HK Grade B Electrical Worker Exam flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

For a resistive circuit, voltage equals current multiplied by resistance (V = I × R); rearranged, I = V/R and R = V/I. It applies to DC and to AC circuits only when impedance replaces resistance.

Three-phase power formula

For a balanced three-phase load, total power P = √3 × line voltage × line current × power factor. The √3 arises from the 120-degree phase displacement between phases and applies to both star and delta loads.

Power factor

The ratio of true power in watts to apparent power in volt-amperes, equal to cos φ for sinusoidal supplies. A low power factor increases current for the same useful power, raising losses and cable sizing.

RMS value

The effective value of an alternating quantity, equal to peak value divided by √2 (about 0.707 × peak) for a sine wave. It produces the same heating effect as an equal DC value in a resistor.

Adiabatic equation

The protective conductor cross-section must satisfy S ≥ √(I²t)/k, where I²t is the let-through energy of the protective device and k depends on conductor material and insulation type.

Earth fault loop impedance

The total impedance of the fault path, Zs = Ze + (R1 + R2), where Ze is the external loop impedance and R1 + R2 are the phase and protective conductor resistances. It must be low enough for disconnection.

TN-C-S system

A supply arrangement where the neutral and protective functions are combined in a PEN conductor from the source and split into separate neutral and protective conductors at the installation origin. Also called protective multiple earthing.

TT system

A supply where the source neutral is directly earthed and the installation's exposed conductive parts are connected to a local earth electrode independent of the source earth. RCD protection is normally required for disconnection.

IT system

A supply with the source isolated from earth or earthed through high impedance, and installation exposed parts earthed locally. A first earth fault does not disconnect but must be detected and alarmed.

Prospective short-circuit current

The current that would flow at a point if a zero-impedance fault occurred there. Protective devices must have a breaking capacity at least equal to this value at their point of installation.

MCB type B

A miniature circuit breaker that trips magnetically between three and five times its rated current. It suits resistive loads such as lighting and socket outlets with low inrush current.

RCD Type A

A residual current device that responds to alternating residual current and to pulsating direct residual current. It is required for circuits supplying equipment with electronic rectifiers and many modern appliances.

Discrimination

Coordination between protective devices so that the device nearest the fault operates first, leaving upstream devices closed. It is achieved by current, time or energy grading between device ratings and characteristics.

Overload protection rule

The design current must not exceed the device rating, which must not exceed the cable capacity (Ib ≤ In ≤ Iz), and the device must operate at 1.45 times cable capacity within the specified time.

Correction factors

Multipliers applied to tabulated cable current-carrying capacity for ambient temperature, grouping, thermal insulation and soil conditions. The corrected capacity must still be at least the protective device rating.

Voltage drop limit

The maximum permitted drop from the origin of the installation is typically three percent for lighting and five percent for other circuits, calculated from cable mV/A/m values, current and length.

Harmonised colour code

Line conductors are brown (single-phase) or brown, black and grey (three-phase), neutral is blue, and the protective conductor is green/yellow. Older installations may use red, yellow, blue and black.

Ring final circuit

A socket outlet circuit that leaves the distribution board, serves outlets in a loop and returns to the same board, giving two paths. Continuity of all three conductors must be verified during testing.

Diversity factor

A fraction applied to connected load to estimate maximum demand, reflecting that not all loads operate simultaneously. Values are taken from Code of Practice tables for each load category.

Insulation resistance test

A DC test at 500 V between live conductors and earth, with a minimum acceptable result of 1 megohm for low-voltage installations. Sensitive equipment must be disconnected before testing.

RCD trip time test

At rated residual current the device must trip within 300 milliseconds; at five times rated current within 40 milliseconds; at half rated current it must not trip. Results are recorded on the schedule.

Polarity test

Verification that line conductors connect to switch and lampholder centre contacts, that socket outlets are wired correctly, and that protective conductors are connected to the intended points.

IP rating

A two-digit code for ingress protection: the first digit rates protection against solids (0-6) and the second against water (0-8). For example, IP44 resists small solids and splashing water.

Class II equipment

Equipment with double or reinforced insulation and no protective earth connection. It relies on insulation alone for protection against electric shock and is marked with a double-square symbol.

SELV

Separated extra-low voltage not exceeding 50 volts AC, supplied from a safety isolating transformer or equivalent source, with the circuit isolated from earth and from other circuits.

Star-delta starting

A motor starting method that connects the winding in star during run-up, reducing starting current and torque to one third of direct-on-line values, then switches to delta for normal running.

Motor slip

The difference between synchronous speed and actual rotor speed, expressed as a fraction of synchronous speed. Slip increases with load and determines rotor frequency and torque.

Transformer efficiency

Maximum efficiency occurs when copper loss equals iron loss. Iron loss is approximately constant with load, while copper loss varies with the square of the load current.

Bathroom zone 1

The area above the bath or shower basin up to 2.25 metres and around the basin to 0.6 metres. Equipment must be at least IPX4 and Class II or SELV, with 30 mA RCD protection.

Reduced low voltage

A construction-site supply of 110 volts with a centre tap earthed, giving 55 volts to earth. It limits touch voltage and is used for hand-held tools on site.

Intrinsic safety Ex i

A protection concept that limits electrical energy in a circuit so that no spark or hot surface can ignite the hazardous atmosphere. It requires a safety barrier and control of circuit capacitance and inductance.

Grade B electrical worker

A person registered by the Electrical and Mechanical Services Department under the Electricity Ordinance to carry out or supervise electrical work within the capacity limits set for that grade.