Tdr cable fault location

Claim 2

In fault location on an insulated electrical cable, the method which comprises: a. applying by way of a series inductor to said cable an alternating voltage at about rated cable voltage to ionize material in the region of failure of said insulation and to thereafter maintain ionization at a reduced current level, b. applying along with said reduced current a D.C. pulse having an abrupt onset to said cable at about a peak of said alternating current to produce a reflection from the discontinuity produced by ionization of said material, and c. measuring the time interval between application of said D.C. pulse and the return of said reflection as a measure of the distance to said fault.

Claim 3

The method of claim 1 wherein said D.C. pulse is of magnitude on the order of the rated cable voltage.

Claim 4

The method of claim 1 wherein said alternating voltage is 60 cycles and said D.C. pulse is applied at the peak of a negative half cycle of the resultant current flowing to said cable.

Claim 5

The method of claim 1 wherein said D.C. pulse has a rise time of about 50 nanoseconds between 10 percent and 90 percent points.

Claim 6

The method of claim 5 wherein said D.C. pulse is of negative polarity and is superimposed on a negative peak of said alternating current in said cable.

Claim 7

In fault location on an insulated electrical cable of known propagation velocity, the method which comprises: a. applying by way of a series inductor to said cable an alternating voltage at about rated cable voltage to ionize material in the region of failure of said insulation and to thereafter maintain ionization at a reduced current level, b. sensing when ionization current flow achieves a predetermined level in successive selected half-cycles in said cable to generate an enable condition, c. in response to said enable condition applying a D.C. pulse having an abrupt onset to said cable concurrently with the flow of said ionizing current to produce a reflection from the discontinuity produced by ionization of said material, and d. measuring the time interval between application of said D.C. pulse and the return of said reflection to indicate the distance to said fault.

Claim 8

In fault location on an insulated electrical cable of known propagation velocity, the method which comprises: a. applying by way of a series inductor to said cable an alternating voltage at about rated cable voltage to ionize material in the region of failure of said insulation and to thereafter maintain ionization at a reduced current level, b. sensing when ionization current flow achieves a predetermined level in successive selected half-cycles in said cable to generate an enable condition, c. in response to said enable condition applying a D.C. pulse having an abrupt onset to said cable concurrently with the flow of said ionizing current to produce a reflection from the discontinuity produced by ionization of said material, and d. measuring the time interval between application of said D.C. pulse and the return of said reflection to indicate the distance to said fault.

Claim 9

The method according to claim 6 wherein said current and said pulse are applied concurrently to said cable and said pulse is applied to a companion caBle and wherein reflections from like impedances in both cables are canceled.

Claim 10

The method according to claim 6 wherein identical pulses are applied to two like cables only one of which is faulted, and from which like returns are canceled to eliminate the effect of the mismatch at the point of connection of said cable upon said pulse.

Claim 11

The method according to claim 6 wherein each said D.C. pulse produces an incident control pulse for use in measuring said interval and wherein a predetermined time delay is applied to said control pulse to compensate for the travel time of said D.C. pulse and the return thereof through the measuring apparatus and connections to said cable.

Claim 12

A cable fault locator for a conductor encased in insulation which has failed at a location spaced from an accessible terminal of said conductor which comprises: a. means for applying an alternating voltage by way of a series inductor at about the rated cable voltage to said terminal to produce limited flow of alternating current at said voltage to said conductor to ionize material at said location, b. means responsive to flow of ionizing current of predetermined level in each successive selected half-cycle for applying an electrical pulse to said terminal concurrently with flow of said ionizing current and at a voltage on the order of rated cable voltage, and c. means for measuring the time interval between said pulse and the return of a reflection from the discontinuity produced at said location by the ionization of said material.

Claim 13

A cable fault locator for a conductor encased in insulation which has failed at a location spaced from an accessible terminal of said conductor which comprises: a. means for applying an alternating voltage by way of a series inductor at about the rated cable voltage to said terminal to produce limited flow of alternating current at said voltage to said conductor to ionize material at said location, b. means responsive to flow of ionizing current of predetermined level in each successive selected half-cycle for applying an electrical pulse to said terminal concurrently with flow of said ionizing current and at a voltage on the order of rated cable voltage, and c. means for measuring the time interval between said pulse and the return of a reflection from the discontinuity produced at said location by the ionization of said material.

Claim 14

The cable fault locator of claim 10 wherein said means for applying said pulse to said terminal includes means to produce a negative pulse and to apply the same in the region of a peak of a negative half cycle of said alternating current.

Claim 15

A cable fault locator for a conductor encased in insulation which has failed at a location spaced from an accessible terminal of said conductor which comprises: a. means for applying by way of a series inductor about the rated cable voltage to said terminal to produce limited flow of power frequency current in said conductor and to ionize material at said location, b. means for sensing flow of ionizing current in each successive selected half-cycle to said cable through said terminal to generate an enable condition, c. means responsive to said enable condition for applying a unipolar electrical pulse to said terminal in the region of a peak of one half cycle of said current, and d. means for measuring the time interval between said pulse and the return of a reflection from the discontinuity produced at said location by the ionization of said material.

Claim 16

A cable fault locator for a conductor encased in insulation which has failed at a location spaced from an accessible terminal of said conductor which comprises: a. means for applying by way of a series inductor about the rated cable voltage to said terminal to produce limited flow of power frequency current in said conductor and to ionize material at said location, b. means for sensing flow of ionizing current in each successive selected half-cycle to said cable through said terminal to generate an enable condition, c. means responsive to said enable condition for applying a unipolar electrical pulse to said terminal in the region of a peak of one half cycle of said current, and d. means for measuring the time interval between said pulse and the return of a reflection from the discontinuity produced at said location by the ionization of said material.

Claim 17

The combination set forth in claim 12 wherein said pulse is less than 1 microsecond in length with rise time of about 50 nanoseconds between 10 percent and 90 percent points.

Claim 18

The combination set forth in claim 12 wherein said pulse is on the order of several hundred microseconds in length and has an onset with a rise time of about 50 nanoseconds between 10 percent and 90 percent points.

Claim 19

A cable fault locator for a conductor encased in insulation which has failed at a location spaced from an accessible terminal, said locator which comprises: a. a high voltage source of ionizing current for applying alternating current to ionize material in the region of said fault, b. a current limiter connected between said source and said terminal for permitting only limited flow of current at said voltage to said terminal, c. a pulse generator for producing a unipolar pulse of a voltage at about the rated voltage of said cable, d. a coupling circuit connecting said pulse generator to said terminal, e. a connection between said current limiter and said pulse generator for supplying an enable condition to said pulse generator in response to flow of ionizing current through said current limiter to apply said unipolar pulse to said cable in the region of the peak of said current for reflection from the mismatch produced at said fault by ionization of said material, f. a timing means, g. means for connecting an output of said pulse generator to initiate actuation of said timing means in predetermined time relation with the application of said unipolar pulse to said cable, and h. means connected between said coupling circuit and said timing means for terminating operation of said timing means to end the timing period in response to receipt of a reflection from said terminal by said coupling circuit.

Claim 20

A cable fault locator for a conductor encased in insulation which has failed at a location spaced from an accessible terminal, said locator which comprises: a. a high voltage source of ionizing current for applying alternating current to ionize material in the region of said fault, b. a current limiter connected between said source and said terminal for permitting only limited flow of current at said voltage to said terminal, c. a pulse generator for producing a unipolar pulse of a voltage at about the rated voltage of said cable, d. a coupling circuit connecting said pulse generator to said terminal, e. a connection between said current limiter and said pulse generator for supplying an enable condition to said pulse generator in response to flow of ionizing current through said current limiter to apply said unipolar pulse to said cable in the region of the peak of said current for reflection from the mismatch produced at said fault by ionization of said material, f. a timing means, g. means for connecting an output of said pulse generator to initiate actuation of said timing means in predetermined time relation with the application of said unipolar pulse to said cable, and h. means connected between said coupling circuit and said timing means for terminating operation of said timing means to end the timing period in response to receipt of a reflection from said terminal by said coupling circuit.

Claim 21

The combination set forth in claim 15 wherein said timing means includes a digital counter and wherein several oscillators each of frequency calibrated with reference to the propagation velocity in a given cable are selectably connectable to said counter for conducting successive tests on different cable types.

Claim 22

The combination set forth in claim 15 wherein said timing means includes a digital counter, an oscillator connected to said counter, and a programmable calculator programmed to modify the output of said counter so as to compensate for variations in propagation velocity for different types of cables.

Claim 23

A cable fault locator for a conductor encased in insulation which has failed at a location spaced from an accessible terminal of said conductor which comprises: a. means for applying a transient voltage at about the rated cable voltage to said terminal to produce limited flow of ionizing current at said voltage to said conductor to ionize material at said location, b. means responsive to flow of ionizing current of predetermined level for applying an electrical pulse to said terminal concurrently with flow of said ionizing current and at a voltage on the order of rated cable voltage, and c. means for measuring the time interval between said pulse and the return of a reflection from the discontinuity produced at said location by the ionization of said material.

Claim 24

A cable fault locator for a conductor encased in insulation which has failed at a location spaced from an accessible terminal of said conductor which comprises: a. means for applying a transient voltage at about the rated cable voltage to said terminal to produce limited flow of ionizing current at said voltage to said conductor to ionize material at said location, b. means responsive to flow of ionizing current of predetermined level for applying an electrical pulse to said terminal concurrently with flow of said ionizing current and at a voltage on the order of rated cable voltage, and c. means for measuring the time interval between said pulse and the return of a reflection from the discontinuity produced at said location by the ionization of said material.