Gfi circuit having tuned circuit fault detector

Claim 2

In an electrical power distribution system comprising at least two current carrying conductors for connecting a source of current to load terminals, one of said conductors being grounded at said source and the other conductor being above ground potential, means for detecting a fault connection to ground on at least one of said conductors and in response thereto interrupting the current flow path from said source, comprising current interrupter means in at least one of said conductors and operable between a current conducting and a current interrupting condition, a tuned circuit inductively coupled to said conductors, an oscillator coupled to said tuned circuit for exciting said tuned circuit with an oscillator signal having a frequency higher than the frequency of the current on said conductors so that the tuned circuit is not resonant at the frequency of said source of current, said tuned circuit being constructed and arranged to have therein a signal of a first characteristic in the absence of a fault to ground on either of said conductors and to have a signal therein of different characteristic in the presence of a fault to ground below a given resistive magnitude on either one of said conductors, means coupled to said tuned circuit for operating said current interrupter means to its interrupting condition when a characteristic of the signal in the tuned circuit changes by a given value from said first characteristic, and means responsive to the signal in said tuned circuit for causing the resonant frequency of the tuned circuit and the frequency of the oscillator output signal to be substantially equal to each other.

Claim 3

The combination claimed in claim 1 wherein the means responsive to the signal in said tuned circuit includes means for comparing the signal in said tuned circuit and the output signal of said oscillator and for producing a tracking signal which has a value which is a function of the phase relationship between the compared signals, and means responsive to the tracking signal for causing the frequency of the oscillator output signal and the resonant frequency of the tuned circuit to be substantially equal to each other.

Claim 4

The circuit claimed in claim 1 wherein said oscillator is a variable frequency oscillator and wherein the means responsive to the signal in said tuned circuit comprises a phase locked loop for comparing the signal in the tuned circuit and the oscillator output signal and for producing a tracking signal having a value which is a function of the relative phases of said compared signals, and means for coupling said tracking signal to said oscillator to make the frequency of said oscillator output signal substantially equal to the resonant frequency of said tuned circuit.

Claim 5

The circuit claimed in claim 1 wherein the means responsive to the signal in said tuned circuit comprises an active circuit loop which presents a negative value of capacitance to said tuned circuit, means in said active circuit loop responsive to a tracking signal for changing the value of said negative capacitance as a function of the value of the tracking signal, said means responsive to the signal in the tuned circuit further including means for comparing said oscillator output signal and the signal in said tuned circuit and for producing in response thereto said tracking signal whose value is a function of the relative phases of said compared signals, said active circuit loop responding to said tracking signal to produce a value oF negative capacitance which makes the resonant frequency of the tuned circuit substantially equal to the frequency of said oscillator.

Claim 6

The combination claimed in claim 1 wherein said tuned circuit is inductively coupled to said conductors by a transformer which has first and second multi-turn windings respectively connected in said two conductors and has a third multi-turn winding which is included in said turned circuit, said windings being wound on a core of magnetic material that exhibits relatively low reluctance and relative high permeability at the frequency of said oscillator.

Claim 7

The combination claimed in claim 5 wherein the ratio of the number of turns between said first or second windings and said third winding does not exceed approximately two.

Claim 8

The combination claimed in claim 6 wherein the ratio is one.

Claim 9

In an electrical power distribution system comprising at least two current carrying conductors for connecting a source of current to load terminals, one of said conductors being grounded at said source and the other conductor being above ground potential, means for detecting a fault connection to ground on at least one of said conductors and in response thereto interrupting the current flow path from said source, comprising current interrupter means in at least one of said conductors and operable between a current conducting and a current interrupting condition, first and second multi-turn transformer windings connected in series relationship, respectively, in said two conductors, said windings being wound in balanced relationship on the transformer, a third multi-turn transformer winding inductively coupled to said first and second windings, circuit means connected to said third winding for providing a tuned circuit having a nominal resonant frequency, an oscillator coupled to said tuned circuit for exciting the tuned circuit with an oscillator output signal having a frequency higher than the frequency of current on said conductors so that the tuned circuit is not resonant at the frequency of said source of current, said tuned circuit being constructed and arranged to have therein a signal of a first magnitude in the absence of a fault to ground on either of said conductors and to have a signal therein which is different in magnitude when a fault to ground below a given resistive value is present on either one of said conductors, means coupled to said tuned circuit for operating said current interrupter means to its interrupting condition when the magnitude of the signal in said tuned circuit changes by a given value from said first magnitude, and means responsive to the output signal of said oscillator and to the signal in the tuned circuit for causing the resonant frequency of the tuned circuit and the frequency of the oscillator output signal to be substantially equal to each other.

Claim 10

In an electrical power distribution system comprising at least two current carrying conductors for connecting a source of current to load terminals, one of said conductors being grounded at said source and the other conductor being above ground potential, means for detecting a fault connection to ground on at least one of said conductors and in response thereto interrupting the current flow path from said source, comprising current interrupter means in at least one of said conductors and operable between a current conducting and a current interrupting condition, first and second multi-turn transformer windings connected in series relationship, respectively, in said two conductors, said windings being wound in balanced relationship on the transformer, a third multi-turn transformer winding inductively coupled to said first and second windings, circuit means connected to said third winding for providing a tuned circuit having a nominal resonant frequency, an oscillator coupled to said tuned circuit for exciting the tuned circuit with an oscillator output signal having a frequency higher than the frequency of current on said conductors so that the tuned circuit is not resonant at the frequency of said source of current, said tuned circuit being constructed and arranged to have therein a signal of a first magnitude in the absence of a fault to ground on either of said conductors and to have a signal therein which is different in magnitude when a fault to ground below a given resistive value is present on either one of said conductors, means coupled to said tuned circuit for operating said current interrupter means to its interrupting condition when the magnitude of the signal in said tuned circuit changes by a given value from said first magnitude, and means responsive to the output signal of said oscillator and to the signal in the tuned circuit for causing the resonant frequency of the tuned circuit and the frequency of the oscillator output signal to be substantially equal to each other.

Claim 11

The combination claimed in claim 8 wherein said first and second windings are wound in balanced relationship.

Claim 12

The combination claimed in claim 9 wherein the turns ratio between a first or second winding and the third winding does not exceed two.

Claim 13

The combination claimed in claim 8 wherein said windings are wound on a core of magnetic material that has a low reluctance and high permeability at the frequency of said oscillator.

Claim 14

The combination claimed in claim 8 wherein said means responsive to the output signal of the oscillator and to the signal in the tuned circuit includes means for comparing the phases to the two signals and for producing a tracking signal whose value is a function of the relative phases of the compared signals.

Claim 15

The combination claimed in claim 12 wherein the oscillator is a variable frequency oscillator and wherein the means responsive to the output signal of the oscillator and to the signal in the tuned circuit further includes means for coupling the tracking signal to said oscillator to change the frequency of the oscillator to maintain a desired phAse relationship between said compared signals.

Claim 16

The combination claimed in claim 12 wherein the means responsive to the output signal of the oscillator and to the signal in the tuned circuit further includes an active circuit loop which presents a negative value of capacitance to said tuned circuit, means in said active circuit loop responsive to said tracking signal for changing the value of said negative capacitance as a function of the value of the tracking signal thereby to maintain a desired phase relationship between said compared signals.

Claim 17

In an electrical power distribution system comprising at least two current carrying conductors for connecting a source of current to load terminals, one of said conductors being grounded at said source and the other conductor being above ground potential, means for detecting a fault connection to ground on at least one of said conductors and in response thereto interrupting the current flow path from said source, comprising current interrupter means in at least one of said conductors and operable between a current conducting and a current interrupting condition, first and second multi-turn transformer windings connected in series relationship, respectively, in said two conductors, said windings being wound in balanced relationship on the transformer, a third multi-turn transformer winding inductively coupled to said first and second windings, circuit means connected with said third winding for providing a tuned circuit having a resonant frequency fo, a variable frequency oscillator having a free running frequency fo, means for coupling said oscillator to said tuned circuit to excite said tuned circuit with the output signal of the oscillator, means for comparing the phase of the output signal of the oscillator with the phase of the signal in said tuned circuit and for producing a tracking signal having a characteristic which is a function of the phase relationship between said compared signals, means for coupling said tracking signal to said variable frequency oscillator for changing the frequency of oscillation of the oscillator to maintain a desired relationship between the two compared signals, means for sensing the magnitude of the signal in said tuned circuit and for producing a sensing signal whose magnitude is a function of the signal in said tuned circuit, and means operable in response to said sensing signal for operating said current interrupting means when said sensing signal changes in a predetermined manner.

Claim 18

In an electrical power distribution system comprising at least two current carrying conductors for connecting a source of current to load terminals, one of said conductors being grounded at said source and the other conductor being above ground potential, means for detecting a fault connection to ground on at least one of said conductors and in response thereto interrupting the current flow path from said source, comprising current interrupter means in at least one of said conductors and operable between a current conducting and a current interrupting condition, first and second multi-turn transformer windings connected in series relationship, respectively, in said two conductors, said windings being wound in balanced relationship on the transformer, a third multi-turn transformer winding inductively coupled to said first and second windings, circuit means connected with said third winding for providing a tuned circuit having a resonant frequency fo, a variable frequency oscillator having a free running frequency fo, means for coupling said oscillator to said tuned circuit to excite said tuned circuit with the output signal of the oscillator, means for comparing the phase of the output signal of the oscillator with the phase of the signal in said tuned circuit and for producing a tracking signal having a characteristic which is a function of the phase relationship between said compared signals, means for coupling said tracking signal to said variable frequency oscillator for changing the frequency of oscillation of the oscillator to maintain a desired relationship between the two compared signals, means for sensing the magnitude of the signal in said tuned circuit and for producing a sensing signal whose magnitude is a function of the signal in said tuned circuit, and means operable in response to said sensing signal for operating said current interrupting means when said sensing signal changes in a predetermined manner.