Earth Loop test

[TheBrick]

Hello I would like some help understanding the earth loop test.

I'm only training at the moment but I recently had the opportunity to pick up some testing kit very cheaply so thought I'd give it a whirl as I could then experiment at home getting my head round some of the tests. The insulation test and low ohmic meter seem to be ok from a little experimentation. Not tried the RCD tester yet.

First as I understand it the the earth loop test measures the impedance along the phase to the supply transformer then back though the earth to the point at which the tester is plugged in. I guess this is the Zs of that socket / fused supply point e.t.c.

Please correct me if any of these point are incorrect.

I am also guessing that it obtains this measurement allowing a small current to pass between phase and earth. A friend tested the installation last week with his kewtech and obtained a reading of 15.1 Ohms, apparently still within regs with the limit being 22.0 ohm for TT. Is this correct? (The ground is stony and very dry at the moment).

The earth loop testers is a Di log Lt22 the instruction with the meter refer to the 15th edition regs (see pic). The system I am testing is a TT system. The CU is an old Wylex.

The problem I have is that when ever I try and use the tester the RCD trip out. Is this just because for some reason the loop tester is allowing too large a current to flow between phase and earth, hence RCD tripping? Dose this mean the test is broken? The instruction just say select 0-10 ohms, plug in, check LN and LE neons illuminate, press test button for reading.

I just thought maybe I should be trying 10 -100 ohm to get a reading as I'm on a TT system but if it's tripping it's tripping regardless of if I am managing to get a reading!


Sorry for the long post, I'm just getting my head around some of this testing. Thanks for any help.

 

[Deleted member 26818]

Earth Fault Loop Impedance Testers operate by taking current from the line conductor and sending it down the earth conductor.
If the test is conducted down stream from an RCD, the RCD is very likely to operate.
You will need a setting in milliohms, something like 0 - 10mΩ, in order not to trip the RCD.

 

[TheBrick]

Thanks spins London.

The lead with the unit is a three pin plug. Dose this mean that if I had a lead with test probes I would be-able to obtain Ze by taking a reading by connecting to the phase at the CU and to the the main earthing terminal?

So with this particular meter because it dose not seem to have a 0- 10 milli ohm setting I could not obtain Zs directly without first finding R1 and R2 (there is another meter which I have been able to find R1 and R2 by connection phase and earth at the meter and taking a reading at socket).

The lead with the unit is a three pin plug. Dose this mean that if I had a lead with test probes I would be-able to obtain Ze by taking a reading by connecting to the phase at the CU and to the the main earthing terminal.

 

[Deleted member 26818]

Yes if you had individual leads, you would be able to obtain Ze, and on TT systems Ra.
You would be able to obtain Zs if there was no RCD, I imagine that the tester is quite old, as most now have an No Trip option.

 

[IQ Electrical]

I am also guessing that it obtains this measurement allowing a small current to pass between phase and earth. A friend tested the installation last week with his kewtech and obtained a reading of 15.1 Ohms, apparently still within regs with the limit being 22.0 ohm for TT. Is this correct? (The ground is stony and very dry at the moment).

The 21 Ohm value for a TT system is the limit for the suppliers electrode, not the consumers electrode which is recommended to be below 200 Ohms for stability.

 

[TheBrick]

Thanks spin London that is clearer.

The 21 Ohm value for a TT system is the limit for the suppliers electrode, not the consumers electrode which is recommended to be below 200 Ohms for stability.

Ah thanks, I just reread the note I made my friend did with his tester and it was 151 ohms no 15.1, I think I added in the decimal place because of finding the 21 ohm limit some where and concluding I must have copied down the reading wrong.

21 ohm supplies electrode

220 ohm consumers electrode.

Thanks