Earth leakage current issue (no load connected)

[kr82]

I've come across a problem today that has completely stumped me. I used my clamp meter on the main protective conductor and found there was over 4A flowing. Checking the line and neutral conductors, they each had exactly 2A. Obviously these numbers do not add up as if there was leakage to earth we wouldn't have the line and neutral conductor currents balanced. Things got more strange when I began isolating circuits as I found that even with the main switch turned off I was still seeing a current of over 4A to earth.

Maybe I am overlooking something silly but I can't think of a situation where I can have current flowing in the protective conductor when the board has been isolated unless it is somehow connected to an external source.

Can anybody enlighten me?

 

[Leesparkykent]

What's the earthing arrangement?

 

[kr82]

It's a TNC-S system

 

[davesparks]

It's not uncommon to get a little bit of current flowing between the bonding and the suppliers earth terminal. If the extraneous parts have a very low resistance to earth then with a TNCS supply you may get a little bit of diverted neutral current.

 

[nowaypedro]

I've had exactly the same problem today.

Called out to domestic 3 bed semi by new owners with history of nuisance RCD tripping from day 1 (lived there about a month). TNS. No regular pattern to tripping and most appliances new. First - Ramp Tested RCD unit with no loads = 25mA (can live with that). Popped leakage tester on earthing conductor =0.2A (yes AMP!). Started opening MCBs one by one and finally main incomer - still 0.2A (see photo). Any thoughts? (please ignore the obvious other errors on the board)

 

[mattg4321]

I thought the idea with an earth leakage clamp meter was to clamp L&N together, so you see the difference in current flow, as the RCD would. Clamping the earth doesn't tell the full story.

However I'm willing to be put right on that. I don't own an ELCM yet. Haven't yet failed to find a fault with IR testing and a bit of experience.

 

[nowaypedro]

Thanks Matt. Agree about IR testing however my point on this post is that the meter is still seeing 0.2A in the earth conductor even when there is no potential between L-N in the whole installation. I suspect there maybe current flowing in the system from the DNO's earth and the installations bonding conductors. Might be a red herring when it comes to nuisance tripping tho.

 

[plugsandsparks]

Try clamping the earth in a block of flats, you will see rather large currents buzzing around. TBH clamping earth is largely a waste of time to detect RCD problems. IMHO. Yes if you have large currents passing through earth cables locally this can have an impact but only if the RCD is close to threshold anyway. Just clamp LN and take it from there