Circuit isolating switching arrangement

Claim 2

A circuit isolating switching arrangement for isolating a balanced transmission line from an equipment connected thereto, comprising a four-terminal network having a pair of input terminals for connection to the transmission line and a pair of output terminals for connection to the equipment, a pair of series-connected elements forming a potential divider connected between said input terminals, each of said elements comprising a series-tuned circuit tuned to a predetermined frequency which is higher Than the maximum frequency normally to be transmitted by the line, a pair of normally closed relay contacts connected between said input terminals and output terminals, and relay operating means connected between the midpoint of the potential divider and ground for opening said contacts in response to a signal at said predetermined frequency being applied to the line.

Claim 3

In a telephone system comprising a balanced transmission line having an equipment connected thereto at a subscriber''s station, means for isolating the equipment from the line comprising a four-terminal network having a pair of input terminals connected to the line and a pair of output terminals connected to the equipment, a pair of series-connected elements forming a potential divider connected between said input terminals, each of said elements comprising a series-tuned circuit tuned to a test frequency which is higher than the maximum frequency normally to be transmitted by the line, transmitter means located at a central exchange for impressing a signal at said test frequency on the line, a pair of normally closed relay contacts connected between said input and output terminals, and relay operating means connected between the midpoint of the potential divider and ground for opening said contacts in response to an impressed test signal.

Claim 4

In a telephone system comprising a balanced transmission line having an equipment connected thereto at a subscriber''s station, means for isolating the equipment from the line comprising a four-terminal network having a pair of input terminals connected to the line and a pair of output terminals connected to the equipment, a pair of series-connected elements forming a potential divider connected between said input terminals, each of said elements comprising a series-tuned circuit tuned to a test frequency which is higher than the maximum frequency normally to be transmitted by the line, transmitter means located at a central exchange for impressing a signal at said test frequency on the line, a pair of normally closed relay contacts connected between said input and output terminals, and relay operating means connected between the midpoint of the potential divider and ground for opening said contacts in response to an impressed test signal.

Claim 5

In combination with a balanced telephone transmission line having an equipment connected thereto at a subscriber''s station, i. a four-terminal network having a pair of input terminals connected to the line and a pair of output terminals connected to said equipment, a pair of series-connected elements forming potential divider connected between said input terminals, each of said elements comprising a series-tuned circuit tuned to a predetermined frequency higher than the maximum frequency normally to be transmitted by the line, a pair of normally closed relay contacts connected between said input terminals and output terminals, and relay operating means connected between the midpoint of the potential divider and ground for opening said contacts in response to a signal at said predetermined frequency, ii. transmitter means located at a central exchange for applying to the line a signal at said predetermined frequency, whereby to operate the relay to open said contacts, iii. and selective testing means located at the central exchange for testing for continuity from each of the line conductors to ground and to each other.

Claim 6

In combination with a balanced telephone transmission line having an equipment connected thereto at a subscriber''s station, i. a four-terminal network having a pair of input terminals connected to the line and a pair of output terminals connected to said equipment, a pair of series-connected elements forming potential divider connected between said input terminals, each of said elements comprising a series-tuned circuit tuned to a predetermined frequency higher than the maximum frequency normally to be transmitted by the line, a pair of normally closed relay contacts connected between said input terminals and output terminals, and relay operating means connected between the midpoint of the potential divider and ground for opening said contacts in response to a signal at said predetermined frequency, ii. transmitter means located at a central exchange for applying to the line a signal at said predetermined frequency, whereby to operate the relay to open said contacts, iii. and selective testing means located at the central exchange for testing for continuity from each of the line conductors to ground and to each other.

Claim 7

The combination claimed in claim 3, wherein the selective testing means comprises an insulation tester having first and second terminals, a first selector switch connected to the first terminal for switching the first terminal selectively to ground or to one line conductor, and a second selector switch connected to the second terminal for switching the second terminal to ground or the other line conductor.