Earthing and Bonding
E 4.5
Earthing conductor The earthing conductor connects the system earth bar or main earthing terminal (MET) to the system means of earthing (see Figures E 3.1 and E 6.5). It is sized as for other protective conductors, namely either by the adiabatic equation: I t S= √ k or by using Table 54.7 of BS 7671; this is explained in Section E 4.3. For PME installations the size of the earthing conductor must be no less than the sizes in Table E 4.6. Table E 4.6 PME (TN-C-S) minimum earthing conductor sizes. PME (TN-C-S) main protective bonding conductor sizes (mm2) Supply neutral conductor size (mm2)
Earthing conductor min. size (mm2)
25
35
50
70
95
120
150
Over 150
10
10
16
25
25
35
35
50
Note: Electricity utility distributors may require a minimum size of earthing conductor at the origin of the supply of 16 mm2 copper or greater for TN-C-S (PME) supplies.
E 5
Armoured cables as protective conductors
E 5.1
General It has been common practice in the United Kingdom to use the armouring of steel wire armoured cables as the protective conductor. This practice complies with BS 7671 and is most desirable for a number of reasons. Armouring provides a lowimpedance, robust, protective earthing system and will greatly assist any potential EMC issues compared with the practice of running external cpcs. Checking that the armouring is of sufficient cross-sectional area is complicated by the armouring being of a different material (steel) to that of the conductor (copper or aluminium). It is also complicated by the cross-sectional area for some two- and three-core cables being less than that given by Table 54.7. In the past,
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E