SOLAS Electrical Apprenticeship key terms

A conductor energised in normal service, including line and neutral conductors, that can deliver current and therefore presents a shock or fire risk if touched or faulted.

A conductor provided for safety purposes, such as earthing exposed metalwork or bonding extraneous parts, so that fault current has a low-impedance path back to the supply source.

Deliberate connection of exposed conductive parts and extraneous conductive parts to the general mass of earth, limiting touch voltage and enabling protective devices to operate under fault conditions.

Connecting together metalwork and conductive parts so they sit at substantially the same potential, preventing dangerous voltage differences between simultaneously accessible parts during a fault.

The total impedance of the path followed by fault current from the source through the installation and back to the source, which determines whether protective devices disconnect quickly enough.

The current that would flow if a short circuit or low-impedance earth fault occurred at a chosen point, used to select protective devices and check breaking capacity.

The maximum fault current a protective device can safely interrupt without damage or dangerous failure, usually quoted in kiloamperes for circuit breakers and fuses.

A protective device that compares current in line and neutral conductors and trips when the imbalance exceeds its rated residual operating current, detecting earth leakage.

Coordination of protective devices so that the device nearest a fault operates first, leaving upstream devices closed and limiting the extent of supply interruption.

Cutting off supply from all live conductors of a circuit or equipment and securing the means of disconnection against reconnection, so work can proceed safely.

Confirming by use of a proven voltage indicator that a conductor or circuit is de-energised before work begins, after isolation and before applying earthing or locks.

A test instrument, often two-pole, used to verify the presence or absence of voltage; it must be proved on a known source before and after the test.

A test applying a direct voltage, commonly 500 V, between conductors and earth to measure insulation resistance in megohms and reveal deterioration or moisture.

A low-resistance measurement confirming that protective conductors, ring circuit conductors and bonding connections form an unbroken, adequately conductive path.

Ratio of insulation resistance measured after ten minutes to that after one minute, used to judge whether moisture or contamination, rather than age, is reducing insulation performance.

An instrument that applies a high direct voltage and measures very high resistance values, used for insulation testing of cables, motors and switchgear.

The relationship stating that current through a conductor equals voltage across it divided by resistance, expressed as I equals V divided by R for a linear resistive circuit.

The total opposition to alternating current from resistance and reactance combined, measured in ohms and having both magnitude and phase angle.

Opposition to alternating current caused by inductance or capacitance, measured in ohms, which stores and returns energy rather than dissipating it as heat.

The ratio of real power in watts to apparent power in volt-amperes in an AC circuit, equal to the cosine of the phase angle between voltage and current.

Power that oscillates between source and reactive load without doing useful work, measured in volt-amperes reactive and associated with magnetic fields in motors and transformers.

An AC supply of three voltages equal in magnitude and displaced by 120 electrical degrees, allowing balanced loads to draw constant total power and efficient motor operation.

A three-phase connection in which one end of each winding joins at a common neutral point, giving phase voltage equal to line voltage divided by root three.

A three-phase connection in which windings form a closed loop, so line voltage equals phase voltage and line current exceeds phase current by root three in a balanced load.

The maximum sustained current a cable can carry without its conductor temperature exceeding the rated insulation limit, adjusted by correction factors for ambient temperature and grouping.

Reduction in voltage along a circuit caused by conductor resistance and reactance, which must be limited so equipment at the far end receives adequate voltage.

Reduction of a cable's tabulated current rating when it is surrounded by thermal insulation that traps heat, applied as a factor before selecting protective device rating.

A manually resettable overcurrent device combining thermal protection against overload and magnetic protection against short circuit, with a defined rated current and breaking capacity.

A circuit breaker characteristic that trips magnetically at roughly five to ten times rated current, suited to loads with moderate inrush such as small motors.

A brief high current drawn when energising transformers, motors, capacitors or LED drivers, which must be tolerated without nuisance tripping of protective devices.

A procedure and device that physically secures an isolator or breaker in the off position, with a personal lock and tag, preventing unexpected reconnection during maintenance.

A formal written authorisation specifying the equipment, isolation points, precautions and time limits for hazardous work, signed by the responsible person before work starts.

A structured evaluation identifying hazards, estimating likelihood and severity of harm, and recording control measures before electrical work is carried out.

A document describing the sequence of work, tools, personnel and safety controls for a specific task, used alongside risk assessment to plan safe execution.

An industrial digital controller that executes a stored program to read inputs, apply logic and drive outputs, used for machine sequencing and interlocking.

A graphical programming language representing control circuits as rungs between supply rails, with contacts and coils, widely used to program programmable logic controllers.

A control arrangement preventing an action unless set conditions are met, such as blocking a motor start while a guard is open, protecting people and equipment.

A controller channel that converts a continuous signal, such as four to twenty milliamps or zero to ten volts, into a digital value for processing and control.

A device converting one form of energy into another, such as pressure into an electrical signal, providing measurement for indication, control or protection.

A temperature sensor made of two dissimilar metals joined at a measuring junction, producing a small voltage proportional to temperature difference by the Seebeck effect.

An electronic unit that converts fixed-frequency supply to adjustable voltage and frequency, controlling induction motor speed and reducing energy use at part load.

A switching technique that varies the on-time proportion of a fixed-frequency waveform to synthesise an average voltage, used in inverters and motor drives.

Steady-state currents or voltages at integer multiples of the fundamental frequency, produced by non-linear loads and causing extra heating and neutral overload.

A set of interconnected solar modules converting light into direct current, arranged in series strings and parallel groups to give the required voltage and current.

An electronic control method that continuously adjusts the operating point of a photovoltaic array to extract the greatest available power under changing irradiance and temperature.

A power electronic device converting direct current into alternating current, used for renewable generation, uninterruptible supplies and motor control, often with grid synchronisation.