Ovlašteni inženjer elektrotehnike common mistakes
Ovlašteni inženjer elektrotehnike · Understand the traps and how to avoid them.
Solving a mixed AC circuit by adding the magnitudes of resistive and reactive voltage drops arithmetically (e.g. U = UR + UL + UC) instead of combining phasors, then reporting a supply voltage that is too high.
Candidates carry over DC-series intuition from basic physics; RMS values look like scalars, and the phase relationship between current through R, L and C is not visualised. The result is arithmetically tidy and therefore feels correct.
What to do instead
Before any number is written, draw the phasor diagram or write the complex impedance Z = R + j(XL − XC) and work in rectangular/polar form. Use U = I·|Z| and verify with the phasor triangle: UR is in phase with I, UL leads by 90°, UC lags by 90°. Check that the computed |Z| ≥ R always - if not, the arithmetic is wrong.
Computing a short-circuit current for protection coordination using only the transformer's rated current or an assumed impedance, without referring the impedances to a common base or including cable and source impedance.
The candidate remembers that Isc ≈ In/uk but forgets that uk is per-unit on the transformer base and that the network upstream, cable length and parallel paths change the result by large factors. Time pressure encourages the one-line shortcut.
What to do instead
Set up an explicit per-unit or ohm-valued impedance diagram: source (or utility fault level), transformer uk and copper losses, cable R and X per km times length, then Isc = c·Un/(√3·Ztotal). State the assumed voltage factor c and the fault point. Sanity-check the answer against the transformer's own maximum fault level before using it to set a breaker.
Dimensioning a final circuit by current-carrying capacity alone, choosing a breaker In ≥ Ib and a cable Iz ≥ In, but skipping the voltage-drop check at the far end and the disconnection-time/loop-impedance check.
Ampacity tables are the most practised calculation, so the design stops once the table lookup passes. Voltage drop and automatic disconnection are treated as formalities rather than as conditions that can govern the cable size, especially on long runs.
What to do instead
Apply all three conditions explicitly and show them: Ib ≤ In ≤ Iz; voltage drop Δu within the limit for the installation type (and starting current for motors); and earth-fault loop impedance low enough for the protective device to trip within the required disconnection time. If the loop-impedance check fails, increase the PE or phase conductor, shorten the run, or change the device - do not just note it.
Treating the transformer's rated apparent power as deliverable at any power factor, or ignoring that a load's kVA at low power factor exceeds its kW, when sizing transformers or generators.
kW and kVA are used interchangeably in everyday speech, and the candidate computes load as a simple sum of nameplate kW. The thermal limit of the machine is current-based, which is easily forgotten under exam pressure.
What to do instead
Build the load schedule in kW and kvar separately, apply demand and diversity factors, then take the vector sum to get kVA and the resulting power factor. Compare that kVA against the machine rating and state the correction (capacitor bank, larger unit) if it exceeds it. For motors, use shaft kW/efficiency and the starting kVA for the voltage-dip check.
Specifying a measurement instrument or method without stating the uncertainty, the class of the instrument, or the loading effect on the circuit being measured, and reporting a result to more digits than the measurement justifies.
Candidates focus on getting a number and treat metrology as bookkeeping. Burden and input impedance effects (a voltmeter drawing current, a current transformer burden) are invisible unless deliberately considered.
What to do instead
For every measurement, name the instrument class and range, compute the worst-case error from class × range plus reading error, and check loading: voltmeter input resistance versus circuit resistance, ammeter/CT burden versus rated burden. Report the result with the correct number of significant figures and an explicit ± uncertainty. For commissioning tests, compare against the acceptance criterion rather than just recording a value.
Citing a standard or regulation by number and general title as justification for a design decision without quoting the specific clause, edition, or national deviation that actually governs it.
Standard numbers are memorised as labels; the candidate knows 'it's covered by the wiring rules' but not which clause or which edition applies, and older editions circulate in study material.
What to do instead
For each compliance claim, cite the standard designation with year, the clause number, and the national deviation or amendment if one exists. Verify the edition currently in force for the jurisdiction. If the exact clause cannot be recalled with confidence, say so and mark it for verification rather than asserting a false citation.
Designing or assessing protection against electric shock by relying on a single measure (e.g. RCDs alone, or earthing alone) without checking that basic protection and fault protection are both present, or mixing up the disconnection times for TN and TT systems.
RCDs are familiar and feel like a universal answer; the distinction between basic protection (insulation, barriers), fault protection (earthing and automatic disconnection), and additional protection (30 mA RCD) blurs, as do the different fault-loop paths in TN and TT.
What to do instead
Structure the answer as a hierarchy: basic protection, fault protection, additional protection, and state which measure covers which. For fault protection, identify the system type first (TN-S, TN-C-S, TT), then apply the correct disconnection time and verify the loop impedance or RCD rating accordingly. Check that the earthing system is not mixed within one installation.
Treating a professional engagement as a purely technical task: accepting a design change or a deviation from the design without written instruction, or signing off work performed by others without documented verification.
Candidates prepare for calculations and underestimate the weight of contractual and liability questions. Verbal instructions and 'the contractor said it was fine' feel normal on site and are not recognised as liability exposures in an exam scenario.
What to do instead
In any scenario question, identify the contractual chain first: who instructed, in what form, and what the engineer's authority is. Require written instructions for changes, document verification with records and dates, and refuse to certify work not inspected. State the ethical duty to report non-compliance even when it creates conflict with the client.
Producing a design or as-built package that lacks the required technical documentation: no single-line diagram, no cable schedule, no protection settings, no risk assessment, no commissioning record, or drawings not revised and signed.
Documentation is seen as administrative output produced after the 'real' engineering. The candidate lists the calculations but not the deliverable set, and forgets that technical control examines the package, not the intent.
What to do instead
Memorise and reproduce the deliverable set for the project phase: design basis, single-line and layout drawings with revision and signature, calculations, cable and equipment schedules, protection coordination, risk assessment, and test/commissioning records. For each item state its purpose and who reviews it. Treat a missing revision or signature as a non-conformity, not a detail.
Sizing a grid-connected PV or other renewable system from module nameplate power alone, ignoring inverter DC/AC ratio limits, voltage rise at the point of connection, and the local grid-code conditions for connection.
The candidate multiplies module watts by count and compares to the inverter, treating the array as a simple generator. Voltage rise, inverter clipping, anti-islanding and protection requirements are separate topics that do not get integrated.
What to do instead
Work through the chain: array power at the actual irradiance and temperature, string voltage window against inverter MPPT range and minimum temperature, DC/AC ratio and expected clipping, then AC-side voltage rise at the point of common coupling and the resulting protection settings. Check the grid-code conditions for connection (protection functions, anti-islanding, metering) explicitly rather than assuming the inverter's certificate covers them.