ABSTRACT
This chapter discusses the types of supply system and earthing arrangements. There are three types of system that are in common use and they are referred to as TT, TN-S and TN-C-S. Where the letter T is used when identifying supply systems, it represents earth. A TT system is normally a supply in which the cables are run into the building above ground. The abbreviation TT indicates that the supply system and the installation have two points of earth. A TT system can usually be identified at the service head. This is because there will be no sign of an earthing conductor being connected to the service head, or the supply cable. A TN-S supply system uses only one point of earth and this is supplied to electricians by the district network operator (DNO). TN-C-S systems are often referred to as protective multiple earthing (PME) systems.
