C&G 2391-52 key terms

UK statutory regulations placing duties on employers and employees to prevent danger from electrical systems at work, covering design, construction, maintenance, work activity and the competence of persons involved.

The enabling Act that sets general duties on employers to protect the health and safety of employees and others. Most electrical safety regulations, including the Electricity at Work Regulations, are made under it.

The Building Regulation for England and Wales covering electrical installations in dwellings. It requires reasonable provision for safety and notification of certain notifiable work to a building control body.

The national standard for electrical installations in the UK, published as the IET Wiring Regulations. It is a non-statutory code of practice widely used to demonstrate compliance with statutory duties.

Industry guidance published alongside BS 7671 that explains inspection and testing requirements in detail, including test methods, instrument use and completion of certificates and reports.

A conductive part of equipment that can be touched and is not normally live, but which may become live under fault conditions, such as the metal enclosure of an appliance.

A conductive part, such as a metal water pipe or structural steel, that is not part of the electrical installation but is liable to introduce a potential, usually earth potential, into the location.

Connecting together, and to the main earthing terminal, all simultaneously accessible exposed and extraneous conductive parts so that dangerous potential differences cannot develop between them.

A conductor connecting an extraneous-conductive-part, such as the incoming metallic water or gas service, to the main earthing terminal, sized according to the supply neutral or earthing conductor.

The conductor that connects exposed-conductive-parts of a circuit to the earthing arrangement, providing the earth fault current path and protective conductor continuity for that circuit.

A dead test in which the end-to-end resistance of line and protective conductors is measured and combined to give the resistance of the line-to-cpc loop, confirming continuity and allowing expected Zs to be calculated.

A three-stage continuity test on a ring final circuit that confirms the ring is unbroken and free of interconnections, by checking end-to-end resistances and readings taken with conductors cross-connected.

A check confirming that line conductors connect to the centre contact of lampholders and to the correct side of switches, sockets and protective devices, so single-pole devices operate in the line conductor.

A dead test applying a d.c. voltage between live conductors and earth, and between live conductors, to confirm the insulation is sound and free from damage, moisture or contamination.

The d.c. voltage applied during an insulation resistance test, typically 500 V for circuits rated up to 500 V, with a lower voltage used where connected equipment could be damaged.

The total impedance of the path taken by earth fault current, from the origin through the line conductor, the fault and the protective conductor back to the source. It determines disconnection time.

The impedance of the earth fault loop path external to the installation, measured at the origin with the installation isolated, and used with circuit resistances to predict Zs.

The current that would flow if a short circuit or earth fault of negligible impedance occurred at a point in the installation, used to check protective device ratings and breaking capacity.

The resistance between an earth electrode and the general mass of earth, measured by a suitable method such as fall-of-potential, and used to confirm the electrode is effective.

A formal procedure for taking an installation or circuit out of service: identifying all sources of supply, isolating them, securing against reconnection, proving dead and placing warning notices before work begins.

Checking that a voltage indicator works correctly before and after testing for dead, typically on a known live source or proving unit, so a faulty indicator cannot give a false dead reading.

A device and label applied to an isolator or protective device to prevent reconnection while work is in progress and to warn others that the circuit is being worked on.

The defined order in which tests are carried out, generally continuity and polarity first, then insulation resistance, then live tests such as Zs, RCD and functional checks, so results are valid and safe.

Confirmation that test instruments are within calibration and meet the accuracy required by BS 7671, including checking leads and using a check box or proving unit before use.

Assessment of an existing installation by inspection and testing to determine whether it remains safe to use, resulting in a condition report with observations and a recommended date for the next inspection.

Examining and testing a selected proportion of an installation rather than every circuit or accessory, chosen to give confidence in the condition of the whole where the installation is large.

The report produced after a periodic inspection, recording the extent and limitations of the inspection, observations and their codes, test results and an overall assessment of the installation's condition.

A classification given to each defect or departure found during inspection, such as C1 danger present, C2 potentially dangerous, C3 improvement recommended, or FI further investigation required.

A failure to comply with a requirement of BS 7671, which must be recorded on the certificate or report so that the person ordering the work is informed of the non-compliance.

The certificate used for a new installation, a new circuit or a major alteration, declaring that the design, construction, inspection and testing comply with BS 7671.

The certificate used for small works that do not include a new circuit, such as adding a socket to an existing circuit, confirming that the work complies with BS 7671.

A document forming part of a certificate or report that records the numerical test results for every circuit, together with circuit details such as conductor sizes and protective device ratings.

A protective device that disconnects the supply when it detects residual current to earth above its rated residual operating current, giving additional protection against electric shock.

The residual current at which an RCD is designed to operate, marked on the device. Testing at half this value checks it does not trip; testing at this or higher values checks operating time.

A live test measuring how long an RCD takes to trip at its rated residual current and at five times that current, compared with the maximum times permitted by BS 7671.

A classification of residual current devices by the waveform of residual current they detect, such as type AC for alternating current only and type A for alternating and pulsating d.c. residual currents.

Operating devices such as switches, isolators, RCD test buttons and emergency lighting to confirm they work as intended, carried out as part of inspection and verification.