This is the first TT installation I've worked on and I am adding a couple of sockets to an existing circuit, the main switch is a 80amp Wylex RCD 100ma I cannot make out the BS No , I've had a look at the earth loop impedance at the socket and it reads 10ohms. Is this ok, and will I need to replace the existing contactum CB with a RCBO. The RCD trips within the correct times I've checked that.Any help will be greatly received.