string efficiency of a string insulator using Guard rings
Guard ring increases the string efficiency of an insulator string practically. If we don't use the guard wire then the voltage across different insulator disks will reduce as we move away from the conductor, thus the highest voltage will appear across the insulator disk which is nearest to the conductor. The guard ring helps in making the voltage distribution across various insulator disks uniform and thus improving the string efficiency.
Their arrangement you can see in the picture below.