Surge Arrester Grounding

That should be in the relevant code you must be working to also. Have a look at the grounding section again. You really should know this to be working on substations.

Here's an idea. Look at your as-built drawings and come back here if there is anything on them that you don't understand.

In general you should have a substation grounding grid at the base of the substation and all devices grounds should have a common ground to this grid. It's OK to ground all your PT's, Ct's and Arrestors connected together and routed to the substation common grounding grid. This is standard procedure. Remember that the structure top arrestor points should also be routed to the grounding grid with a separate appropriately sized braided CU conductors (usually 650KcM). There should be at least 18-⅝" Copperweld Gounding rods at the grid including the 4 rods at the structure base. Arrester drops should be sized according to standard lightning arrester manufacturer recommendations.

Each piece of equipment should have its individual tail connected to your facility's grounding bar. You can't use one tail for two pieces of equipment. Daisy chaining grounds renders all upstream grounds ineffective if the first is opened.

This is for any installation.

240kV you say....then learn fast and survive.

Read more as well.

Did I mention you should do some reading?