There are three types of earthing arrangements commonly used within the UK:
TNS – Whereby the neutral and main earth conductors are separated at the supply of the premises; the main earth is normally in the form of the metallic sheath of a supply cable.
TN-C-S- Commonly referred to as ‘PME’, an arrangement whereby the neutral and main earth conductor are combined within the cut out of the supply cable to the premises.
TT – A system whereby the main earth conductor is provided in the form of an earth electrode, commonly an earth spike(s) or a buried copper plate due to the lack of supplier earth.
Whereas an external earth loop impedance reading (Ze) can be ascertained for both TN systems, an RA value is the preferred method for a TT system as the result will be more accurate and far less susceptible to electrical noise; RA values are obtained using an earth resistance tester.
Earth resistance testers are not only used to measure earth resistance, but they can also be used to determine circuit/system current, earth leakage current, static electrical charges and the resistance of a complete earth system. It is common to use an earth resistance tester to find a low resistance area for the installation of an earth rod, or perhaps even to test the capability of a building’s lightning protection system.
There are a variety of accessories available for the different types of earth testers that will expand the overall testing capabilities of the instrument. These include earth spikes, cable reels and attachable optional clamps for stakeless measurements.