Where isolated premises are connected to a local transformer which provides connection to no other customers, for example, it is possible to use Protective Neutral Bonding (PNB). PNB is not PME, and is not subject to the PME provisions of the ESQCR excepting that Regulation 8(3)(b) makes specific provision for this method of earthing.
In this method of earthing, the LV neutral is not earthed at the transformer, but instead is earthed at the cut-out. No bonding is required. In this way a broken neutral anywhere on the system does not result in elevated potentials on metalwork.
PNB is the preferred method of earthing in these cases, and is particularly effective where livestock is involved. All PNB installations shall be recorded on the mains records.
The resistance to earth of the earth electrode shall be such that twice the current necessary to operate the HV protection will flow following an inter-winding fault on the transformer.
A value of 40Ω or less shall be achieved.