PURPOSE AND SCOPE

National Safety Instruction four (NSI4) provides guidance to persons Senior Authorised under the National Grid safety rules with respect to minimum earthing requirements to help achieve safety from the system for specific work schemes on overhead line circuits. The application of drain earths is principally used to manage voltage induced by an adjacent energised circuit, usually the parallel circuit of a double circuit route. When drain earths are applied at two physically separate points on the same phase conductors and there is an adjacent power carrying parallel circuit an induced current will circulate in the loop formed by the phase and earth conductors and the drain earths.

The magnitude of the inducted current is dependant on the power flow in the parallel circuit the tower geometry and the tower earth resistances.

NSI4 defines two purposes of earthing: earthing to manage and control induced voltages at the point of work and earthing to manage and control induced currents on the circuit being worked on.

Currently, based upon the maximum possible circulating current, two drain earth applications or Drain Earth Shorting Schemes (DrESS) are employed: a single DrESS rated at 450 A and a double DrESS rated at 900 A. The double dress is employed where there are two or more sources of induction in parallel with the earthed circuit and therefore potential for a doubling of the induced current close to the point whether the source of induction changes. Historically these circuits have been classed as complex.

The use of the double DrESS significantly increases the manual handling and working at height exposure of overhead linesmen with the application and removal of a double DrESS taking 7 mandays, increasing the length of the circuit outage and therefore the possibility of incurring additional system management costs.

The rating requirement for drain earths used to control induced currents was based upon what, at the time of study, was anticipated to be the highest power flow in the parallel circuit (see References). This was deemed to be the rating of twin 30.5 ηΩ.m redwood (850 mm2) conductor operated at 90ºC i.e. 3190 MVA and gave a rating of 450 A. This requirement was then applied universally.

Due to the additional health and safety risks and the implications for outage length and system access a review of complex circuits was undertaken to identify those where there is a realisable risk that current doubling would result in a flow in excess of 450 A. This report details the findings of that review.