Leakage capacitance (C) monitoring

Ungrounded power systems is adversely affected by leakage capacitance.

Ungrounded power system must meet the following conditions to ensure protection from indirect contact:

R_A x I_d ≤ 50 V AC and R_A x I_d < 120 V DC

R_A is the resistance value of the equipment grounding connection, in Ohms.
I_d 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 I_d is:

I_d = 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 R_A ≤ 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.