The loop tester measures the full loop, i.e. The electrode to terra-firma mass of earth at the substation or in the street, in series with the transformer winding, the live cable between your building and the transformer,  and the wiring and then the resistance between your local electrode and mass of earth as well.

But the assumptipon is that resistance of the mass of soil and rock, dominates by at least a factor of ten, and maybe a factor of a hundred or more over the copper resistances.

(The substation electrodes may be no more than 20 ohms each and in practice single figure ohms are more likely at the DNO end, if your local electrode is a single rod it could be a hundred ohms, it maybe in the low tens. Do not expect much less than that unless you have buried a large grid or are  living in salt marsh.)

So although the loop tester measures the loop - well it would! - the error introduced by calling all of that reading your electrode resistance is small compared to the variation between dry and rainy days.

If you really want to verify the electrode resistance in isolation then you need a tester with additional electrodes - one to measure the potential of the earth far away from your electrode under test, and another to inject current, that way there is no error at the voltage measuring electrode as it is passing no current, and no error at the current injection electrode, as we do not measure the voltage at that point.

Such tests are quite fun and can be informative, depending as they do on soil types and many other parameters but doing the test that way is rather more faff than can be justified on a simple job where there is already a supply and the exam question is not one about the geology, but just if the RCD will trip when is is supposed to.

Mike/

As Mike says ... Z_e measured to the earth electrode on its own is required to verify loop impedances for ADS ... BUT it's also worth remembering that this should NOT be used as the prospective earth fault current, which could be considerably higher when extraneous-conductive-parts are connected to the MET via bonding, along with any earthed equipment with exposed-conductive-parts in contact with the ground.

The usual conventions are a little muddled - by the definitions, the customer's electrode is part of the consumer's installation, so is not external to it, so Ze does NOT include Ra (although it would, for TT, include the supplier's electrode(s) - Rb).  Hence you get things like statements like the max Ze for TT systems is 21Ω yet Ra can often be in the region of 200Ω.

Really things like Zs = Ze + R1+R2 should only be done for TN systems, originally the requirement for TT systems didn't depend on the loop impedance, just Ra (which technically actually included R2 already).

The point about using a loop meter is that it includes all of Ra ... plus extra bits for the supplier. As the supplier's bit are never going to provide a -ve impedance, if the loop meter gives you an acceptable number, you can be sure that Ra will be less than that - so always compliant. So is a quick, easy but still clean, method of showing compliance. The downside is that if the loop meter shows a value that's just a little too high, you can't then tell if it complies or not - the extra bit could be down to the supplier's side in which case Ra would be OK, or it might be down to Ra in which case it would be a fail - but you can't really tell using a test meter - so either just assume the worst or get hold of a more expensive instrument. In the real world though, electrodes buried in the ground are quite variable beasts - actual resistance figures varying considerably with the weather and season so any practical design and installation will have to include very substantial safety margins anyway - so the difference between loop figures and Ra are usually smaller than would be significant anyway.

            - Andy.

Hi Mike thank you for your detailed explanation. It is very helpful.  

Thank you Graham

Thank you Andy. In this case of TT earthing system, is it still necessary to add a "Ze" value on the test certificate if it is not really known unless we could deduct the actual Ra from the loop measurement? Basically we only need to assess In x Ra < 50V. Should we just write down "max 21ohm" or just "n/a" for the Ze? I have also seen people using the loop test results (external impedance + Ra) for the Ze value on the test certificates, which is not correct based on your explanation above. many thanks 

Without a second measurement electrode, you can only honestly say that the Ze is only 'less than' whatever the loop test reported...
Of course the high resistance may not be where you think it is, and then it gets dangerous, though sometimes it is the neighbours in danger..- read this story ..

 RCD failure causes shock in neighbours house 

Mike

mapj1 said:

Without a second measurement electrode, you can only honestly say that the Ze is only 'less than' whatever the loop test reported...

Although, in retrospect, isn't this the case with the majority of earth electrode measurements, one way or another, because they contain some element of another electrode in the loop - except for fall of potential method, which if you can find the actual sweet spot, OK ... but in reality the measurement is affected by external influences of one sort or another.

I think one of the best methods overall (considering safety as well as measurement efficacy) is the 2-clamp method ... but again the reading includes the parallel effective earth electrode resistances of the parallel paths back to MET.

Ding Wang said:

Thank you Andy. In this case of TT earthing system, is it still necessary to add a "Ze" value on the test certificate if it is not really known unless we could deduct the actual Ra from the loop measurement? Basically we only need to assess In x Ra < 50V.

Where RCDs are used for fault protection, compliance with Regulation 411.5.3 is demonstrated by earth fault loop impedance Z_s being less than the values in Table 41.5. Similarly, if an OCPD is used for ADS in TT systems, Note 1 to Regulation 411.5.2 tells us to assess Z_s. So I would argue that earth fault loop impedance Z_s is what's verified (assessed) in TT systems, as required by BS 7671, and we do NOT directly verify (assess) either I_Δn×R_A<50 V or I_n×R_A<50 V.

I agree that strictly Z_e for a TT system does not include the consumer's earth electrode resistance R_EE, because the electrode is part of the installation, and the table of Symbols in Part 2 tells us that Ze is external to the installation.But that doesn't make Z_e less important to the installation, and verifying (Z_e+R_A)

However, for verification, as discussed above we have to verify Z_s (where RCDs are used for ADS, this is required to be less than the values in Table 41.5). Hence, if Z_e is measured to include R_EE, the formula Z_s=Z_e+(R₁+R₂) could still be used rather than measuring Zs directly. (Noting that, for armoured cables, for  the measured values would need modifying by the relevant formula from PD IEC/TR 50480 to get the value of (R₁+R₂) to use, see Guidance Note 6 and OSG ... and also noting that the armour needs a different temperature correction factor to copper conductors in SWA).

If you wanted to be more factual regarding the measured value, why not record '(Includes R_EE)' next to the value of Z_e recorded on the certificate?

Hi Graham thank you for your explanation. It is now clear to me. We will need to meet both 411.5.3 (i) and (ii). For a site with earth leakage protection, 411.5.3 (i) is very easy to meet even considering the worst Ze (as high as 21 ohms, thus conservative Zs). We will also use Ra (measured from the loop tester -> Ze + Ra) to assess if we meet 411.5.3 (ii). We will put down the measured Ze+Ra against Ze on the certificate and note down that it includes Ra.