Earth Potential Rise - High-voltage Protection of Telecommunication Circuits

High-voltage Protection of Telecommunication Circuits

To protect wired communication and control circuits in sub stations, protective devices must be applied. High voltage can damage equipment and present a danger to personnel. Isolation devices prevent high voltages and currents from propagating from the sub station towards the telephone company's central office. Circuits may be isolated by transformers or using non-conductive fiber optic coupling. Surge arresting devices such as carbon blocks or gas tube shunts to ground do not isolate the circuit but divert high voltage currents from the protected circuit. This type of protection will not fully protect against the hazards of high voltage faults and lightning strikes.

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. In North American practice, the zone of influence is considered to be bounded by the "300 volt point", which is the point along a telecommunications circuit at which the GPR reaches 300 volts with respect to distant earth. The 300 volt point defining a zone of influence around a sub station is dependent on the ground resistivity in ohms, the amount of fault current in amperes. It will define a boundary a certain distance from the ground grid of the sub station. Each sub station has its own zone of influence since the variables explained above are different for each location.

In the UK, the Zone of Influence was historically measured as anywhere within 100m of the boundary of the high voltage compound at a Hot Site. Since 2007, it is allowable to use the Energy Networks Association (ENA) Recommendation S34 ('A Guide for Assessing the Rise of Earth Potential at Substation Sites') to calculate Hot Zone. This is now defined as a contour line marking where the Rise Of Earth Potential (ROEP) exceeds 430V for normal reliability power lines or 650V for high reliability lines. The Zone extends in a radius from any bonded metalwork such as the site earth electrode system and boundary fence. This may effectively reduce the overall size of the Hot Zone. 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, for a width of two meters either side of the conductor. It is the responsibility of the owning Electrical Supply Industry (ESI) to calculate the Hot Zone. Openreach (a BT Group company tasked with installing and maintaining the majority of telephone services in the UK) maintains a Hot Site Register, updated every 12 months by voluntarily supplied information from the ESI companies in the UK. Any Openreach engineer working in such an area must be Hot Site trained.

In some circumstances (such as when a 'cold' site is upgraded to 'hot' status), the Zone of Influence may encompass residential or commercial property which is not within the property of the Electrical Supply Industry. In these cases the cost of 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.