What is good ground resistance value?
An arbitrary answer to this in ohms is difficult. The lower the ground resistance the safer, and for positive protection of personnel and equipment, it is worth the effort to aim for less than 1W. It is generally impractical to reach such a low resistance along a distribution system or a transmission line or in small substation. In some regions, resistance of 5W or less may be obtained without much trouble. In other regions it may be difficult to bring resistance of driven grounds below 50W.
Accepted industry standards stipulate that transmission substations should be designed not to exceed 1W. In distribution substations, the maximum recommended resistance is for 5W or even 1W. In most cases, the buried grid system of any substation will provide the desired resistance.
In light industries as in telecommunication central office, 5W is often the accepted value. For lightning protection the arrestor should be coupled with a maximum ground resistance of 1W. The goal in ground resistance is to achieve the lowest ground resistance value possible that makes sense economically and physically.
As per National Electrical Code It is recommended that the value of any earth system resistance shall not be more than 5.0, unless otherwise specified.