Shock Hazard in the Presence of Protective Residual Current Devices

The application of the residual current principle, as carried out by zero sequence current protective devices, is one of the most efficient ways to reduce the hazard of electric shock in case of the failure of equipment's basic insulation-to-ground. Highly sensitive, and regularly tested, residual current operated circuit-breakers without integral overcurrent protection devices (RCCBs) are rightfully recognized worldwide by standards and codes, as an effective means to protect persons against direct and indirect contact with energized parts by disconnecting the supply in a timely fashion. These devices are also referred to as ground fault circuit interrupters (GFCIs). The protective action of the RCCBs, though, can be nullified not only due to internal malfunctions of the device, but also due to particular ground-fault conditions. In these dangerous situations for persons, for example accidental direct contact with two parts at different potentials, the residual current flowing through the RCCB is below its residual operating value, and, therefore, it cannot trip. This hazardous circumstance exposes persons to dangerous touch voltages despite the presence of an efficient protective device, which cannot be blamed for not intervening. This paper seeks to clarify these particular fault conditions, occurring in the presence of healthy RCCBs.