TN-CS main bonding cable typical leakage current question.

[w0z]

OK I'm prepared for the usual onslaught so here goes...

I recently bought a clamp meter Uni-T UT210E.
The first thing I did was check it out against my trusted AVOs and also a couple of other cheapo DMM's, (measure current through wire through probes and compare to clamp reading). The clamp is more accurate than I expected even at low currents (milliamps) or at least mostly the meters all agree with each other (although they could all be wrong!)
So in the spirit of playing with my new toy I thought I'd see how my power monitoring meters stood up. I suspected they were under reading, they are. I never believed them at low power anyway and as consumption increases the discrepancy reduces considerably as you would expect but not too bothered about this as there is no means of calibration anyway.

I also measured around the earth cable which is connected from the cutout to the main earthing block, expecting to measure almost zero, but the reading was higher than I expected - (no clues given here as looking for real world earth return figures and opinions). What figure would you typically expect?

Caveat: I admit I didn't isolate everything I should/could have (I had to avoid incurring the wrath of SWMBO, and so I will be repeating test at a later date ) Consequently there were a few 'elderly' earthed appliances which could have had some bearing by leakage or induced currents, but no lights on, led or otherwise.

Opinions gratefully received.

 

[Marvo]

On industrial installations we sometimes see > 20Amps earth current but I'd expect a domestic installation to be less than an Amp.

With a TNCS earthing arrangement you should do the same test with the power switched off at your CU. The current you're seeing probably isn't all from your installation.

 

[w0z]

On industrial installations we sometimes see > 20Amps earth current but I'd expect a domestic installation to be less than an Amp.

With a TNCS earthing arrangement you should do the same test with the power switched off at your CU. The current you're seeing probably isn't all from your installation.

Good point- thanks, will do when circumstances allow.

 

[Wilko]

Interesting experiment
Domestically I expect to see only a few mA per circuit. The domestic additional protection RCDs will trip somewhere between 15mA and 30mA of earth current.
If I may ask - do you have metal gas and water pipes ?

 

[w0z]

Interesting experiment
Domestically I expect to see only a few mA per circuit. The domestic additional protection RCDs will trip somewhere between 15mA and 30mA of earth current.
If I may ask - do you have metal gas and water pipes ?

Thanks, yes, both metal.
It may be a while before I can de-energise all and measure again.

 

[Lucien Nunes]

Agree with Marvo, there can be all sorts of external circulating currents from your main EB passing through the EC, that's a fact of life with TN-C-S and one of the reasons larger bonding cables are specified. You might see anywhere between zero and tens of amps under normal conditions, there are so many variables it's hard to predict. In any case it bears no direct relation to your leakage current, in fact if you put heavy load on you will probably find it correlates more with the load current.

 

[w0z]

Interesting experiment
Domestically I expect to see only a few mA per circuit. The domestic additional protection RCDs will trip somewhere between 15mA and 30mA of earth current.
If I may ask - do you have metal gas and water pipes ?

I forgot to add that of course the RCD won’t trip unless it measures a L/N imbalance, irrespective of earth leakage (unless the leakage is causing the imbalance) ...apologies for stating the bl**din’ obvious!

 

[Lucien Nunes]

The imbalance is the earth leakage, they are the same thing by definition (at least for all practical purposes). It's the earthing conductor current that is a different kettle of fish and that is what you are measuring here. It consists of the small leakage current from your own installation (that the RCD can detect, as it results in a differential current in the circuit) plus any currents passing to extraneous parts via both the bonding and CPCs. Although the latter are only driven by low voltages, typically the few volts of drop along the distribution and service cables, they are passing through a very low loop resistance maybe of fractions of an ohm, so the currents can dwarf the leakage current by orders of magnitude.

 

[w0z]

The imbalance is the earth leakage, they are the same thing by definition (at least for all practical purposes). It's the earthing conductor current that is a different kettle of fish and that is what you are measuring here. ....

Thanks, yes perhaps I didn't put it well but that was precisely the point I was trying to make - that extraneous current measured on the earth conductor is not necessarily the result of L/N imbalance in the property.