Ground start circuit

In a subscriber loop test system for testing a two wire telephone line, a ground start circuit that is capable of being connected to said telephone line in order to obtain dial tone on said telephone line from a central office connected thereto, said ground start circuit comprising:

• means, connected between an earth ground connection and a first terminal that is to be connected to one wire of a two-wire telephone line and operative in response to a potential difference appearing thereacross, for providing a control signal that changes in value;
• means, connected between said first terminal and the earth ground connection and responsive to said control signal, for establishing a low impedance path between said first terminal and the earth ground connection whenever said control signal reaches a threshold value whereby said telephone line is placed in an unbalanced condition;
• and
• means, connected to said providing means and responsive to a potential difference subsequently applied across said first terminal and a second terminal that is to be connected to said second wire of said two-wire telephone line, for decreasing the value of said control signal to a level sufficient for said establishing means to eliminate said low impedance path existing between said first terminal and the earth ground connection, whereby said telephone line is returned to a substantially balanced condition.

In a ground start circuit connected to a telephone line having at least two wires, a method of invoking ground start operation of the line comprising the steps of:

• providing, in response to a potential difference appearing between an earth ground connection and one wire of a two-wire telephone line, a control signal that changes in value;
• establishing, in response to said control signal, a low impedance path between said one wire of the telephone line and the earth ground connection whenever said control signal reaches a threshold value whereby said telephone line is placed in an unbalanced condition;
• and
• decreasing, in response to a potential difference subsequently applied across both of said two wires of the telephone line, the value of said control signal to a level sufficient to eliminate said low impedance path existing between said first wire and the earth ground connection, whereby said telephone line is returned to a substantially balanced condition.