Electrical protection system utilizing reverse polarity line testing with unidirectional current devices having reverse breakdown characteristic

Claim 1

IN A SIGNALLING SYSTEM IN WHICH ONE PLACE IS CONNECTED TO ANOTHER PLACE BY A CONDUCTIVE LINE, SAID OTHER PLACE HAVING A FIRST SOURCE OF DIRECT CURRENT POTENTIAL COUPLED TO SAID LINE AND CURRENT-SENSITIVE MEANS COUPLED TO SAID LINE TO DETECT CHANGES IN CURRENT THEREIN, SAID ONE PLACE HAVING AT LEAST ONE SIGNALLING DEVICE CONNECTED TO SAID LINE AND ARRANGED WHEN OPERATED TO ALTER THE CURRENT FLOW IN SAID LINE; THE COMBINATION COMPRISING A SECOND SOURCE OF DIRECT CURRENT POTENTIAL AT SAID OTHER PLACE, MEANS SELECTIVELY TO CONNECT SAID SECOND SOURCE TO SAID LINE IN OPPOSITE POLARITY TO SAID FIRST SOURCE AND TO DISCONNECT SAID FIRST SOURCE FROM SAID LINE FIRST AND SECOND UNIDIRECTIONAL CURRENT-CARRYING DEVICES EACH HAVING A LOW FORWARD RESISTANCE AND A HIGH REVERSE RESISTANCE EXCEPT WHEN SUBJECTED TO A VOLTAGE IN THE REVERSE DIRECTION IN EXCESS OF A PARTICULAR BREAKDOWN VALUE, MEANS AT SAID ONE PLACE TO CONNECT SAID UNIDIRECTIONAL DEVICES IN SERIES WITH SAID LINE WITH THE FORWARD DIRECTION THEREOF CORRESPONDING TO THE CURRENT FLOW IN SAID LINE PRODUCED BY SAID FIRST SOURCE, SAID COMBINED PARTICULAR VOLTAGE BREAKDOWN VALUE OF SAID UNIDIRECTIONAL DEVICES BEING GREATER THAN THE VOLTAGE OF SAID SECOND SOURCE, THE INDIVIDUAL PARTICULAR VOLTAGE BREAKDOWN VALVES OF SAID UNIDIRECTIONAL DEVICES BEING LESS THAN THE VOLTAGE OF SAID SECOND SOURCE, SAID UNIDIRECTIONAL DEVICES BEING ELECTRICALLY SEPARATED BY SAID ONE SIGNALLING DEVICE, AND MEANS TO DETECT CURRENT FLOW IN SAID LINE WHEN SAID SECOND SOURCE IS CONNECTED TO SAID LINE.