Methods and apparatus for sensing ground leakage and automated self testing thereof

1. An apparatus, comprising:

a leakage current sensor configured to measure leakage current from a source of AC power on a primary side of an electrical isolation boundary, which provides power to at least one load circuit on said primary side of said electrical isolation boundary;

said leakage current sensor including (i) a first circuit on the primary side of the electrical isolation boundary; (ii) a second circuit on a secondary side of the electrical isolation boundary; and (iii) a third circuit that inter-couples the first and second circuits of the leakage current sensor while maintaining the electrical isolation boundary therebetween, including at least

(a) a differential current transformer having a primary winding formed by one or more equal numbers of turns of respective legs about a transformer core as such legs pass from the source of AC power to the at least one load circuit, and a secondary winding formed by a plurality of turns of wire about the transformer core; and

(b) an isolator circuit having:

(1) a primary side circuit operating to induce an unbalanced flux in the transformer core that produces a current in the secondary winding of the differential current transformer, the unbalanced flux being in response to an intermediate control signal crossing the electrical isolation boundary from the secondary side thereof; and

(2) a secondary side circuit operating to produce the intermediate control signal in response to a test control signal produced by a control circuit, said secondary side circuit including a switch circuit that turns on and off in response to the intermediate control signal; a test winding of one or more turns on the differential current transformer; and a resistance; wherein the switch circuit, test winding, and resistance are coupled in series between the legs such that the unbalanced flux is produced in response to the intermediate control signal turning the switch circuit on; and

a control circuit on a secondary side of the electrical isolation boundary configured to: (i) receive measured leakage current values from the leakage current sensor at least at some times other than during a self test of the leakage current sensor, (ii) compare the measured leakage current values against a plurality of predetermined thresholds at least at some times other than during a self test of the leakage current sensor; (iii) provide a status output indicating the status of leakage current with respect to said plurality of thresholds; and (iv) perform a self test to determine whether the leakage current sensor is operative by providing said test control signal.

2. The apparatus of claim 1 , wherein:

the control circuit is programmable by an operator in order to set at least one of the predetermined thresholds;

the predetermined thresholds include at least one of: (i) a first threshold level indicating a measure leakage current that is excessive; and (ii) a second threshold level, higher than the first threshold level; and

the control circuit operates to store and retrieve results of the comparisons of the measured leakage current values against the predetermined thresholds.

3. The apparatus of claim 2 , wherein at least one of:

the control circuit is programmable by an operator in order to set at least one of: (i) periodic self test time intervals; (ii) one or more dates of the self tests, and (iii) one or more times for the self tests; and

the control circuit operates to store and retrieve results of one or more of the self tests for validation of the leakage current sensor.

4. The apparatus of claim 3 , wherein the control circuit operates to store into an event log at least one of: the results of one or more of the self tests; and the comparisons of the measured leakage current values against the one or more predetermined thresholds.

5. The apparatus of claim 4 , wherein the control circuit operates to store the status indicators into the event log, including at least one of:

whether the measured leakage current values are at normal levels, because the comparison indicates that the measured leakage current values are below the first threshold level;

whether the measured leakage current values are at warning levels, because the comparison indicates that the measured leakage current values are above the first threshold level but below the second threshold level;

whether the measured leakage current values are at critical levels, because the comparison indicates that the measured leakage current values are above the second threshold level;

whether a self test is in progress;

whether results of one or more self tests indicate that the leakage current sensor is non-operative; and

whether results of one or more self tests indicate that the leakage current sensor is operative.

6. The apparatus of claim 3 , further comprising a display operatively coupled to the control circuit, wherein the display shows a visual indication of at least one of:

the measured leakage current values;

the first threshold level;

the second threshold level;

the periodic self test time intervals;

the one or more dates of the self tests; and

the one or more times for the self tests.

7. The apparatus of claim 3 , further comprising a display operatively coupled to the control circuit, wherein the display shows a visual indication of the status, the status including at least one of:

whether the measured leakage current values are at normal levels, because the comparison indicates that the measured leakage current values are below the first threshold level;

whether the measured leakage current values are at warning levels, because the comparison indicates that the measured leakage current values are above the first threshold level but below the second threshold level;

whether the measured leakage current values are at critical levels, because the comparison indicates that the measured leakage current values are above the second threshold level;

whether a self test is in progress;

whether results of one or more self tests indicate that the leakage current sensor is non-operative; and

whether results of one or more self tests indicate that the leakage current sensor is operative.

8. The apparatus of claim 3 , further comprising a message transmission system coupled to the control circuit and operating to transmit one or more messages indicating at least one of: the results of one or more of the self tests; and the comparisons of the measured leakage current values against the one or more predetermined thresholds.

9. The apparatus of claim 8 , wherein the message transmission system is operable to send the one or more messages in at least one of: (i) a simple network management protocol, (ii) a packet switched network protocol, (iii) an electronic mail protocol, (iv) an instant messaging protocol, and (v) a telephone protocol.

10. The apparatus of claim 9 , wherein the message transmission system is programmable by an operator to select the messaging protocol.

11. The apparatus of claim 10 , further comprising a display operatively coupled to the control circuit, wherein the display shows a visual indication of the selected messaging protocol.

12. The apparatus of claim 9 , wherein the status indicators include at least one of:

whether the measured leakage current values are at normal levels, because the comparison indicates that the measured leakage current values are below the first threshold level;

whether the measured leakage current values are at warning levels, because the comparison indicates that the measured leakage current values are above the first threshold level but below the second threshold level;

whether the measured leakage current values are at critical levels, because the comparison indicates that the measured leakage current values are above the second threshold level;

whether a self test is in progress;

whether results of one or more self tests indicate that the leakage current sensor is non-operative; and

whether results of one or more self tests indicate that the leakage current sensor is operative.

13. A method, comprising:

measuring leakage current from a source of AC power, which provides power to at least one load circuit, said source of AC power and said load circuit being on a primary side of an electrical isolation boundary;

comparing the measured leakage current values against a plurality of predetermined thresholds;

providing a status output indicating the status of leakage current with respect to said plurality of thresholds; and

automatically performing self tests to determine whether the leakage current measuring and comparing operations are operative,

wherein comparing the measured leakage current values and providing a status output are performed on a secondary side of said electrical isolation boundary electrically isolated from the primary side, and performing self tests is triggered by a test control signal from said secondary side of said isolation boundary.

14. The method of claim 13 , further comprising permitting an operator set at least one of:

the plurality of predetermined thresholds; and

timing at which the self tests are performed.

15. The method of claim 14 , wherein

the predetermined thresholds include at least one of: (i) a first threshold level indicating a measure leakage current that is higher than desired; and (ii) a second threshold level, higher than the first threshold level; and

the timing at which the self tests are performed include at least one of: (i) periodic self test time intervals; (ii) one or more dates of the self tests, and (iii) one or more times for the self tests.