What does ‘wiring system’ and ‘accessory’ mean within Part 2 of BS 7671? | Scolmore

What does ‘wiring system’ and ‘accessory’ mean within Part 2 of BS 7671? | Scolmore

Jake Green, Technical Engagement Manager at Scolmore, briefly discusses two simple terms defined in Part 2: ‘wiring system’ and ‘accessory’ as they are to be understood within BS 7671.

The use of language when describing electrical terms matters. Confusion over terms has been the cause of many errors as people seek to interpret what a word or phrase could mean. Part 2 of BS 7671 is an essential read insofar as understanding what a specific regulation means. It details the specific definitions to be used within the Standard and provides clarity of meaning to ensure that all those involved in electrical installation work are ‘singing from the same song sheet’.

Wiring system

A wiring system is described within Part 2 as: ‘an assembly made up of cable or busbars and parts which secure and, if necessary, enclose the cable or busbars.’

There are other definitions detailed within Part 2 for:

● Flexible wiring systems

● Prefabricated wiring systems

A flexible wiring system is, ‘a wiring system designed to provide mechanical flexibility in use without degradation of the electrical components.’ A prefabricated wiring system is, ‘a wiring system consisting of wiring sections incorporating the means of interconnection designed to allow sections to be connected together to form a given system and incorporating installation couplers conforming to BS EN 61535.’

A wiring system is, therefore, by definition, a combination of elements that when taken together form a wiring system, and it is within that context that any regulation referencing wiring systems must be considered. This combination might be, for example, a sheathed and insulated cable clipped directly to a surface or single-insulated conductors within conduit or trunking.

Chapter 52 covers the selection and erection of wiring systems. In addition to any external influences and installation methods, the designer and installer should consider the following:

● Cables and conductors

● Connections, terminations, and jointing

● Associated supports or suspensions

● The enclosures or methods of protection against external influences. (Regulation 520.4)

It is clear therefore, that it is essential for any wiring system to be adequately supported. This will include support for any trunking/conduit systems along with clips and cleats for cables. Regulation 521.10.202 is a particular example worth considering. This regulation is relatively new and requires wiring systems to be supported ‘such that they will not be liable to premature collapse’. This requirement came about to limit the risk to firefighters from entanglement as they entered premises.

To address the requirement of Regulation 521.10.202, Unicrimp has a range of products that are fully capable of supporting wiring systems and cables. These include all-round banding, fire safe cable clips, stainless steel roller ball cable ties and metal LSF cable clips.


An accessory is defined in Part 2 of BS 7671 as: ‘a device, other than current-using equipment, associated with such equipment or with the wiring of an installation.’

An accessory, therefore, does not require current to function. Examples of accessories are commonplace and include amongst others:

● Switches

● Socket-outlets

● Fused connection units

● Connection units

● Cooker control units and the like.

There are several regulations that relate specifically to accessories that a designer and installer should be conscious of. These include:

● Regulation 522.2.201 requires that parts of a cable within an accessory is suitable for the likely temperatures to be encountered. This requires the designer and installer to be conscious of where a rise in temperature may exist and to provide protective measures to mitigate their effects. Manufacturers have data sheets detailing the temperature operating ranges of their products.

● Regulation 526.3 requires every connection to be accessible for inspection, testing and maintenance, with certain exceptions. One such exception is for maintenance-free accessories suitably marked, conforming to BS 5733, and installed in accordance with manufacturer’s instructions.

It is important that the designer and installer selects those accessories which are suitable for the environment within which they are to be installed. Therefore, as with wiring systems, the factors to consider will include, amongst other things, temperature, humidity, the presence of dust and the aesthetics required.

Consider Scolmore’s Click Definity 20 A Control Switch. This range has interchangeable faceplates and comes in a series of double-pole switches with or without neon indication.

The technical guidance issued by Scolmore details critical factors that should be adhered to. These are detailed such that the products conform to the relevant Standard. Included amongst them are:

– Voltage/frequency: 240 V / 50 Hz

– Current rating: 20 A

– Terminal size: 5 mm diameter

– Torque value: 1.8 Nm

– Cable size: 4 x 2.5 mm2

– IP rating: IP20

– Operational temperature: -5˚C to +40˚C

No accessory should be installed outside of the technical specification of the manufacturer.


The selection and erection of wiring systems and accessories are the ‘nuts and bolts’ of electrical installation work. The designer and installer should be constantly aware of the requirements of BS 7671 regarding both. Furthermore, manufacturer’s guidance should always be considered and care given to ensure that they are suitable for their environment.

To browse Unicrimp’s Q-Fire range of products, click here

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