You have to select the resistance so that it can withstand 200A at least for 10 seconds.Earthing is a continuous headache in power distribution systems, especially in small or medium grids with considerable power, several consumer groups and transformers to distribute electrical energy at different voltages. I assumed your system is LRG and you would like to limit it to 200A (one of the common values in use for LRG). For LRG, it is 25A to 600A, while for HRG, it is less than 10A. The important parameter that is missing here is amount of current you would like to limit with the Neutral Grounding Resistor for your system. There are other two types such as Corner Grounding (for Delta system) and Ungrounded system but they are not commonly used. The ground fault current is limited to 10A or less. This way, the process plant is not interrupted. When ground fault occurs in HRG system, the monitoring systems gives alarm and the plant operators start the standby motor and stop the faulty one for the maintenance. The use of line to neutral (single phase) is prohibited (NEC, 250.36(3)) in HRG system, however, phase to neutral is used with using the additional transformer having its neutral grounded. This system must have a ground fault monitoring system. In the case of ground fault of one phase, the faulty phase goes to the ground potential but the system doesn't trip. With HRG, the neutral is grounded through a high resistance so that very small current flows to the ground if ground fault occurs. (3) High Resistance Grounding (HRG) - It is used where service continuity is vital, such as process plant motors. The ground fault current is limited to in the rage from 25A to 600A. In this system, the use of line to neutral (single phase) is prohibited. In this case, the system trips for the ground fault. (2) Low Resistance grounding (LRG) - This is used for limiting the ground fault current to minimize the impact of the fault current to the system. Its use is very common in low voltage system, where line to neutral voltage is used for single phase loads. If the ground fault occurs, high ground current passes through the fault. (1) Solid grounding – The neutral point of the system is grounded without any resistance. There are primarily three types of grounding system which are:

Earthing resistance calculate