Key telephone system

A key telephone system comprising a first channel transformer a primary winding of which is connected through a DC sink circuit to a set of central office lines, an impedance compensating circuit connected to a secondary winding of said first channel transformer, a second channel transformer a primary winding of which is connected through a channel switch to said impedance compensating circuit, at least a key telephone set connected through a current supply circuit to a secondary winding of said second channel transformer, and an oscillation preventing circuit inserted between the ground and an electrically isolated circuit, said isolated circuit being electrically isolated from said central office lines and said key telephone set by being connected between said secondary winding of said first channel transformer and said primary winding of said second channel transformer, said isolated circuit including said impedance compensating circuit and said channel switch, said DC sink circuit, said first and second channel transformer, said isolated circuit and said current supply circuit forming a signal transmission line when said channel switch is closed, and said oscillation preventing circuit normally being in an OFF condition and operating in response to detected significant impedance fluctuations in said signal transmission line to change to an ON condition to prevent oscillation of said impedance compensating circuit.