This is the method that we use to ensure a higher accuracy on very low Ze readings.
At first glance, a difference of say 0.09 Ohms does not seem much on a Ze reading but when this value is used (by the instrument or by calculation) to determine PFC then the difference can be massive.
For example: 230/0.10 = 2.3KA 230/0.01 = 23KA
It goes without saying that any connections must be made safely and live testing carried out with equipment in accordance with GS 38.
1 Find a suitable impedance to be used-this should be around about 0.5 Ohms, R2 leads are good for connectivity reasons.
2 Find an outlet on the installation (fused connection unit, socket outlet etc.) that is likely to have a Zs of around 0.20 Ohms.
3 Measure that Zs with the instrument test leads (not the BS 1363 plug lead) and note the value.
4 Measure the Zs again but with the selected impedance (SAFELY) in series with the earth test lead.
5 Subtract the value measured in step 3 from that measured in step 4 and you now have a value for your test impedance (only for that particular installation).
6 Now return to the origin and measure Ze with the test impedance in series with the earth lead.
7 Subtract the value obtained in step 5 and you now have a far more accurate figure for Ze.
Obviously ambient temperature, harmonics, transformer noise etc. all affect readings but unless you have the transformer impedance and the details of the distribution circuit length and CSA etc. then this is about as accurate a reading as you can hope to measure!