If this is a TT system for the charge point, then normal BS 7671 rules apply (exact value depends on a number of factors - mostly RCD sensitivity e.g. 1667Ω for a 30mA device) but good practice and the need to ensure reliability would normally reduce that to below 200Ω. Some EVs apparently insist on a lower value (e.g. Zs < 100Ω) for correct operation (<100Ω is normal for certain parts of the continent (e.g. France where the incomer includes a 500mA RCD)). In most soils a 4' rod or two seem to suffice.

If it's an additional electrode on a PME system attempting to keep touch voltages down in the event of a break in the supply PEN, you're probably onto a loser (but A722.3  can give you the figures).

If it's a larger install, say with its own HV transformer, then it's a different kettle of fish altogether - you'd be looking at something <20Ω for an LV source earth, much less again if it has to be a combined HV & LV earth. The local DNO tend to have their own rules about such things.

   - Andy.

If this is a TT system for the charge point, then normal BS 7671 rules apply (exact value depends on a number of factors - mostly RCD sensitivity e.g. 1667Ω for a 30mA device) but good practice and the need to ensure reliability would normally reduce that to below 200Ω. Some EVs apparently insist on a lower value (e.g. Zs < 100Ω) for correct operation (<100Ω is normal for certain parts of the continent (e.g. France where the incomer includes a 500mA RCD)). In most soils a 4' rod or two seem to suffice.

If it's an additional electrode on a PME system attempting to keep touch voltages down in the event of a break in the supply PEN, you're probably onto a loser (but A722.3  can give you the figures).

If it's a larger install, say with its own HV transformer, then it's a different kettle of fish altogether - you'd be looking at something <20Ω for an LV source earth, much less again if it has to be a combined HV & LV earth. The local DNO tend to have their own rules about such things.

   - Andy.

As always Andy, answered everything in one hit there, brilliant and huge thanks. 

Is there a way to give an approx estimate on the size of the lattice Mat needed based on an estimate conductivity of the soil (say dry and wet conditions) and the Surface area of the mat. I can't seem to find anywhere that gives values of conductivity / resistance of soil in certain conditions. 

Soil conductivity is horribly variable - over 100:1 variation between dry sandy soils and wet clays. There are figures in things like Mil Handbook 419  that you do not need to pay for and are essentially the same as those in the 'official texts' and also cover a wide range of electrode configurations and geometries.

The easiest way to measure the soil conductivity is to drive in electrodes and measure them ;- )

Mike