Missing phase detector

Claim 1

An unbalance protection apparatus for a multiple transmission line, multiphase electrical transmission system comprising: means for providing a signal voltage representative of a vector sum of voltages on individual transmission line of said multiphase electrical transmission system; means for providing a voltage source; a first relay having a coil and a wiper arm coupled at a first end to said voltage source means and in movable contact at a second end between first and second terminals; switch means coupled in series with said voltage source and said coil of said first relay and switchable between current conductive and nonconductive states in response to said signal voltage for energizing and deengerizing said coil of said first relay such that said wiper arm is movably coupled to said first and second terminals in response to said current conductive and non-conductive states; a second relay having a coil coupled to said first terminal of said first relay, a multiplicity of wiper arms respectively coupled in said transmission lines to control current flow in said transmission lines in response to terminal contact positions of said wiper arm of first relay, and having at least one wiper arm with one end coupled to said switch means and a second end contact movable between two terminals in response to said terminal contact positions of said wiper arm of said first relay, one of said two terminals being coupled to said voltage source means, whereby once said signal voltage exceeds a predetermined threshold, causing a responsive state for said switch means and thereby actions of said first and second relays to place said at least one wiper arm of said second relay in contact with said one terminal coupled to said voltage source means, said switch means remains in said signal voltage responsive state; and reset means coupled between said at least one wiper arm of said second relay and said switch means for resetting said switch means to a pre-signal voltage responsive state.