when the coil 85 is unenergized the contacts 81 82 and 83 are in electrical contact with a first set of contacts 81a 82a and 83a and when the relay coil is in the energized state the actuating mechanism 84 is moved to its other position so that the contacts 81 82 and 83 come into electrical engagement with a second set of contacts 81b 82b and 83b the output of contacts 81a and 82a are connected to the power input of a high frequency generator 86 which produces for example a 400 hz output signal which is connected to two conductors 26a and 26b of the three conductor cable 26 leading downhole the third contact 83a is open the three lower contacts 81b 82b and 83b are connected respectively to all three of the conductors 26a 26b and 26c of the downhole cable 26 one phase of the three phase input power from the interrupt relay 80 is connected to a power supply 87 the regulated dc output voltage of which is connected to a valve position signal monitor 88 the monitor 88 is connected to the two conductors 26b and 26c of the cable 26 to detect signals thereon from the downhole unit 43 and in response thereto provides a current to activate the coil 85 of the relay 75 when the downhole solenoid operated safety valve is in a full open.