Symmetrical fault current detector

A fault current detector comprising:

• current transformer means adapted to be inductively coupled to an alternating current power line to provide a signal representative of current flowing in the alternating current power line;
• and
• means responsive to the signal to provide a fault current indication if the alternating current flow represented by the signal exceeds a predetermined threshold in each polarity but not to provide the fault current indication if the alternating current flow exceeds the predetermined threshold in only one polarity , said means responsive to the signal being independently responsive to the alternating current flow in half-cycles of respective polarity of the alternating current flow to provide a respective indication of the flow in each polarity exceeding the predetermined threshold and comprising a full wave rectifier for rectifying the output signal of the current transformer means, a resistance connected across the output terminals of the rectifier and a respective threshold switch connected to each input terminal of the rectifier and arranged to be triggered in response to the voltage between the respective input terminal and a common output terminal of the rectifier exceeding the predetermined threshold voltage .