When mounting the surge counter the connection of the counter to the arrester bottom terminal should be isolated cable as per manufacturer instruction but what about the connection of counter opposite terminal that will be connected with the grounding grid, it also should be with insulated cable????????

Thanks

There is no need that I know of to use insulated cable between counter and ground.

It certainly will not affect current flowing through counter so it will not affect operation of the counter.

Before counters came along, arrester bases were not even insulated from transformer tank and ground cable was not insulated from tank (that is the configuration of ours transformer arresters... uninsulated ground cable from arrester base to ground point with no attempt to insulate the cable from the tank).

So I certainly don't see any reason to insulate the cable between counter and ground.

One small caution - you may have some different configuration or code requirements than I am aware of. In the end, you are responsible to make sure it is safe and legal for your installation.