Earthing and Bonding
Figure E 6.5 shows a typical domestic dwelling, and main protective bonding requirements. The radiator served via plastic pipe does not require a bond. The radiator served via metal pipes will need a bond if the boiler is not itself bonded. However, it would be better practice to bond the boiler or a pipe close to the boiler.
E 6.4
Sizing protective bonding conductors The sizing of bonding conductors can get confusing when reading Chapter 54 of BS 7671. This is due to the fact that the suggested minimum sizes relate to the size of the line and earthing conductor for TN-S supplies, and the supply neutral conductor for TN-C-S (PME) supplies. Tables E 6.1 and E 6.2 provide an easy look up for main protective bonding conductor sizes.
Table E 6.1 TN-S, PNB, TT main protective bonding conductor sizes. Line conductor size (mm2) 25
35
50
70
95
120
150
185
240
300
400
500
600
Earthing conductor min. size (mm2)
16
16
25
35
50
70
95
95
120
150
240
300
300
Bonding conductor min. size (mm2)
10
10
16
16
25
25
25
25
25
25
25
25
25
Notes: Smaller sizes are possible by using S = (I√t)/k to calculate earthing conductor size; main bonding is half the size of this up to a maximum of 25 mm2. A minimum conductor size of 6 mm2 shall be used and conductors of 10 mm2 or less shall be copper. For conductors of other material, the material must be sized to achieve the equivalent conductance of the copper of the relevant size. See Appendices for data. IT system earthing conductors may be sized using this table.
Table E 6.2 PME (TN-C-S) main protective bonding conductor sizes. Supply neutral conductor size – mm2 25
35
50
70
95
120
150
Over 150
Earthing conductor min. size (mm )
10
10
16
25
25
35
35
50
Bonding conductor min. size (mm )
10
10
16
25
25
35
35
50
2
2
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.
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E