Glossary of terms

Acceleration	Acceleration is the rate of change in velocity with time. $\text{Acceleration}=\frac{\text{Velocity}}{\text{Time}}=(\text{m}/{\text{s}}^2)$
Accident	An accident may be defined as an uncontrolled event causing injury or damage to an individual or property.
Alarm call points	Manually operated alarm call points should be provided in all parts of a building where people may be present, and should be located so that no one need walk for more than 30 m from any position within the premises in order to give an alarm.
Alloy	An alloy is a mixture of two or more metals.
Appointed person	An appointed person is someone who is nominated to take charge when someone is injured or becomes ill, including calling an ambulance if required. The appointed person will also look after the first aid equipment, including re-stocking the first aid box.
Approved test instruments	The test instruments and test leads used by the electrician for testing an electrical installation must meet all the requirements of the relevant regulations. All testing must, therefore, be carried out using an ‘approved’ test instrument if the test results are to be valid. The test instrument must also carry a calibration certificate; otherwise the recorded results may be void.
Basic protection	Basic protection is provided by the insulation of live parts in accordance with Section 416 of the IET Regulations.
Bonding	The linking together of the exposed or extraneous metal parts of an electrical installation.
Bonding conductor	A conductor providing protective bonding.
Cable tray	Cable tray is a sheet-steel channel with multiple holes. The most common finish is hot-dipped galvanized but PVC-coated tray is also available. It is used extensively on large industrial and commercial installations for supporting MI and SWA cables which are laid on cable tray and secured with cable ties through the tray holes.
Capacitive reactance	Capacitive reactance (XC) is the opposition to an a.c. current in a capacitive circuit. It causes the current in the circuit to lead ahead of the voltage.
Centrifugal force	Centrifugal force is the force acting away from the centre, the opposite to centripetal force.
Centripetal force	Centripetal force is the force acting towards the centre when a mass attached to a string is rotated in a circular path.
Circuit protective conductor (CPC)	A protective conductor connecting exposed conductive parts of equipment to the main earthing terminal.
Cohesive or adhesive force	Cohesive or adhesive force is the force required to hold things together.
Compact fluorescent lamps (CFLs)	CFLs are miniature fluorescent lamps designed to replace ordinary GLS lamps.
Compressive force	Compressive force is the force pushing things together.
Conductor	A conductor is a material, usually a metal, in which the electrons are loosely bound to the central nucleus. These electrons can easily become ‘free electrons’ which allows heat and electricity to pass easily through the material.
Conduit	A conduit is a tube, channel or pipe in which insulated conductors are contained.
Corrosion	Corrosion is the destruction of a metal by chemical action.
Delivery notes	A delivery note is used to confirm that goods have been delivered by the supplier, who will then send out an invoice requesting payment.
Duty holder	This phrase recognizes the level of responsibility which electricians are expected to take on as a part of their job in order to control electrical safety in the work environment. Everyone has a duty of care, but not everyone is a duty holder. The person who exercises ‘control over the whole systems, equipment and conductors’ and is the electrical company’s representative on-site is a duty holder.
Earth	The conductive mass of the earth, the electrical potential of which is taken as zero.
Earthing	The act of connecting the exposed conductive parts of an installation to the main protective earthing terminal of the installation.
Efficiency of any machine	The ratio of the output power to the input power is known as the efficiency of the machine. The symbol for efficiency is the Greek letter ‘eta’ ( ). In general, $\text{η}\hspace{0.17em}=\frac{\text{Power}\text{output}}{\text{Power}\text{input}}$
Electric current	The drift of electrons within a conductor is known as an electric current, measured in amperes and given the symbol I.
Electric shock	Electric shock occurs when a person becomes part of the electrical circuit.
Electrical force	Electrical force is the force created by an electrical field.
Electrical industry	The electrical industry is made up of a variety of individual companies, all providing a service within their own specialism to a customer, client or user.
Emergency lighting	Emergency lighting is not required in private homes because the occupants are familiar with their surroundings, but in public buildings people are in unfamiliar surroundings. In an emergency people do not always act rationally, but well-illuminated and easily identified exit routes can help to reduce panic.
Emergency switching	Emergency switching involves the rapid disconnection of the electrical supply by a single action to remove or prevent danger.
Escape/standby lighting	Emergency lighting is provided for two reasons: to illuminate escape routes, called ‘escape’ lighting; and to enable a process or activity to continue after a normal lights failure, called ‘standby’ lighting.
Expansion bolts	The most well-known expansion bolt is made by Rawlbolt and consists of a split iron shell held together at one end by a steel ferrule and a spring wire clip at the other end. Tightening the bolt draws up an expanding bolt inside the split iron shell, forcing the iron to expand and grip the masonry. Rawlbolts are for heavy-duty masonry fixings.
Exposed conductive parts	The metalwork of an electrical appliance or the trunking and conduit of an electrical system which can be touched because they are not normally live, but which may become live under fault conditions.
Extraneous conductive parts	The structural steelwork of a building and other service pipes such as gas, water, radiators and sinks.
Faraday’s law	Faraday’s law states that when a conductor cuts or is cut by a magnetic field, an e.m.f. is induced in that conductor.
Fault protection	Fault protection is provided by protective bonding and automatic disconnection of the supply (by a fuse or circuit breaker, CB) in accordance with IET Regulations 411.3 to 6.
Ferrous	A word used to describe all metals in which the main constituent is iron.
Fire	Fire is a chemical reaction which will continue if fuel, oxygen and heat are present.
Fire alarm circuits	Fire alarm circuits are wired as either normally open or normally closed. In a normally open circuit, the alarm call points are connected in parallel with each other so that when any alarm point is initiated the circuit is completed and the sounder gives a warning of fire. In a normally closed circuit, the alarm call points are connected in series to normally closed contacts. When the alarm is initiated, or if a break occurs in the wiring, the alarm is activated.
First aid	First aid is the initial assistance or treatment given to a casualty for any injury or sudden illness before the arrival of an ambulance, doctor or other medically qualified person.
First aider	A first aider is someone who has undergone a training course to administer first aid at work and holds a current first aid certificate.
Flashpoint	The lowest temperature at which sufficient vapour is given off from a flammable substance to form an explosive gas–air mixture is called the flashpoint.
Flexible conduit	Flexible conduit manufactured to BS 731-1: 1993 is made of interlinked metal spirals often covered with a PVC sleeving.
Fluorescent lamp	A fluorescent lamp is a linear arc tube, internally coated with a fluorescent powder, containing a low-pressure mercury vapour discharge.
Force	The presence of a force can only be detected by its effect on a body. A force may cause a stationary object to move or bring a moving body to rest.
Friction force	Friction force is the force which resists or prevents the movement of two surfaces in contact.
Functional switching	Functional switching involves the switching on or off, or varying the supply, of electrically operated equipment in normal service.
Fuse	A fuse is the weakest link in the circuit. Under fault conditions it will melt when an overcurrent flows, protecting the circuit conductors from damage.
Gravitational force	Gravitational force is the force acting towards the centre of the earth due to the effect of gravity.
Hazard	A hazard is something with the ‘potential’ to cause harm, for example, chemicals, electricity or working above ground.
Hazard risk assessment	Employers of more than five people must document the risks at work and the process is known as hazard risk assessment.
Hazardous area	An area in which an explosive gas–air mixture is present is called a hazardous area, and any electrical apparatus or equipment within a hazardous area must be classified as flameproof to protect the safety of workers.
Heating, magnetic or chemical	The three effects of an electric current: when an electric current flows in a circuit it can have one or more of the following three effects: heating, magnetic or chemical.
Impedance	The total opposition to current flow in an a.c. circuit is called impedance and given the symbol Z.
Inductive reactance	Inductive reactance (XL) is the opposition to an a.c. current in an inductive circuit. It causes the current in the circuit to lag behind the applied voltage.
Inertial force	Inertial force is the force required to get things moving, to change direction or stop.
Inspection and testing techniques	The testing of an installation implies the use of instruments to obtain readings. However, a test is unlikely to identify a cracked socket outlet, a chipped or loose switch plate, a missing conduit-box lid or saddle, so it is also necessary to make a visual inspection of the installation. All existing installations should be periodically inspected and tested to ensure that they are safe and meet the IET Regulations (IET Regulations 610 to 634).
Instructed person (electrically)	An instructed person (electrically) is a person adequately advised or supervised by electrically skilled persons to be able to perceive risks and avoid the hazards which electricity can create.
Insulator	An insulator is a material, usually a non-metal, in which the electrons are very firmly bound to the nucleus and, therefore, will not allow heat or electricity to pass through it. Good insulating materials are PVC, rubber, glass and wood.
Intrinsically safe circuit	An intrinsically safe circuit is one in which no spark or thermal effect is capable of causing ignition of a given explosive atmosphere.
Intruder alarm systems	An intruder alarm system serves as a deterrent to a potential thief and often reduces home insurance premiums.
Isolation	Isolation is defined as cutting off the electrical supply to a circuit or item of equipment in order to ensure the safety of those working on the equipment by making dead those parts which are live in normal service.
Job sheets	A job sheet or job card carries information about a job which needs to be done, usually a small job.
Lamp	A lamp is a device for converting electrical energy into light energy.
Lever	A lever is any rigid body which pivots or rotates around a fixed axis or fulcrum. Load force × Distance from fulcrum = Effort force × Distance from fulcrum.
Levers and turning force	A lever allows a heavy load to be lifted or moved by a small effort.
Line conductor	A conductor in an a.c. system for the transmission of electrical energy other than a neutral or protective conductor. Previously called a phase conductor.
Luminaire	A luminaire is equipment which supports an electric lamp and distributes or filters the light created by the lamp.
Magnesium oxide	The conductors of mineral insulated metal sheathed (MICC) cables are insulated with compressed magnesium oxide.
Magnetic field	The region of space through which the influence of a magnet can be detected is called the magnetic field of that magnet.
Magnetic force	Magnetic force is the force created by a magnetic field.
Magnetic hysteresis	Magnetic hysteresis loops describe the way in which different materials respond to being magnetized.
Magnetic poles	The places on a magnetic material where the lines of flux are concentrated are called magnetic poles.
Maintained emergency lighting	In a maintained system the emergency lamps are continuously lit using the normal supply when this is available, and change over to an alternative supply when the mains supply fails.
Manual handling	Manual handling is lifting, transporting or supporting loads by hand or by bodily force.
Mass	Mass is a measure of the amount of material in a substance, such as metal, plastic, wood, brick or tissue, which is collectively known as a body. The mass of a body remains constant and can easily be found by comparing it on a set of balance scales with a set of standard masses. The SI unit of mass is the kilogram (kg).
Mechanics	Mechanics is the scientific study of ‘machines’, where a machine is defined as a device which transmits motion or force from one place to another.
Metallic trunking	Metallic trunking is formed from mild steel sheet, coated with grey or silver enamel paint for internal use or a hot-dipped galvanized coating where damp conditions might be encountered.
Mini-trunking	Mini-trunking is very small PVC trunking, ideal for surface wiring in domestic and commercial installations such as offices.
Mobile equipment	Electrical equipment which is moved while in operation or can be moved while connected to the supply. Previously called portable equipment.
Movement or heat detector	A movement or heat detector placed in a room will detect the presence of anyone entering or leaving that room.
Mutual inductance	A mutual inductance of 1 henry exists between two coils when a uniformly varying current of 1 ampere per second in one coil produces an e.m.f. of 1 volt in the other coil.
Non-ferrous	Metals which do not contain iron are called non-ferrous. They are non-magnetic and resist rusting. Copper, aluminium, tin, lead, zinc and brass are examples of non-ferrous metals.
Non-maintained emergency lighting	In a non-maintained system the emergency lamps are only illuminated if the normal mains supply fails.
Non-statutory regulations and codes of practice	Non-statutory regulations and codes of practice interpret the statutory regulations telling us how we can comply with the law.
Ohm’s law	Ohm’s law says that the current passing through a conductor under constant temperature conditions is proportional to the potential difference across the conductor.
Optical fibre cables	Optical fibre cables are communication cables made from optical-quality plastic, the same material from which spectacle lenses are manufactured. The energy is transferred down the cable as digital pulses of laser light, as against current flowing down a copper conductor in electrical installation terms.
Ordinary person	An ordinary person is a person who is neither a skilled person nor an instructed person.
Overload current	An overload current can be defined as a current which exceeds the rated value in an otherwise healthy circuit.
Passive infra-red (PIR) detectors	PIR detector units allow a householder to switch on lighting units automatically whenever the area covered is approached by a moving body whose thermal radiation differs from the background.
Perimeter protection system	A perimeter protection system places alarm sensors on all external doors and windows so that an intruder can be detected as he or she attempts to gain access to the protected property.
Person	A person can be described as ordinary, instructed or skilled depending upon that person’s skill or ability.
Personal protective equipment (PPE)	PPE is defined as all equipment designed to be worn, or held, to protect against a risk to health and safety.
Phasor	A phasor is a straight line, having definite length and direction, which represents to scale the magnitude and direction of a quantity such as a current, voltage or impedance.
Plastic plugs	A plastic plug is made of a hollow plastic tube split up to half its length to allow for expansion. Each size of plastic plug is colour-coded to match a wood screw size.
Polyvinylchloride (PVC)	PVC used for cable insulation is a thermoplastic polymer.
Potential difference	The potential difference (p.d.) is the change in energy levels measured across the load terminals. This is also called the volt drop or terminal voltage, since e.m.f. and p.d. are both measured in volts.
Power	Power is the rate of doing work. $\text{Power}=\frac{\text{Work}\text{done}}{\text{Time}\text{taken}}(\text{W})$
Power factor	Power factor (p.f.) is defined as the cosine of the phase angle between the current and voltage.
Pressure or stress	Pressure or stress is a measure of the force per unit area. $\text{Pressure}\text{or}\text{stress}=\frac{\text{Force}}{\text{Area}}(\text{N}/{\text{m}}^{\text{2}})$
Primary cell	A primary cell cannot be recharged. Once the active chemicals are exhausted, the cell must be discarded.
Protective bonding	This is protective bonding for the purpose of safety.
PVC/SWA cable installations	Steel wire armoured PVC insulated cables are now extensively used on industrial installations and often laid on cable tray.
Reasonably practicable or absolute	If the requirement of the regulation is absolute, then that regulation must be met regardless of cost or any other consideration. If the regulation is to be met ‘so far as is reasonably practicable’, then risks, cost, time, trouble and difficulty can be considered.
Relay	A relay is an electromagnetic switch operated by a solenoid.
Resistance	In any circuit, resistance is defined as opposition to current flow.
Resistivity	The resistivity (symbol ρ – the Greek letter ‘rho’) of a material is defined as the resistance of a sample of unit length and unit cross-section.
Risk	A risk is the ‘likelihood’ of harm actually being done.
Risk assessments	Risk assessments need to be suitable and sufficient, not perfect.
Rubber	Rubber is a tough elastic substance made from the sap of tropical plants.
Safety first – isolation	We must ensure the disconnection and separation of electrical equipment from every source of supply and that this disconnection and separation is secure.
Secondary cells	A secondary cell has the advantage of being rechargeable. If the cell is connected to a suitable electrical supply, electrical energy is stored on the plates of the cell as chemical energy.
Secure supplies	A UPS (uninterruptible power supply) is essentially a battery supply electronically modified to provide a clean and secure a.c. supply. The UPS is plugged into the mains supply and the computer systems are plugged into the UPS.
Security lighting	Security lighting is the first line of defence in the fight against crime.
Shearing force	Shearing force is the force which moves one face of a material over another.
Shock protection	Protection from electric shock is provided by basic protection and fault protection.
Short-circuit	A short-circuit is an overcurrent resulting from a fault of negligible impedance connected between conductors.
SI units	SI units are based upon a small number of fundamental units from which all other units may be derived.
Silicon rubber	Introducing organic compounds into synthetic rubber produces a good insulating material such as FP200 cables.
Simple machines	A machine is an assembly of parts, some fixed, others movable, by which motion and force are transmitted. With the aid of a machine we are able to magnify the effort exerted at the input and lift or move large loads at the output.
Single PVC insulated conductors	Single PVC insulated conductors are usually drawn into the installed conduit to complete the installation.
Skilled person (electrically)	A skilled person (electrically) is a person with relevant education, training and practical skills, and sufficient experience to be able to perceive risks and to avoid the hazards which electricity can create.
Skirting trunking	Skirting trunking is a trunking manufactured from PVC or steel in the shape of a skirting board which is frequently used in commercial buildings such as hospitals, laboratories and offices.
Socket outlets	Socket outlets provide an easy and convenient method of connecting portable electrical appliances to a source of supply.
Sounders	The positions and numbers of sounders should be such that the alarm can be distinctly heard above the background noise in every part of the premises.
Space factor	The ratio of the space occupied by all the cables in a conduit or trunking to the whole space enclosed by the conduit or trunking is known as the space factor.
Speed	Speed is concerned with distance travelled and time taken.
Spring toggle bolts	A spring toggle bolt provides one method of fixing to hollow partition walls which are usually faced with plasterboard and a plaster skimming.
Static electricity	Static electricity is a voltage charge which builds up to many thousands of volts between two surfaces when they rub together.
Statutory regulations	Statutory regulations have been passed by Parliament and have, therefore, become laws.
Step-down transformers	Step-down transformers are used to reduce the output voltage, often for safety reasons.
Step-up transformers	Step-up transformers are used to increase the output voltage. The electricity generated in a power-station is stepped up for distribution on the National Grid network.
Switching for mechanical maintenance requirements	The switching for mechanical maintenance requirements are similar to those for isolation except that the control switch must be capable of switching the full load current of the circuit or piece of equipment.
Synthetic rubber	Synthetic rubber is manufactured, as opposed to being produced naturally.
Tensile force	Tensile force is the force pulling things apart.
Thermoplastic polymers	These may be repeatedly warmed and cooled without appreciable changes occurring in the properties of the material.
Thermosetting polymers	Once heated and formed, products made from thermosetting polymers are fixed rigidly. Plug tops, socket outlets and switch plates are made from this material.
Time sheets	A time sheet is a standard form completed by each employee to inform the employer of the actual time spent working on a particular contract or site.
Transformer	A transformer is an electrical machine which is used to change the value of an alternating voltage.
Trap protection	Trap protection places alarm sensors on internal doors and pressure pad switches under carpets on through routes between, for example, the main living area and the master bedroom.
Trunking	A trunking is an enclosure provided for the protection of cables which is normally square or rectangular in cross-section, having one removable side. Trunking may be thought of as a more accessible conduit system.
Velocity	In everyday conversation we often use the word velocity to mean the same as speed, and indeed the units are the same. However, for scientific purposes this is not acceptable since velocity is also concerned with direction.
Visual inspection	The installation must be visually inspected before testing begins. The aim of the visual inspection is to confirm that all equipment and accessories are undamaged and comply with the relevant British and European Standards, and also that the installation has been securely and correctly erected.
Weight	Weight is a measure of the force a body exerts on anything which supports it. Normally it exerts this force because it is being attracted towards the earth by the force of gravity.
Work done	Work done is dependent upon the force applied times the distance moved in the direction of the force. Work done = Force × Distance moved in the direction of the force (J). The SI unit of work done is the newton metre or joule (symbol J).