HK Grade B Electrical Worker Exam key terms

Connecting exposed conductive parts and extraneous conductive parts together so they sit at substantially the same potential, limiting touch voltage during a fault. Main bonding links at the origin; supplementary bonding covers local areas such as bathrooms.

A conductive part of equipment that can be touched and that is not live in normal use but may become live under fault conditions, such as a metal enclosure or motor frame. It must be earthed or otherwise protected.

The total impedance of the path taken by fault current from the point of fault, through the protective conductor and earthing, back through the source and line conductor. It must be low enough for the protective device to operate within the required disconnection time.

A protective measure in which a fault causes the protective device to open the circuit quickly enough to prevent dangerous touch voltage persisting. It relies on correct earthing, bonding, and a verified earth fault loop impedance.

A device that compares current in the line and neutral conductors and trips when the imbalance, typically representing earth leakage, exceeds its rated residual operating current. It gives additional protection and is required for socket outlets and certain special locations.

Protection against direct contact with live parts in normal conditions, achieved by insulation, barriers, enclosures, or placing live parts out of reach. It does not rely on the operation of a protective device.

Protection against contact with live parts that have become live because of a fault. It is provided by earthing and automatic disconnection, double insulation, or equivalent measures, and complements basic protection.

A two-digit classification of an enclosure's resistance to solid objects and water, such as IP44 for splash-proof or IP65 for dust-tight and jet-proof. The rating guides equipment selection for bathrooms, outdoor and construction site use.

Equipment whose basic protection relies on insulation but which also has an exposed metal part connected to the protective conductor of the installation. It depends on a reliable earth connection for fault protection.

Equipment in which protection against electric shock relies on double insulation or reinforced insulation, with no provision for connecting a protective conductor. It is marked with the double-square symbol and is common for portable tools and bathroom fittings.

The inspection and testing of portable and transportable electrical equipment to confirm it remains safe for use, typically including visual checks, earth continuity, insulation resistance, and polarity or function tests at intervals set by risk assessment.

A test applying a DC voltage, usually 500 V, between live conductors and earth, or between conductors, to confirm insulation is sound. Results are recorded in megohms and compared with minimum acceptable values.

A low-voltage measurement confirming that protective conductors, bonding conductors, and ring final circuit conductors form an unbroken low-resistance path. Results also help verify that conductors are correctly connected at accessories.

A check that line conductors are connected to the correct terminals, that switches are in the line conductor, and that socket outlets are wired with line, neutral, and earth in the right positions. It prevents dangerous reversed connections.

A socket outlet circuit in which the cable forms a closed loop returning to the distribution board, allowing current to share two paths. Its continuity and loop resistance must be verified during testing to confirm the ring is intact.

A circuit connected directly to the final protective device in a distribution board and supplying current-using equipment or socket outlets. It is the last stage of the distribution system before the load.

A device such as a fuse, miniature circuit breaker, or residual current device that automatically opens a circuit to prevent dangerous overheating, overcurrent, or electric shock under fault or overload conditions.

Any current exceeding the rated current of a conductor or device, including overload current from excessive connected load and short-circuit current from a fault of negligible impedance. Protective devices must clear it before damage occurs.

The maximum prospective fault current a protective device can safely interrupt without damage or danger. It must equal or exceed the prospective fault current at the point of installation.

The current that would flow if a short circuit of negligible impedance occurred at a given point in the installation. It is calculated or measured during design and testing to confirm device ratings and conductor withstand.

A multiplier applied to connected load when assessing maximum demand, recognising that not all appliances or circuits operate at full load simultaneously. Correct diversity avoids oversizing supply capacity and main switchgear.

The greatest load, after applying diversity, that an installation is expected to draw from the supply. It determines the rating of the main switch, protective devices, and the capacity requested from the supply authority.

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 between voltage and current. Low power factor increases current for the same useful power.

The product of RMS voltage and RMS current in an AC circuit, measured in volt-amperes. It represents the total demand on the supply and includes both true power and reactive power.

Power that oscillates between the source and reactive elements such as inductors and capacitors without doing useful work, measured in volt-amperes reactive. It is associated with magnetic fields in motors and transformers.

An AC supply of three line conductors with voltages displaced by 120 electrical degrees, giving constant power transfer and efficient motor operation. It is described by line and phase voltage relationships and phase sequence.

The conductor connected to the supply neutral point, carrying return current in single-phase circuits and unbalanced current in three-phase systems. It is not a protective conductor, though it may be combined in a PEN conductor.

A conductor provided for safety purposes, such as earthing exposed conductive parts or bonding extraneous conductive parts. It carries fault current only and is identified by green-and-yellow colouring.

The maximum continuous current a cable can carry without exceeding its rated conductor temperature, after applying correction factors for ambient temperature, grouping, thermal insulation, and installation method. It must not be less than the design current.

The reduction in voltage along a conductor caused by its impedance carrying current. It must be limited so that equipment at the far end receives adequate voltage; long runs and undersized conductors are common causes of excessive drop.

The current the circuit is intended to carry in normal service, calculated from the connected load. The protective device rating and cable capacity are selected so that the design current is not exceeded.

The way cables are fixed or enclosed, such as clipped direct, in conduit, in trunking, or buried in thermal insulation. It affects heat dissipation and therefore the current-carrying capacity selected from cable tables.

Enclosures used to route and mechanically protect conductors. Conduit is tubular and may be metal or plastic; trunking is a rectangular enclosure with removable lids, often used for multiple circuits and future capacity.

Defined regions around a bath or shower, numbered from 0 at the water to 2 beyond the immediate splash area, each with permitted equipment, IP ratings, and restrictions on socket outlets. Supplementary bonding may be required.

A temporary or mobile installation subject to harsh conditions, requiring robust cables, impact-resistant enclosures, RCD protection for socket outlets, and regular inspection. Supply arrangements and earthing must suit changing site conditions.

The process of identifying locations where flammable gas, vapour, or dust may be present and assigning zones according to the likelihood and duration of the explosive atmosphere, so that suitable explosion-protected equipment is selected.

Electrical apparatus designed and certified to prevent ignition of a surrounding explosive atmosphere, using methods such as flameproof enclosures, increased safety, pressurisation, or intrinsic safety. Selection depends on the classified zone and gas group.

A protection concept limiting the energy and temperature within a circuit so that it cannot ignite a hazardous atmosphere, even under fault conditions. It uses barriers or galvanic isolators and suits instrumentation and low-power circuits.

A motor starter that connects the stator windings in star for starting, reducing starting current and torque, then switches to delta for normal running. It suits motors designed to run in delta on the available line voltage.

A remotely operated switching device for controlling power circuits, held closed by an electromagnet and returning to the open position when de-energised. It is used with overload relays and pushbuttons in motor control circuits.

A protective device in a motor circuit that detects sustained overcurrent above the motor's rating and opens the control circuit, preventing winding overheating. It is set to the motor full-load current and does not replace short-circuit protection.

A control arrangement that prevents two operations from occurring together when doing so would be dangerous, such as forward and reverse contactors energising simultaneously. It may be mechanical or electrical.

A load whose current lags the applied voltage because of inductance, typical of motors, transformers, and discharge lighting. It draws reactive power and lowers power factor, increasing current for a given useful output.

A manually operated device that rapidly stops dangerous machinery and latches in the off position until deliberately reset. It must be clearly marked, readily accessible, and wired so that failure of the control circuit stops the machine.

The disconnection of all live conductors from the supply and securing against reconnection, so that work can be carried out safely. It is verified by testing for dead before work begins and by locking off the isolating device.

Confirming that a circuit or equipment is de-energised using a voltage indicator proven on a known live source before and after the test. This sequence guards against a faulty or discharged indicator giving a false reading.

A procedure in which an isolating device is locked in the safe position and tagged with the identity of the person working, preventing inadvertent reconnection. Only the person who applied the lock should remove it.

A formal written authorisation controlling high-risk work, recording the isolation applied, the work permitted, precautions required, and the persons responsible for issuing and accepting the permit.

An enclosure containing protective devices and terminals that divides a supply into final circuits. It requires clear labelling, a schedule of circuits, and adequate access for operation and maintenance.

The maximum current the supply authority agrees to provide at the point of supply, determined by the service cable, cut-out fuse, and meter. It constrains maximum demand and the rating of the main switch.

Classification of how the supply source and installation earthing are arranged, determining the fault path and the protective measures permitted. TN systems use a metallic return to the source; TT relies on a local earth electrode.

A conductor, such as a rod or plate, buried in the ground to provide an electrical connection with the general mass of earth. Its resistance must be low enough for protective devices to operate.

A rigid support system, often perforated or ladder type, carrying cables in horizontal or vertical runs. It requires earthing when metallic and affects the grouping and heat dissipation of the cables installed on it.

A built-in mechanical check on an RCD that simulates an earth leakage current to confirm the trip mechanism operates. It verifies function only and does not replace instrument testing of operating time and current.

Current that flows to earth through insulation, capacitance, or a fault path rather than through the intended circuit. Small continuous leakage is normal; a rise may indicate deteriorating insulation or a developing fault.

The sum of the rated currents or power of all equipment connected to an installation, before any diversity is applied. It is the starting point for maximum demand assessment.

An intrinsic safety interface using zener diodes and a resistor to limit voltage and current entering a hazardous area, with a fuse to protect the diodes. It requires a safe-area earth connection.