Are MCBs Becoming Obsolete?


Keith Willcock, Product Manager for Siemens Alpha circuit protection products reviews the implications for designers and installers of electrical installations from the latest requirements for additional protection for socket outlets in BS7671, including Amendment 2 and advises a simple solution.

Regulation 411.3.3 - Its all about the User

Within Chapter 41 (protection against electric shock) Regulation 411.3.3 now tells us: in AC systems additional protection, by use of a 30mA residual current device, must be provided for socket outlets (not rated higher than 32A) in locations where those socket outlets are likely to be used by persons of capabilty BA1, BA2 & BA3. There are no exceptions to this requirement.

Desk sockets

Previously this regulation did not overtly consider user capability. The regulation concerned itself with the equipment i.e. socket outlets. Now the person using the socket outlets is the principal consideration for designers & installers.

What are BA1, BA2, & BA3?

The terms BA1, BA2 & BA3 are explained in BS7671. Appendix 5  of the Wiring Regulations includes a list of external influences, and, in table B, under the sub heading of ‘Utilization’ BA refers to the capability of persons. BA1 is described as an ordinary person (a person who is neither a skilled person nor an instructed person) BA2 as children, and BA3 as disabled persons.

Which locations are included?

Typical obvious locations will include schools, universities, offices, factories, hospitals, nurseries, care facilities, hotels, indoor & outdoor locations. Actually, its hard to exclude any locations, the requirement really means all workplaces, all leisure facilities, and all living accommodation, all locations.  No exceptions permitted.

What is a Socket Outlet?

People probably imagine a common household 13A three pin socket outlet when they read Regulation 411.3.3, but the regulation isn’t limited to 13A sockets, it includes all types of socket outlets up to a maximum rating of 32A.

hanging sockets

The regulations state that a socket outlet is a device, provided with contacts, which is intended to be installed with the fixed wiring, and intended to receive a plug.

Taking account of the number of premises and users covered by this regulation there will be a wide variety of socket outlets that must have additional protection by a 30mA residual current device, including 2A, 5A, 13A, 15A, 16A, 20A & 32A. Round pin. Square pin. Switched or unswitched. Indoor or outdoor sockets.

What is the solution?

The simplest compliant solution is to use an individual Type A 30mA RCBO on each circuit that supplies power to any socket outlet. This will meet the requirement for additional protection, avoid unwanted tripping, and maintain power continuity on healthy circuits.

Type A RCBOs are suitable for circuits with equipment incorporating electronic components that may produce DC currents, such equipment is commonplace in modern installations. Type A RCBOs are designed to trip on alternating sinusoidal residual current and on residual pulsating direct current.  


Are MCBs Becoming Obsolete?

MCBs are incapable of meeting the requirements of BS7671 for additional protection and should only be used with a 30mA residual current device for circuits that supply socket outlets which will be used by ordinary persons, children or disabled persons.


Designers should not group several socket circuits on a single 30mA residual current device as this can actually cause unwanted / nuisance power outages to healthy circuits that share an RCD with a faulty circuit, and is very unlikely to comply with the wider requirements of BS 7671: Chapters 13 & 31, Regulations 531.3.2 etc