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THE CPD ZONE
PROTECTIVE MULTIPLE EARTHING (PME) The aim of this article from the experts at NICEIC is to explain the operating characteristics of the protective multiple earthing (PME) earthing facility, a type of TN-C-S system, which is commonly provided for both new and existing electrical installations. n a PME arrangement the supply neutral conductor functions as both protective and neutral conductors and is connected to multiple earthing points along the route of the supply network, as shown in Fig 1. The supply neutral conductor of a PME earthing facility – often referred to as a PEN (protective earth and neutral) or CNE (combined neutral and earth) conductor – is terminated at the distributor’s switchgear (cut-out) where connection of the earthing conductor to the supply neutral is achieved by means of an internal link provided by the distributor. Regulation 543.4.1 states that within an installation PEN conductors shall not be used, except where one of the conditions described in regulation 543.4.2 is met. Therefore, separate neutral and earth conductors must be used within the consumer’s installation. Because the supply neutral is connected to earth in a PME system, the fault return path for both line to earth and line to neutral faults is via the combined
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“... a PME system is not permitted in hazardous locations such as petrol stations where there is high risk of ignition of explosion.”
Fig 1. PME earthing arrangement
Fig 2. Open-circuit in the PEN conductor
conductor. Using the combined conductor in this way has the advantage of providing the return path with a low impedance, therefore allowing for rapid disconnection of circuit protective devices under fault conditions. The Distribution Network Operator (DNO) will typically specify a significantly lower maximum external fault loop impedance Ze for a TN-C-S system than for a TN-S system of the same current rating. The returning current in a PME system has two possible paths, through the combined conductor and the general mass of Earth. Depending on their relative impedances, some current, often referred to as neutral current diversion (NCD) or circulating neutral current, may return via the general mass of Earth. For this reason, a PME system is not permitted in hazardous locations such as petrol stations where there is high risk of ignition or explosion. Other issues relating to a PME system include the following: Perceived shock A small voltage difference may exist between the PME earthing terminal at the origin of the installation and the general mass of Earth under normal conditions and is dependent on the distribution network configuration and loading conditions. This small voltage above Earth potential could, under certain circumstances result in a ‘perceived shock’ for a person simultaneously in contact with an exposed-conductive-part
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