Earth rod resistance

The 20 ohm figure is sometimes seen as the limit for the LV earth electrodes, when HV side (transformer core and box) and LV side (neutral star point) earths are well separated - such a site could be "hot" in the sense that during an HV to transformer case fault, the rise of voltage may be quite a bit more than the LV network would reasonably tolerate during the short time while the HV side ADS operates.  This tends to be the case for rural pole pig transfomers supplying a few farms, not your situation. The HV earth is a wire running down the pole with the transfomer on it, the LV earth is the wire running down from the neutral line at the next pole along, or an equivalent.

The 20 ohm figure  appears in some of the distribution code guidance for DNOs.


The one ohm figure is about right for a 'cold' site, where HV side and LV side earthing are inter-connected or very close, and  for a moderate HV side prospective fault current.

However, if you are on a 33kV line, or the 11kV line is short and fat for some reason, so the prospective fault current into terra-firma while the HV side disconnects is higher than a few hundred amps, then less than an ohm may well be needed, or in some cases more may be OK - it is only a guide.

Note that underground HV feeds that bring their own earthed armour also need treating a bit differently, as the prospective fault current is then quite a bit higher.

The 20 ohm figure is sometimes seen as the limit for the LV earth electrodes, when HV side (transformer core and box) and LV side (neutral star point) earths are well separated - such a site could be "hot" in the sense that during an HV to transformer case fault, the rise of voltage may be quite a bit more than the LV network would reasonably tolerate during the short time while the HV side ADS operates.  This tends to be the case for rural pole pig transfomers supplying a few farms, not your situation. The HV earth is a wire running down the pole with the transfomer on it, the LV earth is the wire running down from the neutral line at the next pole along, or an equivalent.

The 20 ohm figure  appears in some of the distribution code guidance for DNOs.


The one ohm figure is about right for a 'cold' site, where HV side and LV side earthing are inter-connected or very close, and  for a moderate HV side prospective fault current.

However, if you are on a 33kV line, or the 11kV line is short and fat for some reason, so the prospective fault current into terra-firma while the HV side disconnects is higher than a few hundred amps, then less than an ohm may well be needed, or in some cases more may be OK - it is only a guide.

Note that underground HV feeds that bring their own earthed armour also need treating a bit differently, as the prospective fault current is then quite a bit higher.