WBCHSE HS Vocational Power key terms

The rule that in a resistive circuit the voltage across a component equals the current through it multiplied by its resistance. It lets you find any one of the three quantities when the other two are known.

The principle that the total current entering a junction equals the total current leaving it, because charge cannot accumulate at a point. It is used to write equations for branch currents in networks.

The principle that the algebraic sum of all voltage drops and rises around any closed loop of a circuit is zero. It underpins loop analysis of multi-source networks.

The property of a material that opposes the flow of electric current, measured in ohms. For a conductor it depends on resistivity, length and cross-sectional area, and it rises with temperature in metals.

A material constant describing how strongly a material resists current, expressed in ohm-metres. It allows calculation of a conductor's resistance from its length and cross-sectional area.

The cosine of the phase angle between voltage and current in an AC circuit, equal to real power divided by apparent power. A low value means more current is drawn for the same useful power.

The opposition an inductor offers to alternating current, equal to two pi times frequency times inductance. It increases with frequency, which is why chokes block high-frequency components.

The opposition a capacitor offers to alternating current, equal to one divided by two pi times frequency times capacitance. It falls as frequency rises, so capacitors pass high-frequency signals more easily.

The total opposition to alternating current from resistance and reactance combined, measured in ohms. It is a vector quantity, so resistance and reactance add geometrically rather than arithmetically.

A material such as copper or aluminium that allows electric current to flow readily because it has many free electrons. Conductors form the current-carrying path in cables and windings.

The resistance measured between a live conductor and earth, or between two insulated conductors, using an insulation tester. A high value in megohms shows the insulation is sound and dry.

The non-conducting covering around a conductor that confines current to its intended path and protects people from shock. Its material, thickness and temperature rating must suit the circuit voltage and duty.

Rigid or flexible plastic tubing used to route and protect wiring indoors and in damp locations. It is lightweight and corrosion-resistant but softens at high temperatures and needs support clips at regular spacing.

A tube, metallic or plastic, through which insulated wires are drawn to give mechanical protection and a possible earth path. Metal conduit must be bonded and earthed at all joints.

A rigid open support, usually steel or perforated sheet, carrying several cables in industrial installations. It gives easy access for inspection and allows heat to escape from loaded cables.

A multiplier below one applied to a cable's tabulated current rating to correct for grouping, ambient temperature or installation method. It ensures the chosen cable will not overheat in real conditions.

The maximum steady current a cable can carry without its conductor temperature exceeding the insulation limit. It depends on conductor size, insulation type, ambient temperature and how the cable is installed.

The reduction in voltage along a cable caused by its resistance and reactance when current flows. Excessive drop causes dim lamps and stalled motors, so regulations set a maximum permitted percentage.

A static device with two or more windings on a common magnetic core that changes AC voltage levels by electromagnetic induction. It transfers power at nearly constant frequency with small losses.

The ratio of primary turns to secondary turns in a transformer, which equals the ratio of primary to secondary voltage. It also determines the inverse ratio of primary to secondary current.

A transformer whose secondary has fewer turns than its primary, so the output voltage is lower than the input. Distribution transformers near consumers are of this type.

Power lost as heat when circulating currents are induced in a transformer or machine core by changing flux. It is reduced by building the core from thin laminated sheets insulated from each other.

Energy lost as heat each time the magnetic domains of a core reverse direction during an alternating cycle. It is lowered by using core steel with a narrow hysteresis loop, such as silicon steel.

The difference between synchronous speed and actual rotor speed of an induction motor, usually expressed as a fraction or percentage of synchronous speed. It must be non-zero for the rotor to develop torque.

The speed of the rotating magnetic field in an AC machine, equal to 120 times frequency divided by the number of poles. An induction motor always runs a little below this value.

An AC motor in which current is induced in the rotor by the rotating field of the stator, so no brushes or slip rings are needed. It is rugged and widely used for pumps, fans and conveyors.

The high-voltage overhead or underground link that carries bulk power from generating stations to load centres. High voltage is used so that current, and therefore line loss, stays low for a given power.

The network of feeders, substations and service lines that delivers power from bulk supply points to individual consumers at utilisation voltage. It is the final stage of the power supply chain.

A conductor or line that carries power from a substation or distribution board to a point where it is tapped off to consumers. No consumer service is connected directly along its length.

The tendency of alternating current to concentrate near the outer surface of a conductor as frequency rises, raising effective resistance. It is one reason bundled or hollow conductors are used on high-voltage lines.

A portable instrument that applies a high DC test voltage to measure very high resistance, typically insulation resistance in megohms. Readings are taken with the circuit isolated and discharged first.

A check that a circuit path is unbroken, usually made with a low-reading ohmmeter or a buzzer on an isolated dead circuit. It confirms correct polarity and sound connections before energising.

An instrument transformer whose primary carries the load current and whose secondary supplies a standard low current to meters and relays. Its secondary must never be open-circuited while energised.

An instrument transformer that steps high line voltage down to a safe standard value for voltmeters, relays and metering. It isolates measuring instruments from the high-voltage circuit.

A meter with a split jaw that is clipped around a single conductor to read current by the magnetic field it produces, without breaking the circuit. It is convenient for checking live load currents.

A combined instrument that measures voltage, current and resistance, and often continuity and diode drop, selected by a rotary switch. Correct range and function selection is essential to avoid damage or shock.

A four-layer semiconductor switch that conducts in one direction once a gate pulse triggers it, and turns off only when current falls below a holding level. It is used in controlled rectifiers and AC power controllers.

A circuit of diodes or thyristors that converts alternating current into direct current, often followed by a filter. It is the front end of most DC drives and battery chargers.

A power electronic circuit that converts direct current into alternating current at a chosen frequency and voltage. Inverters allow speed control of induction motors and feed solar or battery supplies into AC loads.

A motor controller that rectifies the supply, then inverts it at an adjustable frequency and voltage to control the speed of an AC motor. It saves energy on pumps and fans running at part load.

A switching technique in which the on-time of a fixed-frequency pulse is varied to control the average output voltage or to synthesise a sine wave. It is central to inverter and drive control.

Deliberate connection of exposed metalwork or a circuit point to the general mass of earth so fault current has a low-resistance path. It makes protective devices operate and limits touch voltage.

The resistance between an earth electrode and the surrounding soil mass, measured in ohms. It depends on soil resistivity, electrode depth and moisture, and must be low enough for protective devices to trip.

A protective switch that compares current in the live and neutral conductors and trips when a small imbalance shows leakage to earth. It protects people against shock but does not replace overcurrent protection.

A small automatic switch that trips on overload by thermal action and on short circuit by magnetic action, and can be reset by hand. Its rating and breaking capacity must suit the circuit it protects.

A protective device containing a metal element that melts and breaks the circuit when current exceeds its rating for a set time. It is cheap and fast but must be replaced after operation.

A safety procedure in which the supply is isolated, locked in the off position and tagged with the worker's name before work begins. It prevents accidental re-energising of equipment under maintenance.

Items worn by a worker such as insulated gloves, safety shoes, helmet and eye protection to reduce injury risk. It is the last line of defence after elimination, isolation and other controls.

The proportion of time a machine or load operates within a defined period, often stated as a percentage or a standard rating class. It determines how much heating a motor or drive can tolerate.

The declared output power, voltage, frequency, speed and duty for which a motor is designed, shown on its nameplate. Operating beyond these values shortens insulation life and may void guarantees.

A starter that connects a motor winding in star for a low-current start, then switches to delta for full running voltage. It reduces starting current to about one third but also reduces starting torque.

Joining metal surfaces with a filler alloy melted at a temperature below the melting point of the parts, wetted by flux. Good joints are bright, smooth and free of voids or excess solder.

Making a permanent electrical joint by compressing a metal sleeve or lug onto a conductor with a crimping tool. The correct die and sleeve size give a mechanically strong, low-resistance connection.

An itemised list of all parts, quantities and specifications needed to build or repair a job. It supports accurate costing, purchasing and stock control in a workshop or small enterprise.

The sales volume or revenue at which total income equals total cost, so neither profit nor loss is made. Knowing it helps a small contractor set prices and judge how much work is needed.

The money available to run day-to-day operations, calculated as current assets minus current liabilities. Shortages cause missed wages and material delays even when the order book is full.

A structured look at a task to identify hazards, judge how likely and how severe harm could be, and decide controls. It is recorded and reviewed before work starts and whenever conditions change.