Abstract A ground fault detector for an ungrounded electrical distribution system responds to a wide range of ground fault current levels by providing the ground fault sensing circuitry with a high impedance at low ground fault signal levels and a low impedance at higher ground fault signal levels. The varying or nonlinear impedance of the sensing circuitry occurs due to the presence of the series combination of a Zener diode and a resistor connected in parallel with a level sensing means. At ground fault signals below the Zener voltage of the Zener diode, the impedance of the sensing circuitry is that of the level sensing means. When the ground fault signal exceeds the Zener voltage, the impedance of the sensing means is significantly reduced since the Zener resistor is now in effective parallel combination with the impedance of the level sensing means. Varying degrees of filtering of the ground fault signal are accomplished through provision of alternate connection terminals of increasing resistance for a filtering capacitor. A ground fault test circuit for the detector provides the equivalent of one-half ampere ground fault. Higher ground faults may easily be simulated by winding additional turns of a test winding on a differential transformer of the detector; each additional turn simulates an additional one-half ampere ground fault.
A ground fault detector for sensing ground faults in an electrical distribution system having at least two conductors of the type wherein the electrical distribution system is ungrounded comprising: a differential current transformer positioned to sense the non-zero summation of currents in the conductors of the system, a secondary winding on the transformer and a rectifying means providing a direct current signal indicative of the magnitude of the non-zero summation of the currents of the system, a sensing circuit responsive to the direct current signal to produce an output, a switching means responsive to the sensing circuit output whenever the sensing circuit output exceeds a selected fraction of the magnitude of the direct current signal, a means for varying impedance of the sensing circuit and thereby the value of said selected fraction of the direct current signal so that the selected fraction is greater when the magnitude of the direct current signal is less than the predetermined level and less when the magnitude is greater than the predetermined level.
Claim 2
Dependent Claim
169 words
A ground fault detector for sensing ground faults in an electrical distribution system having at least two conductors of the type wherein the electrical distribution system is ungrounded comprising: a differential current transformer positioned to sense the non-zero summation of currents in the conductors of the system, a secondary winding on the transformer and a rectifying means providing a direct current signal indicative of the magnitude of the non-zero summation of the currents of the system, a sensing circuit responsive to the direct current signal to produce an output, a switching means responsive to the sensing circuit output whenever the sensing circuit output exceeds a selected fraction of the magnitude of the direct current signal, a means for varying impedance of the sensing circuit and thereby the value of said selected fraction of the direct current signal so that the selected fraction is greater when the magnitude of the direct current signal is less than the predetermined level and less when the magnitude is greater than the predetermined level.
Claim 3
Dependent Claim
76 words
The ground fault detector as in claim 1 wherein the means for varying the impedance comprises a series combination of a Zener diode and a resistor with the combination connected in parallel with the sensing circuit to produce a lower input impedance for the sensing circuit when the direct current signal exceeds the Zener voltage of the Zener diode thereby providing increased loading at higher direct current levels and decreased loading at lower direct current levels.
Claim 4
Dependent Claim
38 words
The ground fault detector as in claim 2 also comprising a capacitor connected in parallel with the sensing circuit and with selectable connection terminals of varying resistance to provide varying degrees of filtering of the direct current signal.