A power filter circuit for protecting electronic equipment from electromagnetic interference introduced by a supply system circuit includes a fault/condition sensing circuit capable of detecting whether the supply system circuit is properly wired. The sensing circuit controls switching circuits in accordance with the determination of whether the supply system is properly wired. The sensing circuit activates a first switching circuit to form a short circuit between the neutral and ground conductors of the power filter circuit when it is determined that the supply system circuit is properly wired. If any wiring fault conditions, including a reverse polarity wiring, is detected by the sensing circuit, the first switch is not activated and the short circuit is not formed between the neutral and ground conductors. The sensing circuit controls a second switching circuit which corrects the polarity when a reverse polarity condition is detected and which does not change the polarity when a correct polarity condition is detected. The sensing circuit can be used with the first and second switching circuits in combination. In this case, the first switching circuit can form a short circuit between the line conductor of the power filter circuit and the ground conductor when a reverse polarity condition is detected to provide superior common mode electromagnetic interference suppression.