FROM UNGROUNDED TO HIGH RESISTANCE GROUNDING

This project was for a previously ungrounded 2,400V system.  A second ground fault would cause multiple portions of the plant to trip off.  Most faults occurred in the 2,400V motors, but instead of tripping off a single motor, large sections of the plant would be tripped.

Prior to the high resistance grounding system, the plant experienced ground faults that were not isolated, and tripped off large sections of the plant.  The system charging current was calculated to be less than 2 amps.  The existing system was changed from ungrounded to high resistance grounding using 10 amp grounding resistors on each of the two main 2,400V services from the utility.  With the main tie closed, total ground fault current is 20 amps (2 x 10 amps). 

The plant was upgrading the plant services and adding a new substation.  The client wanted the protection offered by a grounded system, with protective relaying that would isolate the faulted portion of the power system, instead of continuing to operate with a ground.  High resistance grounding was selected due to the insulation levels of the old 2,400V system.

High resistance grounding was selected due to the insulation levels of the old 2,400V system.

High resistance grounded industrial power systems generally limit the ground fault currents to a range of 5 to 10 amps.  The high resistance grounding will limit the maximum ground fault current and will limit transient overvoltages caused by arcing ground faults to acceptable levels.