Impact of residual harmonic current on operation of residual current devices

Residual current devices (RCDs) are most commonly used as a part of the protection system in low voltage installations. RCDs are supposed to disconnect the supply whenever the imbalance current between the live conductor(s) and the neutral conductor, i.e. residual current, exceeds the predetermined value. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit, which could result in a lethal shock. However, harmonic components in ground fault current may influence the tripping sensitivity of RCDs, and the protection against electrical shock may not be effective. In the paper, results of theoretical analysis and experiments are presented. Several mechanisms which could influence the tripping sensitivity of RCDs are validated, and their dominant conditions are identified.