BS 7671 Status, Voltage Bands and Key Definitions

What BS 7671 Is

BS 7671, the Wiring Regulations, is a British Standard published jointly by the IET and BSI. It is non-statutory, which means it is not law on its own. Following it is a widely accepted way to help meet the relevant duties under the Electricity at Work Regulations 1989. That does not establish compliance with every legal duty: some equipment and hazards fall outside the standard’s scope, and the safety of the actual installation and work must still be assessed.

Approved Document P

In England, Part P of the Building Regulations sets legal requirements for electrical safety in dwellings. Approved Document P gives guidance on meeting those requirements, including using BS 7671 for the technical work. The guidance describes an accepted route to compliance; other solutions may meet the requirements, and following it does not remove the need to check whether it suits the particular work. Some work must be notified to building control. Work that does not need notification must still meet the applicable safety requirements.

Voltage Bands

BS 7671 groups voltages into bands. The two to know are:

• Extra-low voltage (ELV): not exceeding 50 V AC or 120 V ripple-free DC between conductors or to earth.
• Low voltage (LV): above ELV but not exceeding 1000 V AC or 1500 V DC. Standard 230 V single-phase and 400 V three-phase supplies are LV.

SELV, PELV and FELV

These three describe extra-low voltage systems:

• SELV (separated extra-low voltage): supplied from a safety source such as a safety isolating transformer, electrically separated from earth and from other circuits. No live part is earthed.
• PELV (protective extra-low voltage): the same idea as SELV, but the system may be connected to earth.
• FELV (functional extra-low voltage): used where extra-low voltage is needed for functional reasons only. It does not meet the stricter SELV or PELV rules, so it does not give the same protection against shock.

Key Definitions

• Exposed-conductive-part: a conductive part of equipment that can be touched and could become live under a fault, such as a metal enclosure.
• Extraneous-conductive-part: a conductive part not forming part of the installation but liable to introduce a potential, usually earth potential, such as metal water or gas pipework.
• Basic protection: protection against shock under normal, fault-free conditions, for example insulation and barriers.
• Fault protection: protection against shock under single-fault conditions, for example ADS.

The current edition

The current standard is BS 7671:2018+A4:2026. Amendment 4 amends the 18th Edition; there is no 19th Edition, and Amendment 3 remains valid during the transition, until 15 October 2026. City & Guilds 2382 centres move to 2382-26, written to Amendment 4, from 16 April 2026, so revising only older material misses the amendment's additions. These include stationary secondary batteries and Power over Ethernet, meaning extra-low-voltage DC power carried over balanced data cabling. A certificate records the edition it was issued under, so later readers know what the work was judged against.

People, design and definitions

• An ordinary person is neither skilled nor instructed. A skilled person has the knowledge and experience to recognise and avoid the electrical danger of a particular task. Select protective measures and access arrangements for the intended users. Insulation, barriers and enclosures provide basic protection in ordinary use; 30 mA RCD additional protection is required for the circuits and conditions specified in the standard, rather than for every circuit simply because ordinary persons are present.
• A live part is one intended to be energised in normal use. That includes the neutral, though by convention not a PEN conductor.
• A departure is a deliberate, recorded design decision not to follow a particular requirement, where the designer shows the result is no less safe; it is recorded on the certificate.
• Maximum demand is the highest load the installation is expected to draw. Apply diversity where justified by how the loads are used; some loads can run together at full output, so diversity is not an automatic reduction. Check it before finalising a design or adding a sizeable load such as a shower, cooker or heat pump, since the supply, tails and switchgear may not have spare capacity.
• External influences, such as water, heat, corrosion, impact and how the location is used, govern the choice and erection of equipment. Defined terms fix meanings so a requirement is read the same way by every designer, installer and inspector.
• A 230/110 V transformer with a centre-tapped secondary gives about 55 V to earth, the reduced low voltage used for site tools.
• A declared 230 V supply may vary between 216.2 V and 253.0 V at the supply terminals (10% above, 6% below), with 50 Hz held within 1%.
• Part P exists to protect people in dwellings from fire and injury. Responsibility for compliance can rest with the designer, installer, builder or owner, depending on who controls the work, and a registered third-party certifier can inspect and certify notifiable work.
• A generating set is a source of supply, not a load: assess its earthing, isolation and protective arrangement. On battery circuits, follow the specified polarity and connection arrangement. Reversal can damage equipment or defeat polarity-sensitive protection, so DC suitability is more than a voltage rating.
• Identify conductors clearly at their terminations. Markings help trace their intended functions, but are not proof that connections are correct or that a conductor is dead; verify those matters separately.