How BS 7671 fits with UK law, the ELV and LV voltage bands, SELV, PELV and FELV, and the core definitions used throughout the Wiring Regulations.
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. Instead it is the accepted way to meet the legal duties set out in the Electricity at Work Regulations 1989. An installation built to BS 7671 is taken as safe and compliant unless shown otherwise.
Approved Document P
In England, Approved Document P covers electrical safety in dwellings. It forms part of the Building Regulations, so it does carry legal weight, and it relies on BS 7671 as the technical standard. Some jobs are notifiable to building control, while minor work may not be. The aim is to make sure fixed wiring in homes is designed and installed safely.
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.