A 13.8-kV generator and step-up power transformer are connected to a 115-kV power system in the figure below. Specific detail for the 115-kV connection between power system and transformer is omitted for simplicity.
Personal protective grounds (PPG) are to be connected at point (G) for hands-on work on the generator bus and stator winding.
The generator is on an electrical and mechanical clearance that permits workers around and on the rotating parts of the generator. The generator circuit breaker and associated disconnect switches are open.
The transformer is energized from the 115-kV system. The available three-phase fault current at (G) from the 115-kV power system is 20,000 amperes symmetrical with an impedance X/R ratio of 20.
The generator fault current contribution at (G), if it could rotate, is 15,000 amperes with an X/R ratio greater than 20.
What is the minimum conductor size permitted for the protective grounds?
First determine the maximum available fault current at the worksite (G) where protective grounds are to be installed. Since the generator is on clearance and cannot rotate, the only source of available fault current at the worksite is from the 115-kV power system (20,000 amperes).
Next, select a cable ampacity table from paragraph 5.1.1 to determine cable size. Since the power system fault impedance X/R ratio is greater than 10, use Table 2B. The grounds are to be installed in a powerplant, therefore the fault clearing time is assumed to be 15 cycles.
From Table 2B, the minimum cable size with an ampacity equal to or greater than 20,000 amperes for 15 cycles is #2/0 AWG copper (23,000 amperes). From ASTM F855, a grade 3 clamp and ferrule is compatible with #2/0 conductor.
Note that if the cable size had been incorrectly selected from table 2A, a #1/0 conductor (21,000 amperes) would appear adequate. However, for this grounding application the smaller conductor would be undersized due to the additional heating effect of the dc offset component of fault current.
A #2/0 copper cable has adequate ampacity (thermal capacity) for this grounding application. However, having adequate ampacity alone does not ensure the cable is suitable for installation at the worksite. Worker exposure voltage which is dependent on cable length must also be determined.