UPMSP High School Electrician key terms

The passage of electric current through the human body, causing burns, muscle contraction, breathing arrest or heart fibrillation. Severity depends on current magnitude, path, duration and skin condition; even low voltages can be fatal.

The relationship stating that current through a conductor equals the voltage across it divided by its resistance (I = V/R). It underpins calculation of load current, voltage drop and protective device selection in nearly every practical task.

The ratio of true power in watts to apparent power in volt-amperes in an AC circuit, equal to the cosine of the phase angle. Low values increase current for the same load, raising cable and switchgear ratings.

Deliberate connection of exposed metalwork and the neutral point of a supply to the general mass of earth through an electrode and conductor, so fault current flows and protective devices operate quickly while touch voltage stays safe.

A copper, galvanised iron or similar rod, plate or pipe driven into soil to make low-resistance contact with earth. Its resistance depends on soil resistivity, moisture, depth and electrode surface area.

A protective switch that compares current in live and neutral conductors and trips when the imbalance, typically 30 mA for shock protection, indicates leakage to earth. It protects people where overcurrent devices cannot.

A resettable automatic switch that opens on overload or short circuit using thermal and magnetic elements. Rated by current and breaking capacity, it replaces fuses in most modern domestic distribution boards.

A deliberate weak link, usually a metal wire or strip in a cartridge or rewirable carrier, that melts and breaks the circuit when current exceeds its rating for a set time, protecting cables and equipment.

An accidental low-resistance path between conductors of different potential, causing very high current. It can weld contacts, start fires and damage insulation, so breaking capacity and fast disconnection matter.

The resistance measured between live conductors and earth, or between separate circuits, using a high-voltage tester. Healthy wiring reads in megohms; low readings reveal moisture, damage or ageing and must be corrected.

An instrument applying a high DC test voltage, commonly 500 V or 1000 V, to measure insulation resistance in megohms. The supply must be isolated and equipment disconnected before testing to avoid damage.

A combined instrument for measuring voltage, current, resistance and often continuity or diode drop. Correct range, function selection and lead placement are essential; using the current range across a supply can destroy it.

A meter whose split jaws encircle a conductor and measure current from its magnetic field without breaking the circuit. It reads only the conductor inside the jaws, so the live and neutral together cancel to zero.

A low-resistance check, usually with a continuity tester or multimeter, confirming that protective conductors and bonded metalwork form an unbroken path back to the earth bar with acceptably low resistance.

The loss of voltage along a conductor caused by its resistance carrying load current. Excessive drop dims lamps and stalls motors, so cable size is chosen from current rating, run length and permitted percentage drop.

The maximum continuous current a cable can carry without its conductor or insulation exceeding rated temperature under stated installation conditions. It is adjusted by ambient temperature and grouping factors.

Cable insulated and sheathed with polyvinyl chloride, common in domestic and light industrial wiring. It is flame-retardant but softens near 70 degrees Celsius, so it suits normal ambient conditions, not hot locations.

A wiring system in which insulated conductors are drawn through metal or PVC pipes fixed to walls, ceilings or buried in plaster. It gives mechanical protection and allows conductors to be replaced without disturbing the structure.

An enclosure where conductors are joined and branched, protecting connections from dust, moisture and accidental contact. Joints must be mechanically sound and electrically continuous, and boxes must remain accessible for inspection.

A switch with one common terminal and two strapper terminals, used in pairs so a lamp can be controlled from either of two positions, as on a staircase. It is not an intermediate switch.

A switch with four terminals that reverses the two strapper conductors between two two-way switches, allowing a lamp to be controlled from three or more positions in the same circuit.

A static device with two or more windings on a common magnetic core that transfers energy by mutual induction, changing AC voltage in proportion to turns ratio while ideally preserving power and frequency.

An instrument transformer whose primary is the cable or busbar and whose secondary delivers a reduced, proportional current, usually 5 A or 1 A, to meters and protection relays while isolating them from high voltage.

An AC motor in which a rotating field from the stator induces currents in a rotor that runs slightly slower than synchronous speed. Simple, rugged and widely used, it needs no brushes or external excitation.

The difference between synchronous speed and actual rotor speed of an induction motor, expressed as a fraction or percentage of synchronous speed. Slip rises with load and is near zero at no load.

A starting arrangement that connects an induction motor stator in star for reduced starting current and voltage per winding, then switches to delta for normal running. It suits motors designed to run in delta.

A heavy-duty electromagnetic switch for repeated remote switching of motor and lighting loads, with main contacts and a small coil circuit. It is controlled by pushbuttons, timers or relays and needs overload protection.

The luminous flux falling on a unit area of surface, measured in lux. Recommended values vary by task, from about 100 lux for corridors to several hundred lux for workshops and close bench work.

The total light output of a lamp or luminaire in all directions, measured in lumens. It is the starting figure for lighting design, combined with room size, utilisation factor and maintenance factor.

A lighting design calculation giving the number of fittings needed: room area times required lux divided by lumens per fitting times utilisation and maintenance factors. It assumes fairly even general lighting.

The safe practice of disconnecting all supply conductors, locking or tagging the isolator, proving dead with a tester and guarding against reconnection before work begins. It is the first step of any repair.

A procedure in which an isolator is physically locked open and a warning tag records who is working and why, preventing another person from restoring supply to equipment under maintenance.

Preparing a priced list of materials, labour, tools and overheads for a wiring job from drawings and specifications, including wastage, testing time and profit margin, to produce a competitive but safe quotation.

Summing the connected and diversified demand of lamps, sockets, motors and heating to size the supply, main switch, distribution board and cables correctly, avoiding nuisance tripping and overload.

An itemised schedule listing each material and quantity required for an installation, such as metres of cable, numbers of switches and points, used for ordering, costing and comparing quotations.

Wearable protection such as insulated gloves, safety shoes, helmet, goggles and arc-rated clothing, selected after risk assessment. It supplements safe systems of work and isolation; it never replaces them.

Joining conductors or component leads by melting a tin-lead or lead-free alloy with flux so it wets and bonds the surfaces. Clean, hot parts and a correctly tinned iron give a bright, low-resistance joint.

A hand tool that compresses a metal ferrule or lug onto a conductor with a controlled force, producing a permanent low-resistance joint. The correct die and lug size must match the conductor cross-section.

A point where a conductor is clamped or screwed to connect it to equipment, a switch or another cable. Terminals must be tight, correctly sized and free of insulation under the clamp to avoid heating.