UPDA Electrical common mistakes
UPDA Electrical · Understand the traps and how to avoid them.
Computing voltage drop on a low-voltage feeder using only the resistance term and ignoring reactance, then rounding down to the nearest standard cable size.
Candidates memorise a single mV/A/m or simple IR formula from basic circuit theory and forget that for larger LV cables and for any MV/HV circuit the X term dominates; they also skip the correction for conductor operating temperature and load power factor.
What to do instead
Always use the tabulated mV/A/m (or R and X) values at the correct conductor temperature, apply the load power factor, and verify the result against the code's permitted drop (typically 3% lighting / 5% other for LV final circuits). Then select the next larger standard size rather than rounding down, and re-check that the protective device still achieves disconnection within the required time.
Sizing a transformer or generator from the arithmetic sum of connected load kW without applying diversity, demand factor, or load power factor.
Candidates treat connected load as maximum demand because the arithmetic is easy, and they confuse kW with kVA, so they under-size equipment for low-power-factor loads or over-size it badly for diverse loads.
What to do instead
Build a proper load schedule: list connected kVA per item, apply the appropriate demand/diversity factor per category, compute the maximum demand in kVA, then apply a growth margin and check motor starting and harmonic content. State the assumed power factor explicitly in the answer.
Treating a transformer's per-unit or percentage impedance as an ohmic value and using it directly in short-circuit calculations at the wrong voltage base.
Percentage impedance is base-dependent; candidates plug 5% straight into a formula without converting to the specified base kVA or referring it to the correct side of the transformer, producing short-circuit currents wrong by orders of magnitude.
What to do instead
Convert percentage impedance to per-unit on a stated base (e.g. Z_pu = 0.05 on the transformer's own rating), refer all impedances to one common base kVA, add source and cable impedances in per-unit, then convert back to amperes at the fault point using I = base kVA / (√3 × base kV). Show the base values in the working.
Choosing a protective device rating from the load current alone and never verifying that the earth-fault loop impedance allows disconnection within the required time.
Candidates treat protection selection as a single-step 'breaker ≥ load' exercise and forget the second condition (Zs ≤ Ia per the device's time-current curve), especially on long final circuits and TT systems.
What to do instead
For each circuit, after selecting the device, calculate Zs from the supply, transformer, and circuit conductors, read the disconnection current from the manufacturer's curve for the required time (e.g. 0.4 s for TN final circuits up to 32 A), confirm Zs ≤ U0/Ia, and where it fails either increase the CPC, shorten the run, or use RCD protection.
Confusing the roles of earthing and bonding, and mixing up TN-S, TN-C-S, and TT system requirements in written answers.
The terms are used loosely in practice, and candidates describe 'earthing the neutral' or apply TN disconnection rules to a TT installation where an RCD is mandatory.
What to do instead
State the system type first, then the rule: in TN systems the protective conductor is connected to the earthed neutral and disconnection relies on overcurrent devices (with Zs verified); in TT the installation electrode is independent and earth-fault protection must be by RCD with RA × IΔn ≤ 50 V. Keep main protective bonding (extraneous conductive parts) distinct from circuit protective conductors.
Answering a lightning protection question by describing only an air termination and omitting the risk assessment, down-conductor spacing, and equipotential bonding.
Candidates recall the visible hardware from site experience but not the design sequence in the standard, and they assume one rod or one mast protects an entire building.
What to do instead
Follow the standard sequence: determine the need and class of LPS from a risk assessment, then design air termination (rolling sphere, protection angle, or mesh method with stated dimensions), down conductors at the required spacing, earth termination resistance target, and bonding of metal services and internal systems at equipotential bonding bars.
Stating motor efficiency or power factor as a fixed number and calculating motor current as P/(√3·V·η) with the wrong units or omitting the power factor entirely.
Candidates memorise one version of the three-phase power equation and apply it to motors, generators, and transformers interchangeably, and they forget that nameplate kW is shaft output, not input.
What to do instead
Write the equation you are using and its terms before substituting: input kVA = output kW / (η × cos φ); line current I = P_out / (√3 × V × η × cos φ). For transformers use rated kVA directly. Check the answer against typical current densities and stated assumptions.
Describing a variable-speed drive as 'a device that saves energy' without addressing harmonics, cable derating, EMC, or the motor's insulation and bearing issues.
Candidates learn the energy-saving headline and stop there, so they miss the design consequences that exam questions probe: reflected wave/overvoltage on long motor cables, high-frequency earth leakage, and harmonic distortion injected into the supply.
What to do instead
Structure the answer as benefits plus design measures: specify the drive's harmonic limits and any filter/reactor, use screened motor cable with the screen bonded at both ends, respect the manufacturer's maximum cable length, derate cables for harmonics, and address bearing currents and insulation withstand for the chosen motor.
Quoting a code clause number or a specific numerical limit (clearance, burial depth, disconnection time, earthing resistance) from memory without stating the edition or the condition under which it applies.
Candidates rely on remembered figures from training courses or older editions, and many limits are conditional on voltage band, location, or installation type, so a bare number is often wrong in context.
What to do instead
Answer in the form 'according to [named standard] for [stated condition], the limit is X; the value differs when Y'. Where you are unsure of the exact figure, state the principle and the method of verification rather than inventing a number, and always name the standard and edition you are applying.
Answering a project-management or ethics scenario with a purely technical solution, or failing to identify the contractual instruction and the party responsible.
Engineers default to the technical fix and skip the procedural chain: who issued the instruction, whether it is a variation, what notice period applies, and what the engineer's duty to public safety and to report unprofessional conduct requires.
What to do instead
For each scenario, first identify the parties and the contract mechanism (instruction, variation, delay event, defect), then the notice and record-keeping steps, then the technical action. In ethics questions, name the specific duty engaged (competence, public safety, conflicts of interest, confidentiality) and the required disclosure or escalation, in that order.