Grounding resistances can be classified into high and low resistances.
In high resistance grounding, the fault current is limited to less than 10 amperes. While, in low resistance grounding, the current is limited to a value from 25 amperes or more.
The resistances are also categorized on the basis of time they can withstand the fault current. Typical durations are 1 second, 10 second, one minute and 10 minute rating.
The Extended Time rating resistor is used in systems where the reliability of the system is critical. This is true in petroleum industries, mines etc. In these situations, a high resistance which can sustain the fault for a long period is used. When an earth fault occurs of one phase, an alarm is generated. However, the system continues to run until the next scheduled shutdown.
Resistance grounding is not used in systems where the phase voltage exceeds 15kV for cost reasons.
Disadvantages of Solid Grounded Systems
Grounding Resistances - An introduction