| 摘要 |
A line isolation monitor for detecting ground faults in isolated power systems by determining the potential personnel hazard current due to the complex ground fault impedances and a method for accomplishing the same. The total hazard current is determined by dividing the peak line voltage of the isolated power supply by the total impedance between ground and a common mode point, such as the center tap of the primary winding of the line isolation monitor transformer connected to the isolated power supply. This impedance is determined at line frequency by impressing a minute, phase-modulated, power line frequency test signal into the isolated power system and using an extremely selective, coherent filter to remove the signals in phase with the power line signal. The magnitude of the phase modulated test signal passed through the filter is related to the amount of fault impedance. Comparison of this test signal magnitude to the line voltage provides an indication of the total hazard current. The circuitry for accomplishing this procedure includes a closed loop system which adjusts the current level of the test signal until the voltage of the filtered test signal reaches a predetermined level related to the line voltage of the isolated system. The current level of the test signal is then related to the total hazard current.
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