For RCD use, BS7671 states the following regulation:

411.3.3 Additional protection. 

In alternating current systems, additional protection by means of an RCD in accordance with Regulation 415.1 shall be provided for:

(i) socket-outlets with a rated current not exceeding 20 A, and

(ii) mobile equipment with a current rating not exceeding 32 A for use outdoors.

An exception to (i) is permitted:

(a) where, other than for an installation in a dwelling, a documented risk assessment determines that the RCD protection is not necessary, or

(b) for a specific labelled or otherwise suitably identified socket-outlet provided for connection of a particular item of equipment.

NOTE 1: See also Regulations 314.1(iv) and 531.2.4 concerning the avoidance of unwanted tripping.

NOTE 2: The requirements of Regulation 411.3.3 do not apply to FELV systems according to Regulation 411.7 or reduced low voltage systems according to Regulation 411.8.

NOTE 3: See Appendix 2, Item 10 with respect to risk assessment.