The UK Generation, Transmission and Distribution System 13
terminates at the consumer’s intake position. The letters C and S mean ‘combined’ and ‘separate’, respectively. So a TT system has the star or neutral point of the supply transformer directly connected to earth by means of an earth electrode, and the earthing of the consumer’s installation is also directly connected to earth via an earth electrode (Figure 1.5). This system is typical of an overhead line supply in a rural area.
L
Consumer’s installation
Supply
N
N
E Earth electrode
Earth electrode General mass of earth
(Single-phase shown for clarity)
FIGURE 1.5
TT system.
A TN-S system has the star point of the supply transformer connected to earth. Also the outer metallic sheaths of the distribution cable and, ultimately, the service cable are also connected to the star point. Hence, there are separate (S) metallic earth and neutral conductors throughout the whole system (Figure 1.6). A TN-C-S system (also known as protective multiple earthing, PME) has the usual star connection to earth and the metallic sheaths of the distribution and service cables also connected to the star point. In this case, however, the outer cable sheath is also used as a neutral conductor (i.e. it is a combined (C) earth and neutral).