Handbook of Electrical Installation Practice - BeKnowledge

Electricity on Construction Sites 111
Fig. 5.5 TN-S system: separate neutral and protective conductors throughout the system.
Fig. 5.6 TN-C-S system: neutral and protective functions are combined in a single
conductor in a part of the system.
protective conductor (PE) is often the metallic covering of the cable (armouring,
sheathing or conduit) supplying the installations, or a separate conductor. All
exposed-conductive-parts of the installation are connected to this protective
conductor via the main earthing terminal of the installation.
TN-C-S system
The TN-C-S system is more commonly referred to as protective multiple earthing
(PME) with the PEN conductor as the combined neutral and earth (CNE) conductor.The
Electricity Distributor’s regulations demand that all exposed metalwork
is bonded to the protective conductor, which in turn is connected to the neutral conductor
at the supply intake point. Owing to the impracticality of bonding all exposed
metalwork to form an equipotential cage an rcd is normally necessary at the mains
intake position, together with an installation earth electrode, to form a TT system.
The value of the rcd tripping current is dependent on the earth loop impedance
of the final circuits (back to the Electricity Distributor’s transformer), but values
of less than 250mA should not be necessary providing that the maximum