Leakage capacitance (C) monitoring
Ungrounded power system must meet the following conditions to ensure protection from indirect contact in an AC power system:
RA x Id ≤ 50 V
RA is the resistance value of the equipment grounding connection, in Ohms.
Id is the ground fault current, in Amps.
50 V is the maximum acceptable voltage for indirect contacts.
For a three-phase ungrounded power system, the indirect contact fault current Id is:
Id = 2π x F x C x V
F is the frequency of the power system.
C is the earth leakage capacitance.
V is the phase-to-neutral voltage.
Combining these, the ungrounded power system must meet the following condition:
2π x F x C x V x RA ≤ 50 V
It is important that the equipment grounds have low resistance, and that the ungrounded power system leakage capacitance must be monitored and kept to a low value.
For more information, see Cahier Technique No. 178.