Coordination of surge suppressors in low-voltage AC power circuits

A theoretical and experimental study on the coordination of surge suppressors on an indoor low-voltage power system is reported. The system studied was a 120-V three-wire power line, equipped with phase, neutral, and ground conductors. Metal-oxide varistor surge suppressors were applied at three points on the system. These were at the service entrance, at a distribution panel, and at the load. Total line length studied was 30 m (100 ft), with the distribution panel and its suppressors being located at the central point. When unidirectional surges typical of lightning were applied at the service entrance, both experimental and theoretical studies showed similar results: removal of a suppressor at either load or distribution panel resulted in unacceptably large oscillatory voltages. Best load protection was achieved with suppressors in all three locations. Distribution of surge current between the three suppressors was shown to be good for both low and high surge currents. Coordination of protective levels was shown to be achieved, even with the very long surge waves typical of lightning