AM2 / AM2S key terms

A careful look at the work before it starts to identify what could cause harm, how likely and how serious that harm is, and what controls will reduce it to an acceptable level.

A written description of how a particular job will be carried out safely, setting out the sequence of work, who is responsible and the control measures applied at each stage.

A formal written authorisation allowing defined work on or near a system for a limited period, recording the isolations and precautions taken and confirming when the system is handed back.

Items worn or held by the worker, such as safety footwear, gloves, eye protection and a hard hat, that reduce exposure to a hazard and are used alongside other controls, never instead of them.

The practical task in the assessment where containment, cables, accessories and protective devices must be installed to a given specification, safely and within a set time, to a professional standard.

The enclosure at the origin of a domestic installation that holds the main switch, circuit breakers and often RCDs, and from which the final circuits of the premises are supplied.

A small automatic switch that opens on overload or short-circuit current to protect the circuit it supplies, and can be reset by hand once the fault has been dealt with.

A device that continuously compares the current leaving in the line conductor with the current returning in the neutral and trips when the imbalance, caused by earth leakage, reaches its rated value.

A single device that gives a circuit both overcurrent protection and residual current protection, so that an earth fault on that circuit trips only that circuit's own device.

The conductor that connects the exposed conductive parts of a circuit, and any associated extraneous conductive parts, back to the main earthing terminal to provide a path for earth fault current.

A conductor that links incoming metal services such as water, gas and oil to the main earthing terminal so those services sit at substantially the same potential as the earthing system.

A cable in which a layer of steel wires surrounds the insulated cores to give mechanical protection; when the gland is correctly made off, the armour can also serve as the circuit protective conductor.

A cable designed to keep working, carrying current and maintaining circuit integrity, for a stated time while exposed to fire, used for alarms, emergency lighting and other life safety circuits.

The reduction applied to a cable's tabulated current rating when the cable is enclosed in thermal insulation, because trapped heat makes the conductor run hotter for the same load.

A two-digit code marked on an enclosure stating the degree of protection it gives against entry of solid objects and of water, for example IP44 or IP65.

A test made with the supply isolated and loads disconnected, applying a steady DC voltage between conductors and earth to measure, in megohms, how well the insulation resists leakage.

A low-resistance measurement taken with the supply safely isolated to prove that a conductor, such as a protective or bonding conductor, runs unbroken from one end to the other.

A measurement of the line conductor resistance plus the circuit protective conductor resistance for a radial circuit, taken end to end to prove continuity and to predict the earth fault loop impedance.

Three linked resistance readings taken at the consumer unit that prove the line, neutral and protective conductors of a ring final circuit are continuous, correctly connected and form one unbroken loop.

A check confirming that line conductors are connected to the line terminals of accessories and that switches are fitted in the line conductor, so single-pole devices isolate safely.

The total impedance of the earth fault loop, from the origin through the line conductor, the fault and the protective conductor back to the source, used to check disconnection times are met.

The impedance of the supply and the installation's means of earthing, measured at the origin with the main switch open and all final circuits disconnected.

A test with a dedicated instrument that measures how quickly an RCD operates at its rated residual current and at half that value, compared with the permitted maximum times.

The certificate issued for a new installation or for an alteration, declaring that the design, construction, inspection and testing comply with BS 7671 and supported by schedules of inspection and test results.

The shorter certificate used for small additions or alterations that do not include a new circuit, recording the work done and the inspection and test results for that work.

A report produced after inspecting and testing an existing installation, describing its condition, listing departures from BS 7671 and giving each observation a classification code.

The formal sequence for making equipment dead and keeping it dead: identify the supply, isolate, lock off and sign, prove the tester, prove dead at the point of work, then prove the tester again.

Using an approved voltage indicator to show that no dangerous voltage exists between every combination of live conductors and between each live conductor and earth at the exact point of work.

A two-pole test lamp or meter built to the recognised guidance for test leads, with fused leads, finger guards and short exposed probe tips, used to check for the presence of voltage.

A known source of test voltage used immediately before and after the prove-dead step to show that the voltage indicator is working and would actually detect a live supply.

A padlock, lock or locking attachment fitted to an isolator or protective device that physically prevents it being switched back on, usually with a warning notice naming the person working.

A logical method of testing at the middle of a suspect circuit and using the result to decide which half still contains the fault, repeating until the faulty section is narrowed down.

A wiring fault where a circuit's neutral is taken from, or shared with, a different circuit, so that isolating one circuit can leave a neutral live and RCDs may trip unexpectedly.

A loose, corroded or poorly made connection that adds resistance to a conductor, causing local heating, voltage drop and unreliable test readings at that point.

Unwanted operation of an RCD or RCBO when there is no genuine earth fault, often caused by accumulated leakage currents, damp, incorrect wiring or a faulty appliance.

A protective measure in which a fault is cleared by a protective device opening within a specified time, relying on a suitable earth path and the disconnection time set for the circuit voltage.

The way an installation is connected to earth, described as TN-S, TN-C-S or TT according to whether the earth path is a separate supply conductor, a combined neutral and earth, or a local electrode.

Coordination between protective devices so that the device nearest the fault opens first and the rest of the installation stays supplied.

A protective measure in which the circuit is supplied at a safe extra-low voltage from a source such as a safety isolating transformer and is isolated from earth and from other circuits.

The part of the Building Regulations applying in England and Wales that covers electrical safety in dwellings and requires certain notifiable work to be carried out and certified by a competent person.