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 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.

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