However, a lengthy line of 3-phase cable will have inter-phase capacitance so the feed capacitor must be at least an order of magnitude greater than this to ensure reliable operation, say 0.22mF or greater.
Apply to one of the phase 380V through resistor ( calculate the value of resistor so curret is abouth 100mA ) and use AC Leakage Current Sensors.
Just run a parallel trip wire. They won't bother separating wires if you loom or wrap them. They'll cut both.Keep it simple.