Table 1: Minimum csa required for main bonding conductors, except where PME conditions apply Size required for the earthing Size required for copper main conductor bonding conductor mm2 mm2 10 6 16 10 25 16 ≤35 25
PME R O F NOT
practicable the connection is to be made with 600 mm of the meter outlet or union or at the point of entry to the building if the meter is external. Sizing of main bonding conductors Except where PME conditions apply (as is usually the case in a TN-C-S system), Regulation 544.1.1 requires the main bonding conductors to have a cross-sectional area (csa) of not less than half that required for the earthing conductor of the installation and not less than 6 mm2. The csa need not exceed 25 mm2 for copper conductors or a csa affording equivalent conductance in other metals. These requirements are summarised in Table 1 of this article. For installations forming part of a TN-C-S system and having a PME supply, Regulation 544.1.1 requires that csa of the main bonding conductors to be selected in accordance with the neutral conductor of the supply (the combined protective and neutral (PEN) conductor) and Table 54.8 of BS 7671. Table 2 of this article is based on the requirements of Table 54.8. It will be seen by comparing Table 2 with Table 1 that where PME conditions apply, main bonding
conductors are generally required to have a larger csa than where PME conditions do not apply. This is because, where PME conditions apply, the bonding conductors may have to carry diverted neutral currents for long periods in the event of a break occurring in the supply neutral (PEN) conductor. For highway power supplies or street furniture forming part of a TN-C-S system where the supply is PME, Regulation 544.1.1 does not require the csa of any main bonding conductors to be selected in accordance with Table 54.8 as described above (see Table 2). In these cases, the main bonding conductors should be sized in accordance with the requirements of the distributor. Installations serving more than one building Where an installation serves more than one building, the main bonding requirements of Regulation 411.3.1.2, described earlier, are required to be applied to each building (Regulation 411.3.1.2 refers). Main bonding requirements for an installation that serves more than one building will be covered in more detail in a future issue of Connections.
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Table 2 – Minimum cross-sectional area required for the main bonding conductor in relation to the neutral (PEN) of the supply where PME conditions apply (based on Table 54.8 of BS 7671) Copper equivalent csa of the supply Applicable sizes of neutral (PEN) Minimum copper equivalent* csa of neutral (PEN) conductor conductor the main bonding conductor mm2 mm2 mm2 ≤35 6, 10, 16, 25, 35 10 >35 up to 50 50 16 >50 up to 95 70, 95 25 >95 up to 150 120, 150 35 >150 185, 240, 300 etc 50 Note: Local distributor’s network conditions may require a larger main bonding conductor * The minimum copper equivalent csa is given by a copper conductor of the tabulated csa or a conductor of another metal affording equivalent conductance.
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