Earth Loop Resistance

[electrix]

In the U.K. the earth loop resistance is normally measured. Can anyone advise me if this value is the same as the electrode resistance at a building, assuming a TT system of earthing and only the electrode is used for earthing. If not is there an arithmetic relationship.

 

[welshwizard]

If you measure Ze on a TT system then you are taking into consideration the value of the earth electrode

General comment
Typical values for Ze quoted by UK REC's
TT system 21ohms
TN-C-S (PME)system 0.35ohms
TN-S (Cable sheath earth) system 0.8 ohms

If you carryout a loop test at the origin of the installation using a line/earth loop tester between the phase conductor and the earth electrode - with the test link open -the resulting impedance is taken as the electrode resistance

See BS7671 Reg542-02-02 & Reg412-02-20 and BS7671 in general for a more detailed answer

 

[lyledunn]

Welshwizard,
I know value of 21ohms is quoted by REC, but this relates to the impedance of their equipment rather than a typical value of earth electrode resistance. The crucial point is to ensure that for normal TT installations the voltage drop due to the current causing the effective operation of the protective device (be it a fuse, mcb or rcd)flowing via the cpc and earth electrode does not exceed 50v. (25v in some special locations). This is further qualified by GN3 which advises earth electrode resistances in excess of 200 ohms are regarded as unstable. I understand that an engineerig recommendation placing a limit of 100 ohms is being discussed for these installations.
To answer the original question, no it is not the same as the electrode resistance would not include the tx winding, the phase conductor impedance internal and external,nor the cpc impedance. Given the arbitrary nature of electrode resistance I dont think a mathematical link could be established other than at each individual installation.
Welshwizard is right, generally given the practical difficulties most electricians just opt for an assessment using a loop tester. Regards,

Lyledunn