Electrical Glossary

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Class II appliance symbol, indicates an appliance that does not need to be earthed due to extra precautions against shock. Often called 'double insulated', though this is not an entirely accurate term.
2-way switching
A method of wiring switches so that a lamp can be turned on or off from more than one switch.


Sockets, switches etc. All the visible bits of an electrical installation.
A mains powered item other than a light fitting or an independent motor.
electrical conduction across insulation breakdown, often through air.
An obsolete plastic cable, a precursor of pvc. It lasts well, and is usually in good condition.


Current control device used to run a fluorescent tube or other discharge lamp.
Basic insulation
  1. Insulation applied to live parts to provide protection against electric shock rather to ensure the item functions.
  2. Insulation applied to live parts to provide basic protection against electric shock and which does not necessarily include insulation used exclusively for functional purposes.
Basic protection
Basic shock protection from direct contact with live parts. (formally known as "direct contact" protection)
Bayonet Cap, most popular type of lightbulb connection. See also ES, SBC, SES, Bipin.
2 pin connection used on fluorescent tubes & halogen lamps.
Bonding conductor
  1. A cable providing equipotential bonding.
  2. A protective conductor providing equipotential bonding.
Cables are bunched when two or more are contained within a single conduit, duct, ducting, or trunking or, if not enclosed, are close together.


Electrical wire not intended for regular flexing. Has solid single core copper conductors. see also flex. See Cables, Flex, Historic Mains Cables, Low Voltage Wiring.
Cable clips
Clips that hold cable in place. Usually a plastic clip fixed in place with a small masonry nail, but other types also exist.
Cable ducting
An enclosure of metal or insulating material, other than conduit or cable trunking, intended for the protection of cables which are drawn in after erection of the ducting.
Cable ties
Self locking plastic strap to secure cables
Cable trunking
  1. A closed tube, usually rectangular, of which one side is removable or hinged. Used to protect cables.
  2. A closed enclosure normally of rectangular cross-section, of which one side is removable or hinged, used for the protection of cables and for the accommodation of other electrical equipment.
Cartridge fuse
  1. A wire fuse enclosed in a fire-proof cartridge (like a 13A plug fuse). Mains cartridge fuses are usually filled with sand to extinguish arcs.
  2. A device comprising a fuse element or several fuse elements connected in parallel enclosed in a cartridge usually filled with arc-extinguishing medium and connected to terminations (see fuse link).
Compact Fluorescent Lamp. Energy saving lamp used in place of traditional Filament Lamps.
Chocolate block, choc block
screw connector strip for mains cables.
  1. a set of cable & accessories fed from one CU fuse or MCB.
  2. An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s).

  1. Also known as MCB. Device that switches power off in the event of overcurrent. Does approximately the same job as a fuse.
  2. 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.
Circuit protective conductor (CPC)
  1. Earth wire.
  2. A protective conductor connecting exposed-conductive-parts of equipment to the main earthing terminal. AKA earth conductor, earth wire.
Ring shaped fluorescent tube
Class I equipment
  1. Equipment using an earth connection to provide protection against shock.
  2. Equipment in which protection against electric shock does not rely on basic insulation only, but which includes means for the connection of exposed-conductive-parts to a protective conductor in the fixed wiring of the installation.
Class II equipment
  1. Equipment using enhanced levels of insulation to protect against shock (and some other measures). Does not need an earth connection. Also known as double insulated, and marked Logo.
  2. 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 metalwork of the equipment to a protective conductor, and no reliance upon precautions to be taken in the fixed wiring of the installation (see BS 2754).
Class III equipment
Equipment in which protection against electric shock relies on supply at SELV (separated extra low voltage) and in which voltages higher than those of SELV are not generated (see BS 2754).
  1. Tube that protects cables. Unlike trunking, it is not openable at the front, and cables must be threaded through.
  2. A part of a closed wiring system for cables in electrical installations, allowing them to be drawn in andlor replaced, but not inserted laterally.
Consumer unit (CU)
  1. Often called a fusebox. A distribution box that contains a main power switch, fuses or MCBs, and may also contain any of RCD, RCBO, and ocasionally a small transformer for supplying bell circuits etc.
  2. (may also be known as a consumer control unit or electricity control 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 on the incoming circuit(s) and an assembly of one or more fuses, circuit-breakers, residual current operated devices or signalling and other devices purposely manufactured for such use.
Relay (electrical)
Current-carrying capacity of a conductor
The continuous current at which the cable stays within its temperature ratings.
Current-using equipment
Equipment which converts electrical energy into another form of energy, such as light, heat or motive power.
Crossover Switch or Intermediate Switch
Type of switch used only in 3 or more way switching.


Design current (of a circuit)
The max current a circuit is designed to handle in normal service.
device for reducing the efficiency of Filament Lamps.
Direct contact
An electrical shock caused by contact of persons or livestock with intentionally live parts. Note this terminology is now deprecated, and the 17th edition of the wiring regs instead used the phrase "basic protection". See also Indirect Contact.
Distribution board
  1. CU or fusebox
  2. 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.
Discharge Lighting
Lighting using an electrical arc between a pair of electrodes (typically passing through an ionised gas) to produce light. This includes sodium & mercury lights, and metal halide. Other types of discharge lighting also exist.
Ensuring that when a protective device clears a fault, as few circuits as possible (ideally only the affected circuit) are disconnected. In the case of cascaded protective devices such as MCBs or Fuses, there must be sufficient difference in their trip rating such that only one one nearest the fault trips. (with cascaded RCDs discrimination is usually achieved by have devices with progressively faster response times as you move further from the circuits origin).
Double insulation
Insulation comprising both basic insulation and supplementary insulation. See 'Class II'


Electrical connection to damp earth / ground. 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, damp Earth.
Earth electrode resistance
The resistance of an earth electrode to Earth. Note more or less all of this resistance exists in the ground itself, not in the metal rod or other electrode.
Earth fault current
A fault current which flows to Earth
Earth fault loop impedance
The impedance of the earth fault current loop starting and ending at the point of earth fault. This impedance is abbreviated to Zs.
Earth leakage
the flow of mains current to earth, sometimes via a human. Earth current is often fault current, though not always.
Earth wire
electrical wire connected to earth. See CPC.
Earthed equipotential zone
A zone within which exposed-conductive-parts and extraneous-conductive-parts are maintained at substantially the same potential by connecting them together. Under fault conditions, the differences in potential between simultaneously accessible exposed- and extraneous-conductive-parts will not cause electric shock.
Connection of exposed conductive parts to the main earthing terminal of the installation.
Earthing conductor
Any conductor used to connect something to earth. This sometimes describes metal pipe, not only cable.
Device to cut electrical power in the event of certain earth leakage faults. There are 2 different types of ELCB, current operated (aka RCD) and the older voltage operated type. Now superceded by RCDs, the designation 'ELCB' indicates an old device.
Electric shock
A dangerous physiological effect resulting from the passing of an electric current through a human body or livestock.
Death by electric shock.
Appliance case
A part providing protection of equipment against certain external influences and in any direction protection against direct contact.
Equipotential bonding
Connecting exposed conductive parts together to prevent significant potential differences occurring between them.
Edison Screw. The most common size of screw-in lightbulb base. See also SES, MES and BC, SBC, Bipin.
Exposed conductive part
A conductive part of equipment that can be touched. It should not be a live part but could become live under fault conditions.
A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation.


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 very high current flow resulting from a short circuit fault between either Line and Neutral, Line and Earth, or Line to Line in three phase systems.
Fault protection
Protection from contact with extraneous-conductive-parts that have become live through a fault. Formally known a protection from indirect contact.

Filament lamp
Traditional light bulb containing a white hot filament. See also GLS.
Final circuit
A circuit connected directly to current-using equipment, or to a socket-outlet or socket-outlets or other outlet points for the connection of such equipment.
Fixed equipment
  1. Equipment designed to be secured in a specific location.
  2. Equipment designed to be fastened to a support or otherwise secured in a specific location.
Flexible cable (Flex)
A cable whose structure and materials make it suitable to be flexed while in service. The flexibility results from using stranded conductors.
Flexible cord
A flexible cable in which the cross-sectional area of each conductor does not exceed 4 mm2.
Fluorescent lamp
A highly energy efficient type of lighting.
Functional earthing
Connection to Earth necessary for proper functioning of electrical equipment.
Functional switching
An operation intended to switch 'on' or 'off' or vary the supply of electrical energy to all or part of an installation for normal operating purposes.
  1. Wire link designed to melt if excessive curent flows.
  2. A device which by the melting 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.
Box with switch & fuses. see also CU. See House Wiring for Beginners
Fuse carrier
The movable part of a fuse designed to carry a fuse link.
Fuse element
A part of a fuse designed to melt when the fuse operates.
Fuse link
  1. The wire in a fuse.
  2. A part of a fuse, including the fuse elerment(s), which requires replacement by a new or renewable fuse link after the fuse has operated and before the fuse is put back into service.


Specifies the number of duplicated components that are found on an accessory. Hence a dual gang socket or switch has two sockets or switches, a single gang, only one.
Grid switch
A switch assembly usually made up of a grid or yoke, individual switches (fuses, neons, other devices) and a front faceplate.
rubber ring-shaped item that protects cables etc from the sharp edges of a hole. Also see blind grommet
Mains voltage twist fit spot light type bulb mainly used in down lights. Similar in size and shape to low voltage (12 Volt) push fit MR16 bulbs.


Halogen lamp
a high temperature filament lamp. Popularly used as downlighters.
Harmonized Standard
  1. A standard agreed across the EU.
  2. A standard which has been drawn up by common agreement between national standards bodies notified to the European Commission by all member states and published under national procedures.
Device that switches electrical power according to humidity level. Usually switches power on when RH (relative humidity) goes above its threshold value.


Intermediate Switch
Type of switch used only in 3 or more way switching.
Intumescent Hood
See Halogen Lighting and Intumescent Materials
Indirect contact
Contact of persons or livestock with exposed-conductive-parts which have become live under fault conditions. Note this terminology is now deprecated, and the 17th edition of the wiring regs instead uses the phrase "fault protection".
Non-conductive material surrounding or covering a conductor.
  1. Disconnection of both L and N lines.
  2. A function intended to cut off for reasons of safety the supply from all, or a discrete section, of the installation by separating the installation or section from every source of electrical energy.
  1. Switch that isolates, ie disconnects both L and N lines.
  2. A mechanical switching device which, in the open position, complies with the requirements specified for isolation. An isolator is otherwise known as a disconnector.


Junction box
Box containing connection points for cables. Fire resistant.



Miniature solid state light emitter. Very low powered devices available in white and assorted monochromatic colours.
Leakage current
  1. Current leaking outside of the L/N circuit to earth.
  2. Electric current in an unwanted conductive path under normal operating conditions.
Live wire
Live part
  1. A conductor or conductive part intended to be energised in normal use, including a neutral conductor but, by convention, not a PEN conductor.
  2. A part that is live
Low noise earth
An earth connection in which the level of conducted or induced interference from external sources does not produce an unacceptable incidence of malfunction in the data processing or similar equipment to which it is connected. The susceptibility in terms of amplitude/frequency characteristics varies depending on the type of equipment.
  1. Light fitting
  2. Equipment which distributes, filters or transforms the light from one or more lamps, and which includes any parts 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.


the size & thread of screws used with standard UK electrical accessory faceplates.
Main earthing terminal
  1. Earth connection in CU
  2. 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.

Miniature Circuit Breaker, a pushbutton overcurrent cutout that replaces a fuse
Mercury Lamp
A high efficiency white discharge lamp with poor light quality. Sometimes used as white streetlights. Not as popular as they were 20-30 years ago.
Miniature Edison Screw: screw-in bulb base widely used on torch bulbs.
Metal Halide Lamp
High efficacy white discharge light available in various versions of white.
Mineral insulated copper clad cable. A very high temperature rated rigid cable. Widely used for fire critical circuits. Known as pyro. Hygroscopic.
Low voltage (12Volt)two pin push fit spotlight bulb. Similar in size and shape to the mains voltage version the GU10.
electrical meter that measures voltage, current (amps) and resistance. A worthwhile purchase for almost any DIYer.


Neon screwdriver
Simple voltage indicator. It is unsafe to rely on these when dealing with mains, as they give both false positive and false negative results.
Neutral conductor
  1. Black or blue conductor in domestic mains wire.
  2. 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 and also identifies either the mid-wire of a three-wire d. c. circuit or the earthed conductor of a two-wire earthed d,c. circuit.


Origin of an installation
The position at which electrical energy is delivered to an electrical installation.
A generic term encompassing overload currents (i.e. sustained current draw above the design current for the circuit), and fault currents (i.e. a high current flow resulting from a short circuit fault, the magnitude of which is typically only limited by the supply or earth loop impedance.
Overcurrent detection
A method of establishing that the value of current in a circuit exceeds a predetermined value for a specified length of time.
Overload current
An overcurrent situation arising from a sustained current demand being made on a circuit that exceeds its design current.


PAR38 & other numbers
Parabolic Aluminium Reflector spotlight bulbs.
* PAR number gives bulb diameter in 1/8ths of an inch
Portable Appliance Test, electrical goods safety test.
Backbox for electrical accessories (light switches, sockets, etc)
PEN conductor
  1. A conductor combining the functions of both protective conductor and neutral conductor.
Phase conductor
  1. Live conductor
  2. 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. The term in now deprecated in favour of Line conduction (17th edition)
  1. Connector with pins (compare socket)
  2. A device, provided with contact pins, which is intended to be attached to a flexible cable, and which can be engaged with a socket-outlet or with a connector.
Portable equipment
  1. Movable equipment
  2. Electrical equipment which is moved while in operation or which can easily be moved from one place to another while connected to the supply.
Protective conductor
  1. Earth conductor, earth wire.
  2. A conductor used for some measures of protection against electric shock and intended for connecting together any of the following parts: (i) exposed-conductive-parts (ii) extraneous-conductive-parts (iii) the main earthing terminal (iv) earth electrode(s) (v) the earthed point of the source, or an artificial neutral.
Prospective fault current
The maximum possible fault current.
The value of fault current at a given point in a circuit resulting from a fault of negligible impedance between live conductors and a circuit protective conductor (aka earth conductor).
Prospective short circuit current
The maximum possible short circuit current.
The value of short circuit current at a given point in a circuit resulting from a fault of negligible impedance between live conductors and a circuit neutral conductor or between live conductors of different phases.
Protective conductor current
Electric current which flows in a protective conductor (earth wire) under normal operating conditions.
Protective multiple earthing (PME)
  1. Type of supplier earthing. Has implications for some domestic earth arrangements.
  2. 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 Electricity Safety, Quality and Continuity Regulations 2002 (see also figure 5).
Solar PhotoVoltaic, an electricity producing solar panel.



50mm diameter spotlight bulb.
63mm diameter spotlight bulb.
80mm diameter spotlight bulb.
95mm diameter spotlight bulb.
Radial circuit
A final circuit using a length of cable feeding one socket outlet and then going on to the next. Also the usual way of arranging a domestic lighting circuit.
an MCB & RCD in one device. See RCD Electricity Basics & Rewiring Tips
device that cuts the power if it detects earth leakage. See also Rewiring Tips. Compare ELCB.
Electrically controlled switch. The first electrical computers were made from thousands of relays. Larger relays are called contactors.
Residual current
  1. Algebraic sum of the currents in the live conductors of a circuit at a point in the electrical installation.
  2. Any difference between L & N current
Residual current device
  1. RCD
  2. 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.
Residual operating current
Residual current which causes the residual current device to operate under specified conditions.
Ring final circuit (Ring circuit)
A final circuit arranged in the form of a ring and connected to a single point of supply.


Small Bayonet Cap. Miniature version of the more popular bayonet cap used on mains light bulbs.
Small Edison Screw. The 2nd most common size of screw-in lightbulb base, smaller than ES.
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.
Shock current
A current passing through the body of a person or livestock such as to cause electric shock and having characteristics likely to cause dangerous effects.
Short-circuit current
An overcurrent resulting from a fault of negligible impedance between live conductors having a difference in potential under normal operating conditions.
Simultaneously accessible parts
  1. Conductors or conductive parts which can be touched simultaneously by a person or, in locations specifically intended for them, by livestock.
  2. may be: live parts, exposed-conductive-parts, extraneous-conductive-parts, protective conductors or earth electrodes.
Cable with a single conductor. If the outer sheath of T&E cable is removed you're left with 2 singles. Used inside conduit & trunking.
electrical outlet into which a plug can be inserted.
Socket outlet
  1. Socket
  2. See socket. A device, provided with female contacts, which is intended to be installed with the fixed wiring, and intended to receive a plug.
A branch from a ring final circuit or a radial circuit to one device, typically one single or double socket outlet.
Sodium Lamp
2 different types of very high efficacy yellow and pink lighting. Much used for road lighting. Low pressure sodium lighting has the best energy efficiency of all lighting types.
Metal alloy used to join pipes and sometimes wiring. Lead & tin solder has long been used, but is no longer permitted for potable water piping.
Linear lamp, either fluorescent or filament. Also called tubular lamp.
Supplementary insulation
Independent insulation applied in addition to basic insulation in order to provide protection against electric shock in the event of a failure of basic insulation.
  1. Device that connects or disconnects power when operated by hand.
  2. A mechanical device capable of making, carrying and breaking current under normal circuit conditions, which may include specified operating overload conditions, and also of carrying for a specified time currents under specified abnormal circuit conditions such as those of short-circuit. It may also be capable of making, but not breaking, short-circuit currents.
Switch, linked
  1. A switch the contacts of which are so arranged as to make or break all poles simultaneously or in a definite sequence.
  • Unlikely to be encountered in the average house
Bank of 2 or more switches. Permits much greater control over lighting etc than just a single switch. Compare 'dimmer.'
  1. An assembly of switches
  2. An assembly of switchgear with or without instruments, but the term does not apply to groups of local switches in final circuits.
Switch & fuse. Historic forerunner of the fusebox
  1. Devices that switch
  2. An assembly of main and auxiliary switching apparatus for operation, regulation, protection or other control of an electrical installation.


T4, T5, T8, T12
Diameters of fluorescent tubes, in 1/8ths of an inch. Thus T12 is 1.5" diameter.
Twin & Earth, 3 core PVC cable used for most house wiring.
Any button or switch operated electrical cut-out, such as MCB, RCD, RCBO. A colloquial term.
Earthing system using a local earth, usually a rod but may sometimes be a pipe or grid.


A method of creating a very low resistance local earth connection, and eliminating potential differences between concrete floors and CPC during fault conditions, at no significant cost.


Volts multiplied by amps gives VA. This equals power for some loads, but is not equal to power for non-resistive loads.
India rubber electrical wiring. Common half a century ago.


Whole House RCD
A deprecated way of installing an RCD such that a single low trip threshold device (typically 30mA) protects all the circuits in a property. While counter to the advice given in the present wiring regulations. installations of this type are still commonly found. Whole house RCDs are very vulnerable to nuisance trips, and any such trips remove all power to the property.




See Also



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