Earth potential rise
The resulting potential rise can cause hazardous voltage, many hundreds of metres away from the actual fault location.
An EPR event at a site such as an electrical distribution substation may expose personnel, users or structures to hazardous voltages.
An engineering analysis of the power system under fault conditions can be used to determine whether or not hazardous step and touch voltages will develop.
Several methods may be used to protect employees from hazardous ground-potential gradients, including equipotential zones, insulating equipment, and restricted work areas.
The creation of an equipotential zone will protect a worker standing within it from hazardous step and touch voltages.
The surface layer provides a high resistance between feet and the ground grid, and is an effective method to reduce the step and touch voltage hazard.
Difficulties in calculation arise from the extended and irregular shape of practical ground grids, and the varying resistivity of soil at different depths.
The simplest case of the potential at a distance is the analysis of a driven rod electrode in homogeneous earth.
It can, however, reliably be said that the resistance of a ground grid is inversely proportional to the area it covers; this rule can be used to quickly assess the degree of difficulty for a particular site.
80-2000 is a standard that addresses the calculation and mitigation of step and touch voltages to acceptable levels around electrical substations.
This protects equipment and personnel who might otherwise be exposed simultaneously to both ground potentials, and also prevents high voltages and currents propagating towards the telephone company's central office or other users connected to the same network.
Telecommunication standards define a "zone of influence" around a substation, inside of which, equipment and circuits must be protected from the effect of ground potential rise.
[4] The 300 volt point defining a zone of influence around a sub station is dependent on the ground resistivity and the amount of fault current.
Since 2007, it is allowable to use the Energy-Networks-Association (ENA) Recommendation S34[6] ('A Guide for Assessing the Rise of Earth Potential at Substation Sites') to calculate the Hot-Zone.
The 'Zone' extends in a radius from any bonded metalwork, such as the site earth electrode system or boundary fence.
However, strip earth electrodes, and any non-effectively insulated metallic-sheath/armouring of power cables which extend out of this zone would continue to be considered as 'hot' for a distance of 100m from the boundary, encompassing a width of two meters on either side of the conductor.
Openreach (a BT Group company tasked with installing and maintaining a significant majority of the physical telephone network in the UK) maintains a Hot-Site Register, updated every 12 months by voluntarily supplied information from the ESI companies in the UK.
In these cases the cost of retroactively protecting each telephone circuit may be prohibitively high, so a drainage electrode may be supplied to effectively bring the local Earth Potential back to safe levels.
[1] ACIF Working Committee CECRP/WC18, AS/ACIF S009:2006 Installation Requirements for Customer Cabling (Wiring Rules), Australian Communications Industry Forum, North Sydney, Australia (2006) ISBN 1-74000-354-3
