Guide to the Wiring Regulations
Some relevant points to note about the various systems are now considered.
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TN-C This system is only used in the UK by utilities and usually includes earthed sheath return wiring (ESRW) or earthed concentric wiring. TN-S A true TN-S earthing system has the neutral-earth point at the source or right at the transformer. In practice this is rare and these systems are considered to be TN-C-S PNB (see Figures E 2.6 and E 2.9).
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TN-C-S (PME) PME systems are used by the utility DNOs (distribution network operators) in most modern networks, mainly for economy and safety. A Government licence is required to own and operate this type of system.
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TN-C-S (PNB) Figure E 2.6 is something that should be studied, as it is a variation of a TN-C-S system known as PNB, meaning protective neutral bonding. This variation of a TN-C-S system is not defined in BS 7671 but is defined in BS 7430: 1998. The electrical utility companies commonly use this PNB terminology, although it is not so common with electrical contractors or consulting engineers. The TN-C-S PNB system is often used to describe the earthing arrangement in installations with on-site transformers; whilst some describe these arrangements as TN-S they are very close to the TN-C-S PNB system of Figure E 2.6, depending upon the position of the neutral earth link. A typical arrangement for an on-site transformer with the neutral earth link made in the main switchgear is shown in Figure E 2.9.
L
L2
L1
N L3
Connection A E
Transformer
Main Switchgear
Figure E 2.9 Earthing arrangement for on-site substation.
E
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Point of neutral source earthing