Taken from wiring matters magazine
The phrase ‘similar switchgear assemblies’ in Regulation 421.1.201 means those assemblies used for the same fundamental application as a consumer unit. A consumer unit is defined in Part 2 of BS 7671:
‘Consumer unit (may also be known as a consumer control unit or electricity control unit). A particular type of distribution board comprising a type-tested co-ordinated assembly for the control and distribution of electrical energy, principally in domestic premises, incorporating manual means of double-pole isolation on the incoming circuit(s) and an assembly of one or more fuses, circuit-breakers, residual current operated devices or signalling and other devices proven during the type-test of the assembly as suitable for such use.’
An example of a similar switchgear assembly is a three phase distribution board that is intended to be operated by ordinary persons. This would have to have isolation that interrupts the three incoming line conductors and the neutral, rather than just double-pole isolation as mentioned in the above definition.
So I would say a plastic enclosure for just an rcd stand alone is the same as a double pole isolator as used by the DNO is not applicable to the regulation.
I know the DNO isolator is outside the scope of bs 7671 but you get my point