Sch 2 para 15 Safety accessories
(1) Safety accessories must— (a) be so designed and constructed as to be reliable and suitable for their intended duty and take into account the maintenance and testing requirements of the devices, where applicable; (b) be independent of other functions, unless their safety function cannot be affected by such other functions; (c) comply with appropriate design principles in order to obtain suitable and reliable protection, including, in particular, fail-safe modes, redundancy, diversity and self-diagnosis.