HK Grade B Electrical Worker Exam common mistakes
HK Grade B Electrical Worker Exam · Understand the traps and how to avoid them.
Treating the nominal current rating (In) of a circuit breaker or fuse as the only current the circuit can safely carry, and selecting the protective device without checking that In does not exceed the current-carrying capacity (Iz) of the cable after all correction factors are applied.
Candidates memorise the rule 'In ≤ Iz' as a single inequality but forget that Iz must be recalculated for the actual installation conditions. They use the tabulated current rating from the cable table directly, ignoring ambient temperature, grouping, thermal insulation, and the method of installation. This leads to selecting a device that appears compliant on paper but leaves the cable overloaded in service.
What to do instead
For every final circuit calculation, write out the full chain: It (tabulated) → apply Ca, Cg, Ci, etc. → Iz → then verify In ≤ Iz and also Ib ≤ In ≤ Iz. Show each correction factor explicitly and state the installation reference method used. Practise at least ten cable-sizing exercises where the installation conditions differ from the reference method, and always cross-check the protective device's actual tripping characteristic against the cable's overload withstand.
Assuming that a residual current device (RCD) provides protection against all electric shock, and therefore omitting or under-specifying the earthing and bonding requirements in special locations such as bathrooms and construction sites.
The phrase '30 mA RCD protection' is repeated so often in study material that candidates treat it as a universal substitute for earthing and supplementary bonding. They forget that an RCD does not reduce touch voltage to a safe level on its own, and that supplementary equipotential bonding is still required in locations containing a bath or shower, and that construction-site installations have additional requirements for earth electrodes and protective conductor continuity.
What to do instead
Revise the requirements for each special location separately, listing the protective measures in the order they must be applied: automatic disconnection of supply, supplementary bonding, RCD additional protection, and limitation of exposed conductive parts. For bathrooms, state the zones and what is permitted in each. For construction sites, state the requirement for a reliable earth electrode and the reduced touch voltage. In exam answers, never write 'RCD is sufficient' without also stating the disconnection time and the bonding requirement.
Calculating the earth fault loop impedance (Zs) using only the phase conductor resistance and ignoring the protective conductor resistance, or using the wrong conductor temperature correction.
Candidates often remember Zs = Ze + (R1 + R2) but then use the line conductor resistance for both R1 and R2, or forget that R1 and R2 are at different temperatures under fault conditions. They also forget that the tabulated mV/A/m values are at 20 °C and must be corrected for operating temperature, and that the protective conductor may be a different size or material from the phase conductor.
What to do instead
Always calculate R1 and R2 separately from the actual conductor sizes and materials, apply the correct temperature correction factor (usually 1.2 for the phase conductor and 1.2 for the protective conductor at fault temperature, but check the specific standard), and add Ze. For a quick check, use the measured R1+R2 from testing and add Ze. In written answers, show the formula, the values substituted, and the maximum permitted Zs from the protective device's time-current characteristic. State that the measured Zs must not exceed the tabulated maximum.
Confusing the roles of overcurrent protective devices for overload versus short-circuit protection, and failing to check that the device's breaking capacity is adequate for the prospective fault current at its installation point.
Candidates learn that a circuit breaker protects against overload and short circuit but do not distinguish between the two duty cycles. They select a device with a breaking capacity (Icn) lower than the prospective short-circuit current (Ipsc) because they only check the thermal setting and forget the electromagnetic breaking capacity. They also forget that the device must be suitable for the fault level at the origin and that backup protection may be needed.
What to do instead
For every protective device, record two separate checks: (1) Ib ≤ In ≤ Iz and I2 ≤ 1.45 Iz for overload; (2) the device's rated short-circuit breaking capacity must be ≥ the prospective fault current at that point, or a suitable backup device must be declared. Calculate the prospective fault current at the origin and at the end of each submain, and compare with the device's Icn or Ics. In exam answers, state both the overload and short-circuit conditions explicitly.
Applying diversity to a final circuit or to a submain without checking the actual load characteristics, and using the maximum demand figures from a table without justifying the assumed diversity factor.
Candidates memorise diversity factors for domestic installations and then apply them indiscriminately to commercial or industrial loads. They also forget that diversity cannot be applied to a fixed load such as a motor or a water heater, and that the maximum demand must be based on the actual connected load and its duty cycle. This leads to undersized submains and main switches.
What to do instead
For each load, classify it as fixed, cyclic, or intermittent, and state the basis for any diversity applied. Use the standard diversity tables only as a starting point and justify any departure. Calculate the maximum demand by summing the full load of fixed equipment and applying diversity only to loads that are genuinely unlikely to operate simultaneously. In exam answers, show the connected load, the diversity factor used, and the resulting design current, and state that the protective device and cable are selected from that design current.
Describing the inspection and testing sequence incorrectly, for example carrying out insulation resistance testing before continuity of protective conductors, or failing to perform a visual inspection before any live testing.
Candidates learn the tests as a list but not the logical order. They forget that continuity of protective conductors must be confirmed before insulation resistance testing because the protective conductor is used as a reference, and that a visual inspection must precede all testing to identify obvious defects that would make testing unsafe or meaningless. They also omit the safe isolation procedure at the start.
What to do instead
Memorise the sequence as a workflow: safe isolation and prove dead, visual inspection, continuity of protective conductors, continuity of ring final circuit conductors, insulation resistance, polarity, earth electrode resistance (if applicable), earth fault loop impedance, prospective fault current, RCD operation, and functional testing. For each test, state the instrument used, the expected result, and the action to take if the result is unsatisfactory. Practise writing the sequence from memory and explain why each test precedes the next.
Selecting a wiring system or cable type without checking its suitability for the environmental conditions, such as using PVC-insulated cables in a location subject to high ambient temperature or direct sunlight, or using ordinary cables in a hazardous area.
Candidates focus on current-carrying capacity and voltage drop and treat the wiring system as an afterthought. They do not consider the effect of UV radiation, chemicals, mechanical damage, or temperature on the cable sheath and insulation. In hazardous areas, they may not realise that the cable and its glands must be certified for the zone and gas group.
What to do instead
For each installation, list the environmental conditions: ambient temperature range, presence of water, dust, corrosive substances, mechanical impact, UV exposure, and fire risk. Then select a wiring system and cable type that is rated for the most severe condition. For hazardous areas, identify the zone and the gas or dust group, and select cables, glands, and enclosures that are appropriately certified. In exam answers, state the environmental condition and the specific cable or enclosure property that addresses it.
Misunderstanding the role of the protective conductor in a TT system and incorrectly applying the disconnection time formula for TN systems.
Candidates learn one earth fault loop impedance formula and apply it to all systems. In a TT system, the fault loop includes the earth electrode resistance and the supply earth, so the fault current is much lower and an RCD is usually required. Candidates may calculate Zs as if it were a TN system and conclude that the disconnection time is met when it is not. They also forget that the electrode resistance must be measured and that the RCD's rated residual operating current and tripping time must be verified.
What to do instead
For each earthing system, state the fault path and the protective measure. For TN-S, TN-C-S, and TT, write the correct Zs formula and the maximum permitted disconnection time from the standard. For TT, calculate the expected fault current using the actual electrode resistance and show that the RCD will operate within the required time. In exam answers, identify the system first, then apply the correct rule. Practise converting a TN calculation into a TT calculation and explain why the protective device changes.
Confusing the requirements for electrical equipment in bathrooms with those for other locations, particularly the zone boundaries and the permitted equipment in each zone.
Candidates memorise that bathrooms need RCD protection but do not learn the zone dimensions or the IP ratings required. They may place a socket-outlet inside zone 2 or install a luminaire without the correct IP rating. They also forget that supplementary equipotential bonding is required for metallic parts in the zones and that the location of the consumer unit or switchgear must be outside the zones.
What to do instead
Draw the bathroom zones from memory: zone 0 is inside the bath or shower basin, zone 1 is above the bath or shower to a height of 2.25 m, zone 2 is the area within 0.6 m of the zone 1 boundary, and zone 3 was removed in recent editions but may still appear in older material. For each zone, list the permitted equipment and its minimum IP rating. State that supplementary bonding is required for metallic parts that are simultaneously accessible, and that all circuits in the bathroom require 30 mA RCD additional protection. In exam answers, name the zone and the specific requirement.
Writing vague or incomplete answers to statutory and code-of-practice questions, such as stating 'the installation must comply with the regulations' without naming the specific regulation, the requirement, or the responsible person.
Candidates treat statutory questions as common sense and do not learn the specific duties imposed by the Electricity Ordinance and the Code of Practice for the Electricity (Wiring) Regulations. They do not know the classes of electrical worker, the notification requirements, or the records that must be kept. As a result, answers lack the precision that examiners expect.
What to do instead
Build a one-page summary of the statutory framework: the Ordinance, the Regulations, the Code of Practice, and the relevant classes of work. For each duty, state who is responsible (registered electrical worker, contractor, owner), what must be done, and what record or notification is required. Practise answering questions in the form 'State the requirement for ...' and always include the specific regulation or code clause where possible. If you cannot recall the exact clause, state the principle precisely and avoid vague generalisations.