Page 372 - Handbook of Electrical Engineering

360 HANDBOOK OF ELECTRICAL ENGINEERING

The main advantage of using a high impedance is that the system will function satisfactorily
if only one line is faulted to earth. This is highly beneficial for emergency and essential services
such as process shut-down supplies, computer supplies, fire protection systems, telecommunications
and public address systems. These consumers must be maintained whenever possible. The earth fault
would be detected and the operating staff alerted. The staff would then be in a position to decide
whether or not to defer the shutting down of the supply to a later more convenient time. The single
fault cannot develop into an explosive or damaging state because the current is far too small. A
solidly earthed system does not have this benefit.
Reference 9 recommends Reference 15 for further reading on this subject.

13.3.3 IEC Types of Earthing Systems

The international standards IEC60364, part 4, and Reference 10 use a set of diagrams to clarify five
basic methods of earthing and providing the neutral where it is required. Three of these methods are
most commonly applied to oil industry installations. The five methods are abbreviated TNC, TNS,
TNCS, TT and IT, and are shown in Figures 13.5 to 13.9. The three common ones are TN, TT and
IT. The first letter is T or I. The second letter is N or T.

• The first letter denotes the source of power from a star-connected winding. T denotes that the star
point of the source is solidly connected to earth, which is usually at a location very near to the
winding. I denotes that the star point and the winding are isolated from earth by their design and

Figure 13.5 IEC earthing system type TNC.