"Tim W" wrote in message
...

A time delayed (type S) RCD may be used in place of the main CU isolator,
or
before it in a separate box, if you have a TT (earth rod) installation
earth. This is not and RCBO - it doesn't deal with current overloads and
it
doesn't need to, any more than the main 100A switch in a CU would. The
supplier's fuse does that.

You could also have a non delatey 100A RCD - but this would be essentially
stupid along with RCBOs.

Why would you want a main RCD if you have double pole RCBOs on all circuits?
I can't see the need for an RCD in this situation.