PEC EPE Electrical common mistakes
PEC EPE Electrical · Understand the traps and how to avoid them.
Solving three-phase power problems by plugging line voltage straight into S = V²/Z or S = VI without converting to phase quantities, then reporting the wrong apparent power.
The candidate memorised the single-phase formulas and applies them to delta or star circuits without tracking whether V and I in the formula are line or phase values. Delta/star conversion is the step most often skipped under time pressure.
What to do instead
Before substituting numbers, write on the paper which quantities are given (line or phase) and which the formula requires, then do the conversion explicitly: V_phase = V_line/√3 in star, I_phase = I_line/√3 in delta. Practise 15 mixed star/delta load problems until the conversion is automatic, and always state the conversion in the working so partial credit survives.
Treating a transformer or induction machine problem as ideal, ignoring copper and core losses, magnetising current, or slip, and then choosing the answer that matches the ideal result.
Textbook derivations often start from the ideal model, and candidates stop reading once they recognise the standard formula. Real machine questions usually hinge on the non-ideal term, so the ideal answer is deliberately among the distractors.
What to do instead
Build a one-page model sheet per machine: for a transformer, the equivalent circuit with R_c, X_m, R_eq, X_eq referred to one side; for an induction motor, the slip-dependent rotor resistance R2/s. For every problem, first identify which non-ideal elements the data supports (loss figures, efficiency, slip) and include them before computing.
Applying the wrong per-unit base or forgetting to convert impedance to the new MVA and kV base when combining a generator, transformer, and line in a single-line diagram.
Per-unit work requires two conversions (old base to new base, and each side of a transformer to the chosen base), and candidates do one and skip the other. The arithmetic looks right, so no alarm is raised.
What to do instead
Adopt a fixed procedure: choose S_base once, choose V_base per voltage level using transformer turns ratios, convert every given reactance with X_new = X_old (S_new/S_old)(V_old/V_new)², then redraw the diagram with all values on the common base before any fault or load calculation. Check that the per-unit voltage at every bus is near 1.0 pu as a sanity test.
Computing fault current without correctly handling the sequence networks, or assuming a bolted three-phase fault when the question describes a single line-to-ground or line-to-line fault.
Three-phase fault is taught first and dominates practice, so candidates default to it. Sequence-network interconnection rules (series for LLG, parallel paths for LL) are easy to misremember.
What to do instead
Memorise the interconnection table: LG = positive + negative + zero in series with 3Z_f; LL = positive and negative in parallel; LLG = all three in parallel. Draw the three sequence networks for each fault type before substituting, and state the fault type explicitly at the top of the solution. Note that the zero-sequence network is open at delta windings and depends on grounding.
Sizing protective devices by picking the next standard rating above the calculated load current, without checking cable ampacity, voltage drop, or the coordination requirement the question actually asks for.
Candidates learn one rule of thumb (round up to the next breaker size) and apply it universally, missing that design questions usually constrain two or three quantities simultaneously.
What to do instead
For every protection or cable-sizing question, list all constraints first: design current, correction factors for temperature and grouping, voltage drop limit, and discrimination with upstream devices. Then select the device that satisfies all of them, not just the first. Practise reading derating tables and applying correction factors in the correct order.
Mixing up real, reactive, and apparent power in power-factor correction problems, or computing the capacitor kVAR from the load kW instead of the load kVAR.
The triangle of P, Q, S is simple in isolation but the correction formula Q_c = P(tan φ1 − tan φ2) uses P, and candidates often substitute S or confuse the two angles.
What to do instead
Always sketch the power triangle before and after correction, label P as unchanged, and derive Q_c from the tangent difference. Verify by checking that the new apparent power equals P/cos φ2 and is smaller than the original. Do five problems with leading and lagging loads to see how the sign of Q changes.
Answering electronics questions by recalling a formula without checking the operating region of the device (cut-off, active/saturation, triode, or breakdown).
Bias circuits look like linear algebra, so candidates solve the linear equations and accept a physically impossible result such as a negative collector-emitter voltage or a drain current beyond the supply rail.
What to do instead
After solving, always test the assumed region: for a BJT check V_BE ≈ 0.7 V and V_CE > 0.2 V for active; for a MOSFET check V_GS > V_th and V_DS > V_GS − V_th for saturation. If the test fails, re-solve with the correct region model (V_CE = 0.2 V saturated, or I_D = 0 in cut-off). Make the region check a mandatory written step.
Ignoring the time constant and transient term in RC/RL and first-order circuit problems, giving only the steady-state answer.
Candidates are comfortable with DC steady state and treat the circuit as resistive, forgetting that the question asks for the value at t = 0+, at a specific time, or after a switch operates.
What to do instead
Use the three-point method for every switched circuit: value at t = 0− (capacitor voltage and inductor current are continuous), value at t = ∞ (capacitor open, inductor short), and time constant τ = RC or L/R with the resistance seen from the reactive element's terminals. Write v(t) = v(∞) + [v(0+) − v(∞)]e^(−t/τ) explicitly before substituting.
Skipping units and prefixes when converting between milli, micro, kilo, and mega in calculations, producing answers off by factors of 1000.
Exam pressure and calculator entry make prefix errors the single most common source of lost marks that candidates do not notice, because the resulting number often resembles a distractor.
What to do instead
Convert every given value to base SI units in a written line before calculating, then convert the result back at the end. Use a consistent notation (for example 4.7 mH written as 4.7 × 10⁻³ H) and re-read the answer choices to see whether one matches a missed prefix, which is a warning sign that the conversion was wrong.
Answering measurement and instrumentation questions by assuming ideal meters, ignoring voltmeter loading, ammeter burden voltage, or the effect of the meter on the circuit.
Introductory problems use ideal meters, so candidates never develop the habit of checking meter resistance against circuit resistance, and the error is invisible in the arithmetic.
What to do instead
Before computing, compare the meter's internal resistance with the impedance it is connected across or in series with. If the voltmeter resistance is not at least 100 times the circuit resistance, include the parallel loading in the calculation; similarly include ammeter series resistance. Practise one problem each with a low-impedance voltmeter and a high-burden ammeter to see the size of the error.