The regs in the UK say that (pretty much) every circuit must be RCD protected. But they also say that you can't use a single RCD to protect everything, because

The regs in the UK say that (pretty much) every circuit must be RCD protected. But they also say that you can't use a single RCD to protect everything, because you might need power to fix whatever has tripped the RCD. RCDs are expensive, so the most common solution is to have two RCDs, each protecting half of the circuits.