Yep, they're the ones, also known as "Type S" here.

The typical split-bus board on a TN-S or TN-C-S supply has a 100A isolator switch which feeds the non-RCD side of the board to which MCBs are fitted for lights, water heater, etc. That bus also feeds a sub-main RCD (typically 80A, 30mA trip) and the bus from that carries the MCBs for all the other circuits.

The problem comes on a TT supply where all circuits need RCD protection due to the high loop impedance. We need to replace the plain 100A D.P. switch with a main RCD, but make it a 100mA delay type to achieve discrimination with the 30mA RCD downstream.

The alternative is to use a single bus board with one main RCD. And I think you already know my views on that method! [Linked Image]



[This message has been edited by pauluk (edited 02-08-2005).]