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.
In 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 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 or extraneous-conductive-part and ‘Earth potential’. Such effects may occur in locations where a reduction in body resistance is likely due to the presence of water such as a shower area within a sports facility.
Open-circuit PEN conductor
An open-circuit in the PEN conductor within the network, as shown in Fig 2, can cause the combined neutral/earth terminal at the consumer’s cut-out to rise above Earth potential, due to its carrying of load currents from installations downstream of the open-circuit. Consequently, the protective conductors at the main earthing terminal and subsequently all other protective conductors within the installation are also likely to rise in potential.
Similarly, any metallic parts, such as gas and water pipework connected within the consumers’ installation are also likely to rise above Earth potential, creating a shock risk to any person in simultaneous contact with such parts and the general mass of Earth.
There is also the potential risk of fire and thermal damage to cables, in particular, flat twin and earth cables where the non-insulated protective conductor typically has a smaller cross-sectional area (csa) than the associated live conductors.
For this reason, it may be inappropriate to use a PME system arrangement in some premises and prohibited in certain other installations. For example, regulation 9(4) of the Electricity Safety Quality and Continuity Regulations 2002 (as amended) prohibits the connection of a combined neutral and protective conductor to any metal work in a caravan or boat.
However, connection to the PME terminal is permitted for fixed buildings of the location such as offices, restaurants or shops, although it must not be used for the caravan or boat mooring supplies.
To reduce the risks linked with a PME, the combined conductor is earthed at multiple points along the network and bonding is provided within the consumer’s installation, in accordance with BS 7671.
Bonding conductors
Due to the low earth loop impedance and increased fault currents within a consumer’s installation, typically associated with PME, all main protective bonding conductors must be sized in relation to the PEN conductor of the supply and Table 54.8 of BS 7671, reproduced in Table 1. However, in some cases the local distribution network conditions may require a larger conductor than those listed in Table 1.
Note: Table 54.8 of BS 7671 is also applicable for a PNB system arrangement, a variant to the PME system.
Where the PME supply is extended from a dwelling to a detached outbuilding, such as a garage, these bonding requirements will also need to be applied to any extraneous-conductive parts within that area.
Due to the practicalities for satisfying these bonding requirements at the load end the contractor may decide to convert the earthing arrangement within the garage to a TT system incorporating an earth electrode (542.1.2.3) combined with an appropriate RCD (411.5.2).
Although the TT system earthing arrangement provides less onerous conditions for bonding than that associated with PME, it also presents its own set of issues. There is a potential risk of causing damage to other buried services when installing earth electrodes. In addition, the difficulties in providing suitable electrode separation from other buried metalwork connected to the supply PEN conductor.
Summary
Although the distributor may have initially provided a PME earthing arrangement for an installation, where an addition or alteration is to be made it is the responsibility of the installation designer to assess its suitability and adequacy for the altered circumstances. Where necessary, alternative means of earthing, such as a TT system earthing arrangement, may be used.
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