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AC Alternating Current
Accessory A device, other than current-using equipment, associated with such equipment or with the wiring of an installation
Bonding Conductor A protective conductor providing equipotential bonding
Circuit An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)
Circuit-breaker A device capable of making, carrying and breaking normal load currents and also making and automatically breaking under pre-determined conditions, abnormal currents such as short-circuit currents. It is usually required to operate infrequently although some types are suitable for frequent operation.
Class 1 Equipment Equipment in which protection against electric shock does not rely on basic insulation only, but which includes means for connection of exposed-conductive parts to a protective conductor in a fixed wiring of the insulation
Class 2 Equipment Equipment in which protection against electric shock does not rely on basic insulation only, but in which additional safety precautions such as supplementary insulation are provided, there being no provision for the connection of exposed metal work of the equipment to a protective conductor, and no reliance upon precautions to be taken in the fixed wiring of the installation.
Class 3 Equipment Equipment in which protection against shock relies on supply at SELV and in which voltages higher than those of SELV are not generated.
Consumer Unit A particular type of distribution board comprising a co-ordinated assembly for the control and distribution of electrical energy, principally in domestic premises, incorporating manual means of double pole isolation of the incoming circuit(s) and an assembly of one or more fuses, miniature circuit-breakers, residual current operated devices or signalling and other devices purposely manufactured for such use.
CPC Circuit Protective Conductor - a protective conductor connecting exposed-conductive-parts of equipment to the main earthing terminal
Current Measured in Amperes or Amps it is the flow of a charge when a voltage is applied
Current-using equipment Equipment which converts electrical energy into another form of energy, such as a light, heat or motive power.
DC Direct Current
Distribution Board An assembly containing switching or protective devices (e.g. fuses, circuit breakers, residual current operated devices) associated with one or more outgoing circuits fed from one or more incoming circuits, together with terminals for the neutral and protective circuit conductors. It may also include signalling and other control devices. Means of isolation may be included in the board or may be provided separately.
Earth The conductive mass of the Earth, whose electric potential at any point is conventionally taken as zero
Earth electrode A conductor or group of conductors in intimate contact with, and providing an electrical connection to, Earth
Earthing Connection of exposed-conductive-parts of an installation to the main earthing terminal of that installation.
Earthing conductor A protective conductor connecting the main earthing terminal of an installation to an earth electrode or to other means of earthing
Electric shock A dangerous physiological effect resulting from the passing of an electric current through a human body or livestock
Electrical installation An assembly of associated electrical equipment supplied from a common origin to fulfill a specific purpose and having certain co-ordinated characteristics.
ELV Extra Low Voltage normally not exceeding 60 V a.c. or 120 V ripple free d.c., whether between conductors or to Earth.
Equipotential bonding Electrical connection maintaining various exposed-conductive-parts and extraneous-conductive-parts at substantially the same potential
Exposed-conductive-part A conductive part of equipment which can be touched and which is not a live part but which may become live under fault conditions.
Extraneous-conductive-part A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation
Fault A circuit condition in which current flows through an abnormal or unintended path. This may result from an insulation failure or a bridging of insulation. Conventionally the impedance between live conductors or between live conductors and exposed- or extraneous-conductive parts at the fault position is considered negligible.
Fault current A current resulting from a fault
Fuse A device which by the fusing of one or more of its specially designed and proportioned components, opens the circuit in which it is inserted by breaking the current when this exceeds a given value for a sufficient time. The fuse comprises all the parts that form the complete device.
Insulation Suitable non-conductive material enclosing, surrounding or supporting a conductor.
Isolation A function intended to cut off the reasons of safety the supply from all, or discrete section, of the installation by separating the installation or section from every source of electrical energy.
Isolator A mechanical switching device which, in the open position, complies with the requirements specified for isolation. An isolator is otherwise known as a disconnector.
Live Part A conductor or conductive part intended to be energised in normal use, including a neutral conductor but, by convention, not a PEN conductor.
Luminaire Equipment which distributes, filters or transforms the light from one or more lamps, and which includes any part necessary for supporting, fixing and protecting the lamps, but not the lamps themselves, and, where necessary, circuit auxiliaries together with the means for connecting them to the supply. For the purposes of the regulations a lampholder, however supported, is deemed to be a luminaire.
LV Low Voltage normally exceeding extra-low voltage but not exceeding 1000 V a.c. or 1500 V d.c. between conductors, or 600 V a.c. or 900 V d.c. between conductors and Earth.
Main earthing terminal The terminal or bar provided for the connection of protective conductors, including equipotential bonding conductors, and conductors for functional earthing, if any, to the means of earthing.
MCB Miniature Circuit Breaker acts in a similar way to fuse.
Neutral conductor A conductor connected to the neutral point of a system and contributing to the transmission of electrical energy. The term also means the equivalent conductor of an IT or d.c. system unless otherwise specified in the Regulations.
PELV Protective Extra-Low Voltage - An extra-low voltage system which is not electrically separated from earth, but which otherwise satisfies all the requirements for SELV
PEN conductor A conductor combining the function of both protective conductor and neutral conductor.
Phase conductor A conductor of an a.c. System for the transmission of electrical energy other than a neutral conductor, a protective conductor or a PEN conductor. The term also means the equivalent conductor of a d.c. System unless otherwise specified in the Regulations.
PME Protective Multiple Earthing - An earthing arrangement, found in TN-C-S systems, in which the supply neutral conductor is used to connect the earthing conductor of an installation with Earth, in accordance with the Electrical Supply Regulations 1988 as amended.
Power Measured in Watts it is the rate of energy transfer, Amps, when a voltage is applied
Protective Conductor The earth cable or conductor in a circuit
RCBO Residual Current Breaker with integral Overcurrent protection is similar to an RCD but also has the added function of an MCB
RCCB Residual Current Circuit Breaker similar to an RCBO
RCD Residual Current Device a mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions.
SELV Separated Extra Low Voltage an extra-low voltage system which is electrically separated from Earth and from other systems in such a way that a single fault cannot give rise to the risk of electric shock.
TN System Terre Neutre a system having one or more points of the source of energy directly earthed, the exposed-conductive-parts of the installation being connected to that point by protective conductors.
TN-C-S System Terre Neutre - Combined - Separated a system in which the neutral and protective functions are combined in a single conductor in part of the system.
TN-S Terre Neutre - Separated a system having separate neutral and protective conductors throughout the system
TT A system having one point of the source of energy directly earthed, the exposed-conductive-parts of the installation being connected to earth electrodes electrically independent of the earth electrodes of the source
VAC Volts Alternating Current
VDC Volts Direct Current
Voltage bands

Band 1 covers
- installations where protection against electric shock is provided under certain conditions by the value of voltage;
- installations where the voltage is limited for operational reasons (e.g. telecommunications, signalling, bell, control and alarm installations).
Extra-low voltage (ELV) will normally fall within voltage band 1

Band 2
Band 2 contains the voltages for supplies to household, and most commercial and industrial installations. Low voltage (LV) will normally fall within voltage band 2. band 2 voltages do not exceed 1000 V a.c. rms or 1500 V d.c.

Voltage, nominal

Voltages by which an installation (or part of an installation) is designed. The following ranges of nominal voltage (rms values for a.c. are defined:
- Extra-low. Normally not exceeding 60 V a.c. or 120 V ripple free d.c., whether between conductors or to Earth.
- Low. Normally exceeding extra-low voltage but not exceeding 1000 V a.c. or 1500 V d.c. between conductors, or 600 V a.c. or 900 V d.c. between conductors and Earth.

The actual voltage of the installation may differ from the nominal value by a quantity within normal tolerances