Page 592 - Handbook of Electrical Engineering

Appendix H

Worked Example for the Calculation

of Earthing Current and Electric Shock

Hazard Potential Difference in a Rod

and Grid Earthing System

H.1 WORKED EXAMPLE

A 33 kV overhead line terminates at a pole in a small switching station. The distance to the pole
from the source is 15 km, but of course there are many other poles along the route. The subject pole
is earthed at its footings.
The overhead line ohmic data are:-

Positive sequence impedance Z 1pkm = 0.35 + j0.4 ohms/km.
Negative sequence impedance Z 2pkm = 0.35 + j0.4 ohms/km.

Zero sequence impedance Z 0pkm = 0.7 + j1.5 ohms/km.

The sending end of the line has a 100 MVA transformer that has the following ohmic data:-

Positive sequence impedance Z 1t = 0.1084 + j1.084 ohms/phase.
Negative sequence impedance Z 2t = Z 1t ohms/phase.
Zero sequence impedance Z 0t = Z 1t ohms/phase.

It will be assumed that the source impedance Z s feeding the transformers is small enough to
be neglected. To illustrate the difficulty in finding a suitably low resistance to earth it will be assumed
that the secondary winding of the transformer is solidly earthed and hence the NER resistance R n is
zero, a non-zero value will be recommended at the conclusion of the calculations. However, it will
be assumed that the resistance to earth R en at the source transformer is 1.5 ohms. The resistance to
earth at the far end pole is R ep , which needs to be determined. This requires a suitable grid and rod
system to be chosen. The calculation process will be carried out in a series of steps.

Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical Industry. Alan L. Sheldrake
 2003 John Wiley & Sons, Ltd ISBN: 0-471-49631-6