Circuit Parameters with Faults
The resistance of the fault is normally that of the arc and may be approximated by and
For example, a 735 kV line would have an arc resistance of 4 V with a fault current of 4 kA, assuming no resistance in the ground return path.
The overall grounding resistance depends on the footing resistance (resistance of tower metalwork to ground) of the towers (Rt) and also on the resistance per section of the ground wires (Rs ), where present. The situation is summarized in Figure 7.20, where usually the effective grounding resistance is smaller than the individual
Figure 7.20 Equivalent network of towers with footing resistance (RT) and ground wire sections (resistance RS) tower footing resistance. There is normally a spread in the values of Rt but normally Rt should not exceed 10 V (ground wires present).
Typical values of fault resistance with fault location are as follows: at source, 0 V; on line with ground wires, 15 V; on line without ground wires, 50 V.