MEngC PE Electrical Building Services common mistakes
MEngC PE Electrical Building Services · Understand the traps and how to avoid them.
Sizing the main switchboard and downstream cables from connected load (sum of all nameplate ratings) instead of applying diversity/demand factors per the applicable standard, then failing to check the result against the utility supply capacity and the maximum demand history of similar buildings.
Candidates memorise nameplate formulas and treat 'total connected load' as the design current because it is the only number given in the question; diversity tables are skimmed rather than internalised, and there is no habit of cross-checking against a comparable building's measured maximum demand.
What to do instead
For every load schedule question, build a two-column table: connected load and diversified demand, citing the diversity factor source (standard clause or engineering judgement with justification). Then state the design current, protective device rating, and cable derating chain (ambient, grouping, installation method) as separate steps. Practise on a 20-way board with mixed lighting, socket, motor, and HVAC loads until the sequence is automatic.
Applying cable derating factors in the wrong order or omitting one of them (ambient temperature, grouping, thermal insulation, soil thermal resistivity, depth of laying, protective device type), and then selecting a cable whose tabulated current-carrying capacity is compared directly with the design current without any correction.
Manufacturer tables and standard appendices present correction factors in separate tables, and candidates under time pressure grab the base rating; the correction-factor multiplication chain is easily forgotten because it is rarely exercised in undergraduate coursework.
What to do instead
Write the derating equation explicitly on every calculation: It ≥ In / (Ca × Cg × Ci × Cs × Cd), and show each factor's numeric value with its source table. Do five worked examples covering buried, clipped-direct, in-duct, and grouped-in-trunking cases, and verify one against a cable-sizing software output or manufacturer's calculator.
Computing prospective short-circuit current using only the transformer impedance and ignoring the contribution from motors, the supply authority's declared fault level, and the impedance of the cable run from the origin to the point of fault; then checking the protective device's breaking capacity but not its energy let-through against the cable's withstand.
Candidates default to the simplest formula (Isc = U/(√3·Z)) with a single impedance, because the per-unit or ohmic method with multiple sources is perceived as too slow for an exam; motor contribution and cable impedance are treated as refinements rather than requirements.
What to do instead
Adopt a fixed three-step method: (1) establish source fault level or transformer Z with X/R ratio, (2) add cable and busbar impedances to the fault point using the ohmic or per-unit method, (3) add motor contribution as a multiple of motor full-load current. Then verify both breaking capacity and k²t (or I²t) withstand of the cable and the protective device's current-limiting let-through. Practise one radial and one parallel-fed example.
Designing the earthing and bonding system by quoting a target earth resistance figure alone, without distinguishing protective earthing, functional earthing, main equipotential bonding, and supplementary bonding, and without checking disconnection time versus touch voltage for the relevant earthing system (TN-S, TN-C-S, TT).
Earth resistance is the only earthing number most candidates have practised, so it becomes the whole answer; the loop-impedance/disconnection-time relationship and the different roles of bonding conductors are learned as definitions rather than as design checks.
What to do instead
For each earthing question, state the system type first, then the fault path, then the protective device's disconnection time and the resulting touch voltage at the exposed conductive part. Separately list main equipotential bonding conductor sizing (based on the incoming neutral/phase conductor cross-section) and supplementary bonding conditions. Work one example per system type, including a TT case where an RCD or earth electrode resistance governs.
Treating standby generator sizing as a single kVA figure equal to the total connected load, without separating steady-state block load, transient voltage/frequency dip on motor starting, and the load steps the generator can accept; and without checking the changeover sequence and neutral/earth arrangement for the paralleled or switched source.
Generator sets are often taught as 'kVA must exceed load kVA', and the dynamic behaviour (subtransient reactance, load acceptance, voltage dip limits) is not part of most electrical engineering curricula; the earthing of a switched neutral is an afterthought.
What to do instead
Size generators with three checks: continuous kVA at site ambient and power factor, transient dip on the largest motor start (typically limited to 10–15% voltage dip), and step-load acceptance per the manufacturer's data. Then specify the transfer arrangement (open transition, closed transition, or soft loading), the neutral switching policy, and the earth-fault protection scheme for each source configuration.
Forgetting that UPS and emergency lighting autonomy, battery end-of-discharge voltage, and the actual load power factor at the end of discharge change the required battery capacity, and specifying autonomy minutes without stating the load profile and design margin.
Autonomy is treated as a single number (e.g. '30 minutes') copied from a code table, while the battery sizing calculation (Peukert effect, ageing factor, temperature correction, end voltage) is unfamiliar and time-consuming.
What to do instead
Always state four parameters together: load kW and power factor, required autonomy, end-of-discharge voltage per cell, and design margin/ageing factor. Use the manufacturer's discharge curve rather than a linear kWh conversion. For emergency lighting, separate the escape-route and open-area requirements and check that the UPS or central battery system is certified for the luminaire type and switching mode.
Producing a lighting design that meets the maintained illuminance target but ignores uniformity, glare (UGR), colour rendering, and the lighting power density limit, and not distinguishing maintained versus initial illuminance or the maintenance factor used.
Illuminance average is the easiest output from design software and the only metric candidates habitually report; UGR, uniformity, and LPD are perceived as secondary checks, and maintenance factor is often left at a default without justification.
What to do instead
For every lighting layout, report a five-metric set: average maintained illuminance, uniformity (Emin/Eavg), UGR, Ra, and LPD in W/m² compared with the code limit. State the maintenance factor and its components (lamp lumen depreciation, luminaire dirt depreciation, room surface depreciation). Practise one office, one corridor/escape route, and one industrial high-bay case.
Designing lighting controls and daylight-linked dimming without checking the control zoning against the fenestration and occupancy pattern, without verifying the dimming protocol compatibility and standby power of the control gear, and without quantifying the predicted energy savings against a baseline.
Controls are often added as a compliance item ('we have occupancy sensors') rather than as a designed system; candidates do not model the interaction between daylight dimming, occupancy sensing, and manual override, and they accept vendor claims instead of calculating savings.
What to do instead
Draw the control zoning diagram over the floor plan, define each zone's sensor type, setpoint, and override behaviour, and confirm protocol compatibility (e.g. DALI-2, 0–10 V, wireless) with a driver schedule. Compute annual energy using a simple hour-by-hour or bin method against a no-controls baseline, and state the assumptions. Check that emergency lighting and controls do not conflict (failure mode of the control system must not leave the space dark).
Specifying fire detection and alarm cabling, device spacing, and power supplies without checking the loop loading and voltage drop at the end device, the integrity requirements for the cable category, and the cause-and-effect matrix linking detection to suppression, smoke control, and evacuation.
Device spacing rules are well memorised, but loop power calculations, cable fire-resistance categories, and the cause-and-effect logic are systems-engineering tasks that require reading the standard and coordinating with mechanical and architectural disciplines; candidates skip them because they appear to be 'other people's scope'.
What to do instead
For each detection zone, calculate loop current and worst-case voltage drop with the specified cable, confirm the panel's power supply and battery standby meet the standard's duration, and state the cable category required for each circuit. Produce a cause-and-effect matrix with rows as detection events and columns as outputs (alarm, suppression release, smoke dampers, lifts homing, door release), and check each row against the fire strategy.
Answering codes, standards, contracts, and ethics questions by quoting a clause number or a general principle without identifying the applicable edition, the contractual hierarchy of documents, or the specific professional obligation (e.g. duty to report unsafe conditions, competence limits, conflict of interest disclosure).
Candidates revise standards as lists of numbers rather than as documents with editions, national forewords, and contractual status; professional-practice questions are treated as common sense, so answers stay generic and fail to name the actual obligation or the correct order of precedence.
What to do instead
Build a one-page reference for each core standard listing its edition year, scope, and the clauses most often cited in design. For contract questions, practise ranking documents (contract, specification, drawings, schedules, BOQ) and identifying which governs on conflict. For ethics, memorise the code of conduct's specific duties and apply a three-step answer: identify the obligation, state the action, state the record/disclosure step.