Protective multiple earthing (PME) system: an earthing arrangement, found in TN-C-S systems, in which the supply neutral conductor is used to connect the earthing conductor of an installation with Earth, in accordance with the Electricity Safety, Quality and Continuity Regulations (ESQCR) (see also Figure 1) [this term is defined in the BS7671]. This term is often used in the United Kingdom.
The TN-C-S system is also known as a “protective multiple earthed” (PME) system. The PEN conductor, which is referred to as a combined neutral and earth (CNE) conductor, is earthed at the source and extremities of the distribution mains and points in-between; hence the reference to multiple earthing. Multiple earthing of the CNE conductor ensures that if the conductor becomes open-circuit for any reason, exposed-conductive-parts remain connected to earth; under such conditions the supply voltage between the installation line and neutral conductors is substantially reduced and consumers will experience unacceptable voltage variations.BS 7430:2011+A1:2015
PME can be installed by the Electricity Supply Company only after the supply system and the installations it feeds must have complied with certain requirements.
The great virtue of the PME system is that neutral is bonded to earth so that a phase to earth fault is automatically a phase to neutral fault. The earth-fault loop impedance will then be low, resulting in a high value of fault current which will operate the protective device quickly.
It must be stressed that the neutral and earth conductors are kept quite separate within the installation: the main earthing terminal is bonded to the incoming combined earth and neutral conductor by the electricity supply company. The difficulty of ensuring that bonding requirements are met on construction sites means that PME supplies must not be used. Electricity supply regulations forbid the use of PME supplies to feed caravans and caravan sites.
PME Basic Requirements
- Where an installation has more than one source of supply to which PME conditions apply, a main protective bonding conductor shall be selected according to the largest PEN conductor of the supply [544.1.1, BS7671].
- Where a PME earthing facility is used as the means of earthing for the electrical installation of a swimming pool or other basin, it is recommended that an earth mat or earth electrode of suitably low resistance, e.g. 20 ohms or less, be installed and connected to the supplementary protective equipotential bonding [702.410.3.4.3, BS7671].
- A PME earthing facility shall not be used for the means of earthing for an installation falling within the scope of this section unless all extraneous-conductive-parts are reliably connected to the main earthing terminal in accordance with Regulation 411.3 .1.2 .
- Unless a metal grid is laid in the floor, the use of a PME earthing facility as the means of earthing for the electrical installation is not recommended [705.415.2.1, BS7671].
- The Electricity Safety, Quality and Continuity Regulations (ESQCR) prohibit the connection of a PME earthing facility to any metalwork in a leisure accommodation vehicle (including a caravan). This does not preclude the use of a PME earthing facility as the means of earthing for other purposes, such as to the installations of permanent buildings. [708.411 .4, BS7671].
- Socket-outlet protective conductors shall not be connected to a PME earthing facility [708.553.1.14, BS7671].
- Where an earth connection to a distributor’s PME network has been provided for a street electrical fixture, the earthing and bonding conductor of a street electrical fixture shall have a minimum copper equivalent cross-sectional area of 6 mm2 for supply neutral conductors with copper equivalent cross-sectional areas up to 10 mm2. For larger sized supply neutral conductors the main bonding shall comply with Table 54.8 .
- A PME earthing facility shall not be used as the means of earthing for the protective conductor contact of a charging point located outdoors or that might reasonably be expected to be used to charge a vehicle located outdoors unless one of the methods specified in [722.411.4.1, BS7671].
- Where the PME supply is extended to a detached outbuilding such as, for example, from a house to a detached garage, these bonding requirements will also need to be applied to any extraneous-conductive-parts in the garage. Given such circumstances, it is advisable to use a separate (TT) earthing arrangement for the installation in the detached building.
- An alternative method of supplying and protecting street furniture that may be used is by means of a TN-C-S system. In such cases a combined neutral and earth conductor cable may normally be used at the source of supply (for example to an individual lighting column or for larger installations a local supply authority PME supply into a feeder pillar, with cables using separate line, neutral and circuit protective conductors to feed individual items of street furniture as might be used on footpaths, or to feed items of street furniture in the carriageway) [220.127.116.11, BS7430].
- In the case of circuits feeding more than one item of street furniture, e.g. by looping using a cable with separate line, neutral and protective conductors, an earth electrode should be installed both at the point of supply and at the last or penultimate unit and this electrode should be such as to make the resistance to earth of the neutral at any point less than 20 Ω before the connection of any circuit protective or bonding conductors to the neutral terminal. If a single electrode produces a resistance of more than 20 Ω, other earth electrodes equally spaced along the circuit should be installed.
TABLE 54.8 – Minimum cross-sectional area of the main protective bonding conductor in relation to the PEN conductor of the supply :
|Copper equivalent cross-sectional area of the PEN|
|Minimum copper equivalent* cross-sectional area of the main protective bonding conductor|
|35 mm2 or less||10 mm2|
|over 35 mm2 up to 50 mm2||16 mm2|
|over 50 mm2 up to 95 mm2||25 mm2|
|over 95 mm2 up to 150 mm2||35 mm2|
|over 150 mm2||50 mm2|
* The minimum copper equivalent cross-sectional area is given by a copper bonding conductor of the tabulated cross-sectional area or a bonding conductor of another metal affording equivalent conductance.