Definitions used in BS7671:2008

What it means
Most circuits which require additional protection will need to have Residual Current Devices with a sensitivity of 30 mA.

Solution
Use RCBO’s for individual circuit protection which incorporate overload, fault and additional protection or group circuits fed from a Residual Current Circuit Breaker with individual overload and fault protection for each circuit.

What it means
Sockets which are for use by ordinary persons and under 20 amp (someone who is neither a competent or instructed person) need additional protection as above..

Solution
These sockets need to be protected with 30 mA residual devices unless the use is under the supervision of a competent or instructed person or cannot be accessed by an ordinary person. Various solutions exist depending on the type and use of the installation.