My recent columns on ground fault protection have prompted several questions from readers regarding the practice of resistance grounding of 480-V distribution systems. These systems have been used in industrial applications for many years and are now frequently being considered for power systems supplying data centers and other mission-critical facilities.
On 480/277-V systems, a 27.7-Ohm resistor is common, limiting the maximum ground fault current to 10A and the maximum power dissipation in a ground fault to approximately 700W. At these levels, a ground fault in a piece of equipment such as a motor can be allowed to persist without hazard. The result is a power system that can tolerate insulation failure on a single phase without blowing a fuse or tripping a circuit breaker.