Protective device for AC system with unearthed neutral line - uses test current injected between neutral and earth and potential difference compared with mains potential difference

摘要

The protective system for a multiphase AC distribution system with unearthed neutral prevents overheating of the neutral by a "double short" current overload. It reduces the fault current to a safe value and trip the circuit if the reduction is not effected within a preset time. A test current at a frequency differing from that of the mains (this may be direct current) is injected between neutral and earth from a constant voltage source through a limiting resistance, and the potential difference between the neutral and earth is measured at both frequencies. From the difference between these values a criterion is provided for the triggering of an alarm.