MONITORING CIRCUIT FOR DETECTING LEAKAGE CURRENTS

• US 3,700,966 A
• Filed: 05/19/1971
• Issued: 10/24/1972
• Est. Priority Date: 05/19/1971
• Status: Expired due to Term

• Current Assignee: Paul Morrow
• Original Assignee: Paul Morrow
• Inventors: Morrow, Paul
• Primary Examiner(s): Trammell, James D.
• Application Number:
  US05/144,914
• Time in Patent Office: 524 Days
• Field of Search: 317/18R,18D 340/255
• US Class Current: 361/49
• CPC Class Codes: H02H 3/162 for ac systems

Abstract

The leakage currents to ground from the power input conductors connected to a load are detected. When these leakage currents reach a selected magnitude, an amplifier responds to produce a usable output for indicating dangerous leakage current conditions. Detection takes place by means of a diode connected to each input conductor, the latter carrying an ac voltage, which diodes are connected in common to ground through a load resistor to provide paths for purely resistive leakage currents in phase with the ac voltage. An amplifier is connected to this load resistor to detect the in phase leakage currents. To detect out of phase or wattless leakage currents due to capacitive coupling between the input conductors and ground, a further diode, load resistor and amplifier combination is used and its output is summed with the in phase output to drive a further amplifier whose output is adjusted to operate only when the sum of the in phase and out of phase inputs reaches a selected value.

Claims

1. A monitoring circuit for detecting leakage currents from power input conductors connected to a load, which input conductors carry an ac voltage, comprising:

detector means connected across said input conductors and connected to ground for passing leakage current whenever either of said input conductors has a leakage connection to ground, said detector means including first branch means for passing a first leakage current between said input conductors during alternate half cycle of said ac voltage when one input conductor has a leakage connection to ground and second branch means for passing a second leakage current between said input conductors during the remaining half cycles when the other input conductor has a leakage connection to ground;

amplifier means having signals representing said leakage currents as inputs thereto; and

means connected to said amplifier means for effecting an operative output therefrom only in response to a selected level of input to said amplifier means.

2. A monitoring circuit for detecting leakage currents from power input conductors connected to a load, which input conductors carry an ac voltage, comprising:

detector means connected across said input conductors and connected to ground for passing leakage currents therethrough on alternate half cycles of said ac voltage;

amplifier means having signals representing said leakage currents as inputs thereto; and

means connected to said amplifier means for effecting an operative output therefrom only in response to a selected level of input to said amplifier means;

said first branch means including a diode connected to said other input conductor and poled to pass said first leakage current only during a positive half cycle on said other input conductor, said second branch means includes a diode connected to said one input conductor and poled to pass said second leakage current only during a positive half cycle on said one input conductor, said diodes being connected in common to ground, said detector means also including an amplifier having a common input for the leakage currents passing each diode.

3. A monitoring circuit for detecting leakage currents from power input conductors connected to a load, which input conductors carry an ac voltage, comprising:

detector means connected across said input conductors and connected to ground for passing leakage currents therethrough on alternate half cycles of said ac voltage;

amplifier means having signals representing said leakage currents as inputs thereto;

means connected to said amplifier means for effecting an operative output therefrom only in response to a selected level of input to said amplifier means;

said first branch means including a diode connected to said other input conductor and poled to pass said first leakage current only during a positive half cycle on said other input conductor, said second branch means includes a diode connected to said one input conductor and poled to pass said second leakage current only during a positive half cycle on said one input conductor, said diodes being connected in common to ground, said detector means also including an amplifier having a common input for the leaKage currents passing each diode;

a further diode connected between ground and in common to said input conductors to pass out of phase leakage current due to capacitive coupling of said input conductors to ground;

a second amplifier having signals representing said out of phase leakage currents as input thereto; and

means for summing the outputs of the first mentioned and said second amplifiers.

4. The monitoring circuit according to claim 3 wherein one of said first and said second amplifier is operated in inverting mode and the other is operated in non-inverting mode.

5. The monitoring circuit according to claim 4 wherein said means connected to said amplifier means comprises a further amplifier having an adjustable reference voltage as an input thereto, and a relay actuated by said further amplifier.

6. A monitoring circuit for detecting leakage currents from power input conductors connected to a load, which input conductors carry an ac voltage, comprising:

detector means connected across said input conductors and connected to ground for passing leakage currents therethrough on alternate half cycles of said ac voltage;

amplifier means having signals representing said leakage currents as inputs thereto; and

means connected to said amplifier means for effecting an operative output therefrom only in response to a selected level of input to said amplifier means;

said amplifier means comprising a first amplifier having an output proportional to leakage currents in phase with said ac voltage, and a second amplifier having an output proportional to leakage currents out of phase with said ac voltage.

7. The monitoring circuit according to claim 6 wherein one of said amplifiers is operated in inverting mode and the other amplifier is operated in non-inverting mode.

8. The monitoring circuit according to claim 7 wherein said means connected to said amplifier means comprises a further amplifier having an adjustable reference voltage as an input thereto, and a relay actuated by said further amplifier.

9. The monitoring circuit according to claim 1 including a voltage stabilizing amplifier connected to said amplifier means.

10. The monitoring circuit according to claim 1 including a reference voltage transformer connected across a pair of said input conductors, rectifying means connected to said transformer, and said rectifying means supplying reference voltage to said means connected to said amplifier means.

11. A monitoring circuit for detecting leakage currents from power input conductors connected to a load, which input conductors carry an ac voltage, comprising:

detector means connected across said input conductors and connected to ground for passing leakage currents therethrough on alternate half cycles of said ac voltage;

amplifier means having signals representing said leakage currents as inputs thereto;

means connected to said amplifier means for effecting an operative output therefrom only in response to a selected level of input to said amplifier means; and

reference voltage transformer connected across a pair of said input conductors, rectifying means connected to said transformer, and said rectifying means supplying reference voltage to said means connected to said amplifier means;

said amplifier means comprising a first amplifier having an output proportional to leakage currents in phase with said ac voltage, and a second amplifier having an output proportional to leakage currents out of phase with said ac voltage.

12. The monitoring circuit according to claim 11 wherein one of said amplifiers is operated in inverting mode and the other amplifier is operated in non-inverting mode.

13. The monitoring circuit according to claim 12 wherein said means connected to said amplifier means comprises a further amplifier having an adjustable reference voltage as an input thereto, and a relay actuated by said further amplifier.

14. The monitoring circuit according to claim 13 iNcluding a voltage stabilizing amplifier connected to said amplifier means.