finished wiring a DB to supply milking robots, turned it on and everything was going swimmingly till I put my Earth leakage clamp on and found I had 150-170mA on the main DB earth, but yet no rcbo’s tripping, so switched the board off and it was still there so I assumed it was coming from the old wiring or the farm as this is an add on.

Went to the main intake point and found that when I isolated the main isolater so no current was being drawn from the entire farm I was still get this 170mA of current in the earth. When I disconnected my Earth to my new installation it dropped to 95mA then when I disconnected the rest of the original install Earth it went to 0mA.

Now to my question lol, how with zero current draw across the entire install am I getting a leakage current on my earth? Is it on the Scottish Power incoming side or am I being a bit dim and have missed something?

 

[binky]

Capacitors in the equipment discharging perhaps.

 

[UNG]

What type of mains supply do you have and what is the Ze I'm wondering if you have an external earthing issue and you are reading a current that is finding an earth on the installation you are working on
Have you measured the leakage by clamping the live(s) and neutral to compare to the leakage on the earth

 

[Dairyspark]

Capacitors in the equipment discharging perhaps.

Thought that to begin with but it stayed steady over a period of 5-10 minutes

 

[Geoff1946]

Could there be a ground potential, perhaps from nearby distribution transformer, causing current through earth points on your installation?

 

[Dairyspark]

What type of mains supply do you have and what is the Ze I'm wondering if you have an external earthing issue and you are reading a current that is finding an earth on the installation you are working on
Have you measured the leakage by clamping the live(s) and neutral to compare to the leakage on the earth

I have a single phase 80-100A supply with TN-S. Overhead line to transformer then 300mish of underground cable. My Ze is 0.38Ohms

I am hoping it’s an external issue as then it’s SP’s problem

No I haven’t yet, but I will clamp my Line and Neutral tails in the morning when I return. If I’m getting the same am I to assume it’s the installation that’s causing it some how? Or if I have a don’t have a difference between line and neutral does that mean it’s external?

I have advised the farmer to phone SP in the morning for them to attend as something definitely isn’t right

 

[binky]

Thought that to begin with but it stayed steady over a period of 5-10 minutes

It can take a while to discharge large capacitors, and most modern equipment has smoothing circuitry to protect PCB boards, so leak down the earth

 

[Dairyspark]

It can take a while to discharge large capacitors, and most modern equipment has smoothing circuitry to protect PCB boards, so leak down the earth

The only thing that I could think of that would have largeish capacitors would be the VFD for the new vacuum pump but it hasn’t been powered up yet.

 

[Marvo]

Is there a particular reason you did the live leakage test?

You say the leakage is roughly equal from your new installation and the original installation so it's not cause by a single culprit and depending on the circuits and equipment connected I wouldn't consider 150-170mA leakage as particularly high in the first place for a commercial/industrial supply. It's very possible the 150-170mA is functional leakage not fault current and you're chasing your tail. Maybe your UK regs have a limit to global functional leakage but where I am we wouldn't consider that a problem.

 

[Dairyspark]

I did it out of curiosity, I was about to turn items on to load up the board and get a max demand and with no loads on I got 150-170mA on Earth. (I know I don’t need to clamp Earth when doing max demand, I just stuck in on there to see if I was getting anything)

What I can’t understand is when I disconnect the full farm from the power, I’m still getting this current on the Earth at the cable head which made me think it was incoming issue.

Also to whoever asked about my line and neutral I clamped my Line and Neutral tails straight from the cable head yesterday and got 16mA difference.

It may not be considered high for a commercial/industrial supply but if something develops into an actual fault and his livestock start falling over, and I didn’t highlight it at the time, I wouldn’t be very popular

 

[Mike P]

I wonder if it’s some kind ofPhantom reading, a bit like stray voltage and this is caused by high impedance test equipment. As Marvo says, you could be chasing your tail.