SideWinder, your right, RCDS are, and should be considered as ''Additional protection''
I wonder how we ever got on before RCDs and there derivatives......!!!!
I'm going on to my 5th Reg's edition now, and it just amazes me how some of you younger sparks out there, consider these devices. Not your fault i know, but how you have been brought up, by taking the Reg's to the very last word.
The fuse and the MCB, and a good earthing installation/system.....ARE the main source of protection to a circuit. Sized correctly the fuse and the MCB will disconnect that circuit with out any additional means, and have been doing so since time in memorial.
Like it or not, an RCD or any similar devices, are ADDITIONAL protection, and only required in certain circumstances. Unfortunately these devices are being relied on in too many areas, where there additional protection is taken for granted, to the detriment of having an solid/good earthing system....
An RCD, is not a ''prerequisite'' in general terms for ADS, except in specific circumstances....
Nothing wrong with RCD devices at all, just don't rely on them as a plaster for not having the basics right. Use them for what they were designed for, Additional protection where needed, and you won't go far wrong!!!