HK Grade B Electrical Worker Exam cheat sheet

Electric shock severity depends on current path, magnitude and duration: ~1 mA is threshold of perception, 10-30 mA causes muscular tetany (cannot let go), 50 mA and above across the heart risks ventricular fibrillation; body impedance falls sharply when skin is wet.

Touch voltage is the potential difference between simultaneously accessible conductive parts; a person touching an exposed metal part while standing on earth receives the touch voltage, so bonding and disconnection times are the primary controls.

Step voltage is the potential difference across one pace (~1 m) on the ground near an earth electrode carrying fault current; it is worst at the electrode and decreases with distance.

The three protective measures against indirect contact are automatic disconnection of supply, double insulation (Class II), and electrical separation or SELV/PELV; direct contact is controlled by insulation, barriers, enclosures and obstacles.

Arc flash energy rises with fault current and clearing time; switching off, proving dead, locking off and where required using rated PPE and insulated tools precede all work on or near live parts.

Lockout/tagout sequence: isolate all sources (including back-feeds, UPS, PV and standby generators), lock, prove the tester on a known source, test for dead at the point of work, then earth/discharge.

Basic first aid for electric shock: isolate supply before touching the casualty, call emergency services, start CPR if breathing/ pulse absent, and treat burns; do not move a casualty still in contact with a live conductor.

Portable equipment inspection intervals shorten with harsh use and increased shock risk; Class I earthed equipment in construction or wet environments needs more frequent formal visual and combined testing than office equipment.

The Electricity Ordinance (Cap. 406) and its subsidiary regulations govern electrical work in Hong Kong; the Electricity (Wiring) Regulations set technical requirements and the Code of Practice for the Electricity (Wiring) Regulations gives deemed-to-comply guidance.

Grade B electrical worker registration is issued by the Electrical and Mechanical Services Department (EMSD) under the Electricity Ordinance; grades A, B and C differ in the maximum permissible capacity of installation work.

A Grade B worker may carry out or supervise electrical work on low-voltage installations of higher capacity than Grade A; work beyond the grade limit must be supervised by a higher-grade worker or contractor.

The Electricity (Wiring) Regulations require new installations to be inspected, tested and certified before energisation, and periodic inspection and testing thereafter; the Code of Practice specifies intervals and record keeping.

The Code of Practice for the Electricity (Wiring) Regulations covers design, construction, inspection, testing and certification, and aligns with the IEC 60364 series; it is advisory guidance but is used as the benchmark for compliance.

The Occupational Safety and Health Ordinance and Factories and Industrial Undertakings Ordinance impose duties on employers and employees for safe systems of work, risk assessment, and competent persons for electrical work.

Statutory notices and forms: the Certificate of Completion (Form WR1) and periodic test certificates are prescribed records; retention and availability for inspection by EMSD officers is a legal obligation.

Only registered electrical workers and registered electrical contractors may carry out or arrange electrical work; unregistered work is an offence and may invalidate insurance and supply.

Ohm's law: V = I × R; power: P = V × I = I²R = V²/R; for three-phase balanced loads P = √3 × V_L × I_L × cos φ.

Series: R_total = R1 + R2 + ...; parallel: 1/R_total = 1/R1 + 1/R2 + ...; two resistors in parallel: R = (R1 × R2)/(R1 + R2).

AC impedance: Z = √(R² + (X_L − X_C)²); inductive reactance X_L = 2πfL; capacitive reactance X_C = 1/(2πfC); at resonance X_L = X_C and Z = R.

Power factor = cos φ = P/S; true power P in watts, apparent power S in VA, reactive power Q in var; S = √(P² + Q²); correction capacitors supply Q and raise cos φ toward unity.

RMS value of a sine wave = peak/√2 ≈ 0.707 × peak; average value over a half cycle = 2 × peak/π ≈ 0.637 × peak; form factor = RMS/average ≈ 1.11.

Faraday's law: induced EMF e = −N dΦ/dt; Lenz's law gives the opposing polarity; self-induced EMF e = −L di/dt; energy stored in an inductor W = ½LI².

Transformer EMF equation: E = 4.44 × f × N × Φ_max; turns ratio V1/V2 = N1/N2 = I2/I1 for an ideal transformer.

Three-phase relationships: star V_L = √3 × V_ph and I_L = I_ph; delta I_L = √3 × I_ph and V_L = V_ph; total power is the sum of the three phase powers.

Hong Kong low-voltage public supply is typically 220 V single-phase (phase to neutral) and 380 V three-phase (phase to phase), 50 Hz, from a 380/220 V star-connected distribution transformer.

System earthing arrangements are designated by the first letter (source earthing: T = directly earthed, I = isolated or impedance-earthed) and second letter (installation exposed parts: T = earthed locally, N = connected to neutral).

TN-S: separate neutral and protective earth conductors throughout; TN-C-S: combined PEN conductor split into separate N and PE at the origin; TN-C: combined PEN throughout; TT: installation earth electrode independent of source earth.

In a TN system, fault current returns via the metallic PE/PEN path and is high enough to operate overcurrent devices; in TT the loop resistance is higher, so RCD protection is normally required for disconnection.

IT systems are used where continuity of supply is critical; the first earth fault does not cause disconnection but must be detected by an insulation monitoring device and cleared promptly.

Supply characteristics to confirm before design: nominal voltage, frequency, prospective short-circuit current, earth loop impedance (external), and the type/rating of the supply protective device.

Single-phase loads on a three-phase supply should be balanced across phases to limit neutral current and voltage unbalance; unbalanced loads produce neutral current equal to the phasor sum of phase currents.

The supplier's cut-out, meter and main switch define the origin of the installation; the means of earthing and main protective bonding connect at this point.

Earthing performs three functions: limits touch voltage on exposed metalwork, provides a low-impedance fault path for automatic disconnection, and stabilises system voltage and limits overvoltage.

Circuit protective conductor (CPC) size rule: for phase conductors ≤ 16 mm², CPC ≥ phase conductor; 16-35 mm², CPC ≥ 16 mm²; > 35 mm², CPC ≥ half the phase conductor (subject to adiabatic check).

Adiabatic equation for CPC sizing: S = √(I²t)/k, where k depends on conductor material and insulation (e.g. copper/PVC k ≈ 115, copper/XLPE k ≈ 143); the CPC must withstand the let-through energy.

Main protective bonding conductors connect extraneous conductive parts (water, gas, structural steel, other services) to the main earthing terminal; typical minimum 6 mm² copper for main bonds, 4 mm² for supplementary bonding.

Earth fault loop impedance Zs = Ze + (R1 + R2); disconnection time compliance requires Zs ≤ the maximum value tabulated for the protective device rating and type (or Ia ≤ U0/Zs).

Earth electrode resistance: a single rod R ≈ ρ/(2πL) × (ln(4L/d) − 1); multiple rods in parallel reduce resistance, with spacing ≥ their driven length for effective independence.

Supplementary bonding connects simultaneously accessible exposed and extraneous conductive parts where disconnection times cannot be met or in special locations such as bathrooms; its resistance must satisfy the touch voltage limit.

Protective conductor continuity must be verified by a low-resistance test (typically ≤ 1 Ω for the main earthing terminal to the furthest point, using a calibrated low-reading ohmmeter).

Cable selection considers current-carrying capacity, voltage drop, thermal constraints (Ib ≤ In ≤ Iz), short-circuit withstand, and environmental factors such as temperature, grouping and thermal insulation.

Correction factors: Ca (ambient temperature), Cg (grouping), Ci (thermal insulation), Cd (buried depth), Cs (soil thermal resistivity); the tabulated Iz must be divided by the product of applicable factors.

Voltage drop limits: lighting circuits typically ≤ 3% and power circuits ≤ 5% from the origin of the installation; use mV/A/m values from cable tables and account for three-phase factor √3.

Wiring system selection: PVC/sheathed twin and earth for domestic concealed work; SWA for underground or mechanical protection; MICC or XLPE for high temperature; flexible cords for portable equipment.

Segregation of band I (SELV/PELV) and band II (LV) circuits; band I conductors must not share an enclosure or trunking with band II unless separated by a barrier or insulated to the higher voltage.

Cable support and protection: cables must be supported at intervals per the Code, protected by conduit or trunking where exposed to mechanical damage, and run in safe zones (within 150 mm of corners/ceiling) or protected by 30 mA RCD if buried in walls.

Fire safety: fire-rated cable for life safety systems, fire-stopping of penetrations, and use of low-smoke zero-halogen cable in escape routes and public buildings.

Bending radius: typically 4× cable diameter for armoured or sheathed cable and 6× for MICC; exceeding it damages insulation and conductors.

Conductor materials: copper (high conductivity, ~58 MS/m) and aluminium (lighter, cheaper, lower conductivity); aluminium requires larger cross-section and careful termination to avoid oxidation and creep.

Standard conductor sizes (mm²): 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630.

Stranding classes: solid (class 1), stranded (class 2), flexible (class 5); flexible conductors need suitable terminals or ferrules, and fine-stranded cables need crimped or screw terminals rated for the strand class.

Insulation and sheath materials: PVC (70 °C), XLPE (90 °C), EPR (90 °C), silicone (180 °C), MICC (high temperature, fire resistant); the temperature rating sets the current-carrying capacity.

Cable installation method reference (IEC 60364-5-52 / Code of Practice): method A (enclosed in conduit in thermally insulated wall), B (conduit on wall), C (clipped direct), D (buried), E (free air), F (tray).

Current-carrying capacity depends on installation method, ambient temperature, grouping, and whether the cable is surrounded by thermal insulation; the tabulated value must be corrected before comparing with Ib and In.

Cable joints and terminations must maintain insulation, mechanical strength and continuity; joints in buried or inaccessible cables should be avoided or made with approved kits.

Colour coding (harmonised): line brown, neutral blue, protective conductor green/yellow; three-phase line conductors brown, black, grey; old HK/UK colours (red, yellow, blue, black) still appear in existing installations.

Overcurrent protection comprises overload protection (protects conductors, must satisfy Ib ≤ In ≤ Iz and I2 ≤ 1.45 Iz) and short-circuit protection (must operate within the cable's thermal withstand).

MCB types: B (trips 3-5 × In), C (5-10 × In), D (10-20 × In); type selection matches the inrush of the load and the earth loop impedance available for disconnection.

Fuse classes: gG (general purpose, full-range), aM (motor, back-up only), gM; fuses are current-limiting and provide high breaking capacity; the let-through I²t must be within the cable's withstand.

RCDs: 30 mA for additional protection and socket outlets, 100/300 mA for fire protection or TT systems; Type AC responds to AC only, Type A to AC and pulsating DC, Type B to smooth DC and high frequency.

RCBO combines overcurrent and residual current protection in one device; RCDs do not protect against overload or short circuit and must be backed by overcurrent protection.

Discrimination (selectivity) between devices: the upstream device must not operate before the downstream device for faults in the downstream zone; achieved by current, time or energy discrimination.

Breaking capacity (Icn/Icu) of a protective device must be at least the prospective short-circuit current at its point of installation, otherwise back-up protection is required.

RCD testing: press the test button (mechanical), then use an RCD tester to verify trip current and time at ½×IΔn (no trip), 1×IΔn (trip within 300 ms) and 5×IΔn (trip within 40 ms).

Isolation requires a device that provides a visible or clearly indicated break and can be locked off; a switch-disconnector or isolator must be rated for the load and have a suitable utilisation category.

Utilisation categories: AC-1 (resistive), AC-3 (squirrel-cage motor starting/switching), AC-23 (motor loads with frequent operation), DC-1, DC-3; the device must be rated for the actual duty.

Contactors are designed for frequent switching of load current but not for isolation or short-circuit breaking; they are combined with fuses or an MCCB for fault protection.

Motor starters: DOL (direct-on-line, up to ~5.5 kW typical), star-delta (reduces starting current to 1/3, starting torque to 1/3), auto-transformer, soft starter, and variable-frequency drive.

Star-delta starting requires a motor with six terminals and a delta-rated winding equal to the line voltage; the transition must be timed and the overload relay set for the motor full-load current.

Control circuit voltages: SELV (≤ 50 V AC) preferred for pushbuttons and emergency stop circuits; emergency stop must be a latching, manually reset device that directly opens the control circuit.

Switchgear ratings: rated operational voltage Ue, rated current Ie, rated short-circuit making capacity Icm, rated short-circuit breaking capacity Icu/Ics, and IP rating for the enclosure.

Interlocking: mechanical or electrical interlocks prevent paralleling of supplies, opening under load where not rated, or access to live parts; isolators must be capable of being locked in the OFF position.

Final circuit types: radial (one path from the board), ring (loop returning to the board), and spur (branch from a ring or radial); ring final circuits rely on two paths and need a CPC continuity check.

Socket outlet circuits: 30 mA RCD protection for socket outlets (unless a documented risk assessment justifies omission for specific labelled equipment), and for cables buried ≤ 50 mm in walls.

Lighting circuits: typically 1.5 mm² protected by 6 A or 10 A MCB; the circuit must be arranged so that a lamp failure does not leave exposed live parts.

Fixed equipment circuits: rating based on the equipment's rated current and diversity; motor circuits need overload protection set to the motor full-load current and short-circuit protection sized for starting.

Diversity (demand factors) for final circuits: lighting 66% of total, socket outlets 100% of largest circuit plus 40% of the rest (typical domestic guidance); apply the Code of Practice values for the installation type.

Cable size for a final circuit is chosen from Ib (design current), In (device rating) and Iz (corrected cable capacity); voltage drop and disconnection time must also be verified.

Every final circuit must have a means of isolation, a means of protection against overload and short circuit, and a means of switching; the neutral must be switched only with the line in a multi-pole device.

Special final circuits (showers, cookers, EV chargers) require dedicated circuits with RCD protection and isolation; EV charging points require Type A or Type B RCD and a dedicated CPC.

Design sequence: determine load and diversity, select supply and earthing system, size conductors for Ib, voltage drop and fault withstand, select protective devices, verify disconnection times, then document.

Maximum demand = connected load × diversity factor; apply diversity per load type (lighting, sockets, cooking, water heating, motors) using the Code of Practice tables rather than a single blanket factor.

After diversity, check the supply capacity (cut-out rating) and the main switch rating; if maximum demand exceeds the supply, apply load management or upgrade the supply.

Voltage drop calculation: ΔV = (mV/A/m × I × L)/1000 for single-phase; multiply by √3 for three-phase; verify against 3% lighting and 5% power limits.

Earth fault loop impedance check: Zs = Ze + (R1 + R2) ≤ tabulated maximum for the device; if exceeded, increase CPC size, use a lower-rated device, or add RCD protection.

Short-circuit withstand: verify k²S² ≥ I²t for the protective device let-through energy at the point of installation; check both the cable and the protective conductor.

Protection against thermal effects: keep combustible materials away from equipment, use fire-rated enclosures, and provide thermal insulation clearance or derating where cables pass through insulation.

Documentation: design calculations, cable schedules, protective device coordination, and as-built drawings must be retained and updated; they form the basis for periodic inspection.

Initial verification sequence: inspection (visual) first, then dead tests (continuity, insulation resistance, polarity), then live tests (earth loop impedance, RCD, prospective short-circuit current, phase sequence).

Continuity of protective conductors: low-resistance test, typically ≤ 1 Ω for the main earthing terminal to the furthest point; ring final circuit continuity uses the three-step r1, rn, r2 method and expects r1 ≈ rn and r2 ≈ r1 × (CPC/phase area).

Insulation resistance: 500 V DC test, minimum 1 MΩ (1 MΩ for SELV/PELV at 250 V DC); test between live conductors and earth with sensitive equipment disconnected.

Polarity: verify that line is connected to the centre contact of lampholders, switches are in the line conductor, and socket outlet polarity is correct.

Earth fault loop impedance: measured with a loop tester or calculated from Ze and (R1 + R2); compare with the maximum permitted for the device type and rating.

RCD test: ½×IΔn no trip, 1×IΔn trip within 300 ms, 5×IΔn trip within 40 ms; record the operating time and current for each RCD.

Fault diagnosis: open circuit (no continuity, no voltage at load), short circuit (overcurrent trip, low insulation resistance), earth fault (RCD trip, low loop impedance), and high resistance joint (voltage drop, heat, intermittent operation).

Certification: Electrical Installation Certificate (EIC) for new work, Minor Electrical Installation Works Certificate for small additions, and Periodic Inspection Report/Condition Report for existing installations; all require schedules of test results.

IP rating: first digit (0-6) for solids (6 = dust tight), second digit (0-8) for water (4 = splashing, 5 = jet, 7 = temporary immersion, 8 = continuous immersion); e.g. IP44 for bathroom zones, IP65 for outdoor.

Equipment classes: Class I (basic insulation plus earthed metalwork), Class II (double or reinforced insulation, no earth), Class III (SELV, ≤ 50 V AC); the class determines the protective measure required.

Switchgear and accessories must be rated for the voltage, current, frequency and environment; accessories in special locations need appropriate IP and RCD protection.

Transformers: isolation transformer for electrical separation, safety isolating transformer for SELV, and auto-transformer (not for SELV); the isolating transformer's output must not be earthed.

Motor protection: overload relay (thermal or electronic), short-circuit protection (fuse or MCCB), phase failure/under-voltage protection, and thermistor or PTC protection for embedded windings.

Lighting equipment: luminaires must be suitable for the environment (IP, ambient temperature, fire rating), and lampholders must not be accessible to touch; LED drivers may need derating in enclosed fittings.

Equipment selection considers rated voltage, current, frequency, breaking capacity, IP rating, ambient temperature, and the prospective fault level at the point of connection.

Portable appliance testing: Class I requires earth continuity, insulation resistance and functional tests; Class II requires insulation resistance and functional tests; record results and label with test date.

Three-phase induction motor: synchronous speed Ns = 120f/P; slip s = (Ns − Nr)/Ns; rotor frequency = s × f; torque is proportional to V² at starting.

Motor starting current is typically 5-8 × full-load current for DOL; star-delta reduces it to 1/3, and soft starters/VFDs reduce it further with controlled torque.

Motor full-load current from nameplate; overload relay set to the motor FLC (or 1.05-1.1 × FLC for service-factor motors); the circuit conductor and short-circuit device are sized for starting duty.

Transformer losses: copper loss (I²R, varies with load squared) and iron loss (hysteresis and eddy current, approximately constant); maximum efficiency occurs when copper loss equals iron loss.

Transformer cooling classes: ONAN, ONAF, OFAF; the rating depends on cooling; temperature rise limits are set by insulation class (A, B, F, H).

Control systems: relay logic, PLC, and contactor-based circuits; the emergency stop must directly interrupt the control circuit and latch, and the start circuit must be self-holding (seal-in contact).

Motor protection coordination: Type 1 (damage permitted, must be safe to restart) vs Type 2 (no damage other than welding of contacts) coordination between starter and short-circuit device.

VFD installation: screened motor cable, earthing of the screen at both ends, separation of power and signal cables, and limitation of cable length to control bearing currents and EMI.

Bathroom zones: zone 0 (inside bath/shower basin), zone 1 (above bath to 2.25 m, around basin 0.6 m), zone 2 (0.6 m beyond zone 1 to 2.25 m); each zone specifies IP rating, equipment class and RCD (30 mA) requirements.

Zone 0 requires IPX7 and SELV ≤ 12 V; zone 1 requires IPX4 (IPX5 in public baths) and Class II or SELV; socket outlets are not permitted in zones 0, 1 or 2 (shaver supply units excepted).

Construction sites: 110 V centre-tapped-to-earth reduced low voltage (55 V to earth) for hand-held tools, 30 mA RCD protection for all socket outlets, and robust IP-rated equipment with frequent inspection.

Hazardous areas: zone classification (gas: zone 0, 1, 2; dust: zone 20, 21, 22); equipment must be certified for the zone and gas/dust group, with protection concepts such as Ex d (flameproof), Ex e (increased safety), Ex i (intrinsic safety).

Intrinsic safety (Ex i) limits energy in the circuit so that no ignition can occur; it requires a safety barrier or galvanic isolator and control of circuit capacitance and inductance.

Temporary installations (events, exhibitions) require RCD protection, robust cable routing, inspection before use, and a documented periodic inspection regime.

Swimming pools and fountains: zone 0 (inside the water basin), zone 1 (within 2 m horizontally and 2.5 m above), zone 2; SELV ≤ 12 V in zone 0 and 30 mA RCD protection for all circuits.

Agricultural and outdoor installations: equipment rated for wet conditions, 30 mA RCD protection for socket outlets, and protection against mechanical damage from livestock and machinery.