Sory it is on this web site (Paul)
Well, at least that website is good for something
Let's go into a little more history, starting with the typical domestic installation of the 1960s. The situation is complicated in the U.K. by the different types of earthing arrangements employed (see link to U.K. power diagrams above).
In areas served by TN-S (installation grounded by a completely separate conductor right back to the transformer) or TN-C-S (grounded to the neutral, American-style), ground-fault protection normally relied upon the regular fuses or MCBs (overcurrent circuit breakers). In rural TT areas (installation grounded only to a local rod) though, the loop impedance could be far too high to blow even a 5A fuse on a lighting circuit, nevermind a 30A fuse on a ring or range circuit. So in houses fed by TT, it was necessary to fit a main ELCB to provide ground-fault protection to the whole installation. This was normally the voltage-operated ELCB described above at that time, but current-operated ELCBs could also be used, often with 300 or even 500mA trip in the earlier days.
Since that time, the voltage ELCB has become obsolete, the current-operated ELCB became the RCCB/RCD and the trip current reduced, first to 100mA and then to 30mA which is the norm today.
Under the current Regs., RCD protection at 30mA is required for any outlet which is likely to be used to feed equipment outdoors (it is also needed on any other circuit in which the loop impedance and operating time of the overcurrent device cannot provide disconnection within the specified time, but that would be rare in a domestic setting).
So, we have some circuits which need RCD protection and some which do not. In TN-S/TN-C-S houses, that gives several options. We could fit an individual RCBO for each branch circuit which requires RCD protection. Unfortunately, even though costs are dropping the RCBO is still relatively pricey here, so for anything more than a couple of circuits it can soon become very expensive.
The alternative, and more common solution to date, has been to use a split-load board. There is a simple main isolator switch (not circuit breaker) for the whole board, then circuits which do not
require RCD protection can be put on MCBs straight onto that main bus. The bus also feeds to a sub-main 30mA RCD, then circuits requiring RCD protection are placed on MCBs on that RCD bus. Thus a ground-fault on a circuit still knocks out more than just that one circuit, but not the whole house.
In rural TT areas -- where all circuits need RCD protection simply due to the high loop impedance -- it's still common to find just a single main 30mA RCD which protects the entire system (or a 100mA RCD installations dating back to the 1980s). That's far from ideal though, and it has led to wider use of the split-load board with two
This works in a similar way to the split-load board on a TN-S/TN-C-S system, but with a 100mA delayed-trip RCD in place of the main isolator switch to guarantee that a fault on the 30mA bus will trip that sub-RCD and not the main one.
Below is a typical split-load board, main switch on the right, 30mA RCD protecting the left-hand part of the board. For TT use, we'd swap the main on the right for the 100mA delayed RCD.