For those of you who are familiar with British Standard, especially BS 7430-Code of practice for Earthing, when do you need to perform this calculation?

If you are employing TN-C-S system, which happens to be used here in US, do you still need to perform this calculation? As you know, the earth resistance doesn't mean much to us here in US since the earth is not a part of fault current path.

I am currently working on a project overseas where they adapted British Standard and they are asking for the earthing calculation. What confuses me is that they are saying they will use US earthing system, which is TN-C-S system. Why do you need an earthing calculation if you are using TN-C-S system?