Abstract: An insulation resistance detection method and circuit for an ungrounded DC power supply systems. When the ratio of the voltage of the positive bus to that of the negative bus exceeds a preset range, the DC current is injected into the ungrounded DC power supply systems. DC power supply systems have essentially some grounded stray and external capacitors, so some of the injected DC current will flow into the grounded capacitor, increasing the voltage of the fault bus and the leakage current. The proposed method increases the leakage current by the injection of DC current, so the current sensor can detect the leakage current from which the insulation resistance can be calculated. The DC current injection method can significantly improve the detection of ground insulation faults in ungrounded DC power supply systems.

Abstract: A patrol system comprises: a computer, a receiving and sending communication device, and a plurality of signal elements. The computer and the receiving and sending communication device are wirelessly connected to each other. The receiving and sending communication device serves to sense the signal elements. The signal elements are disposed at designated patrol locations. When a patrolman with a receiving and sending communication device arrives at a designated patrol location and gives feedback to the computer, the computer will determine next patrol location to be patrolled and inform the patrolman. And thus the patrol route of each patrolman is determined randomly by the computer, so that the thief doesn't know the patrol route. Further, even the patrolman doesn't know the next patrol location to be patrolled, thus preventing the patrolman from colluding with the thief and betraying the patrol route.