Protective device for low current transmission circuits

A protective device for low current transmission lines, comprising a transmitting unit connected to one end of said transmission line and a receiving unit connected to the other end of said line, said transmitting unit comprising at least one lightning protector connected to said one end of said line, a filter network across said one end of said line, a transducer for applying signal to said one end of said line, and an emitter for communicating with said transducer through an electrically and magnetically insulated medium, whereby low current signals produced by said transmitting unit can traverse said medium between said emitter and said transducer, and signals produced by energy containing disturbances cannot;

• said receiving unit comprising at least one second lightning protector connected to said other end of said line, a second filter network across said other end of said line, a second transducer for receiving signals on said line, and a receiver for communicating with said second transducer through an electrically and magnetically insulated medium, whereby low current signals on said line can traverse said medium between said transducer and said receiver, and signals produced by energy containing disturbances cannot, the device further comprising means to check the ability of the line to transmit signals.