Pancyprian TS Elec. Eng. III ΘΚ common mistakes
Pancyprian TS Elec. Eng. III ΘΚ · Understand the traps and how to avoid them.
Calculating AC quantities by treating impedance magnitudes as simple resistances (adding |Z_R| + |Z_L| arithmetically) instead of doing complex-number arithmetic with j, so series RL/RC branch currents and phase angles come out wrong.
Candidates learn DC series/parallel rules first and carry the 'just add the values' habit into AC, where the reactance is 90° out of phase with resistance; the phasor relationship is understood conceptually but not practised numerically under time pressure.
What to do instead
Before every AC calculation, write each element as a complex impedance (R + j0, 0 + jX_L, 0 − jX_C), convert to polar only at the end, and do one worked series-RLC and one parallel-RLC problem by hand each study session until the arithmetic is automatic. Always state the phase angle alongside the magnitude.
Applying the wrong turns-ratio direction in transformer problems, e.g. using N1/N2 where V1/V2 is needed for a step-down transformer, or forgetting that current ratio is the inverse of voltage ratio.
The ratio is memorised as a single formula without a physical anchor, and candidates do not check whether the answer is plausible (a step-down transformer must give a lower secondary voltage).
What to do instead
Anchor the ratio in the ideal-transformer power balance: V1/V2 = N1/N2 = I2/I1. After computing, sanity-check direction and magnitude (secondary voltage lower for step-down, primary current lower than secondary). Practise both directions and include a short-circuit/loading variant.
Selecting cable cross-sectional area from current-carrying capacity alone, ignoring voltage drop, correction factors for grouping and ambient temperature, and the protective device's disconnection requirements.
Candidates treat cable sizing as a one-step table lookup and forget that the tabulated Iz must be corrected (Iz' = Iz × factors) before comparing with In, and that volt-drop limits often govern long runs.
What to do instead
Work a fixed checklist for every sizing question: design current Ib → device rating In → corrected tabulated capacity Iz' ≥ In → volt drop check → earth-fault/disconnection check. Show each step explicitly, including the correction factors applied, so partial credit is visible.
Confusing the roles of the freewheeling (flyback) diode and the controlled switch in a chopper or half-bridge, or assuming a thyristor turns off when its gate signal is removed.
The gate is intuitively treated like a transistor base, and the diode's conduction interval during switch-off is not visualised, so current-path and waveform questions are answered from a faulty mental model.
What to do instead
Draw the current path for each switching state (switch on, switch off) before answering, and memorise the rule that a thyristor commutates only when anode current falls below holding current. Redraw the inductor current waveform for continuous and discontinuous conduction modes.
Reading an analogue instrument or calculating a measurement result without accounting for the meter's internal resistance, burden, or the loading effect on the circuit under test.
Ideal-meter assumptions (ammeter zero resistance, voltmeter infinite resistance) are taught first and then applied to real instruments, so the effect of a low-resistance voltmeter or high-resistance ammeter on the reading is overlooked.
What to do instead
For every measurement question, state the meter's internal resistance and compute the loading error explicitly (e.g. voltmeter in parallel with R changes the equivalent resistance). Also practise calculating accuracy from full-scale deflection and class index, not from the reading.
Choosing or setting a protective device without verifying discrimination and the required disconnection time, e.g. picking a fuse or MCB by nominal rating only and ignoring the time-current characteristic and the earth-fault loop impedance.
Protection is studied as device types rather than as a coordination problem; candidates do not connect the time-current curve, the fault loop impedance and the permitted disconnection time into one check.
What to do instead
For each protection question, state the fault current, the required disconnection time from the relevant wiring rule, and read the device's operating time from its characteristic at that current. Confirm the upstream device does not operate first (discrimination). Present the reasoning as a short chain.
Mixing up the roles of the neutral, protective earth, and equipotential bonding, or treating TN-S, TN-C-S and TT systems as interchangeable when describing fault paths.
The three conductors look similar on a schematic and the earthing arrangements differ only in where the neutral is earthed and how the protective conductor is derived, which is easy to blur under exam pressure.
What to do instead
Sketch each system (TN-S, TN-C-S, TT, IT) showing the source earthing point, the PE/PEN conductor and the fault loop, and state for each whether an RCD is needed and what limits the fault current. Answer system-identification questions by naming the loop path explicitly.
Miscalculating per-unit or percentage quantities in transmission and distribution problems, e.g. forgetting to convert base values when changing base MVA or kV, or ignoring the square of the voltage ratio for impedance referral.
Per-unit arithmetic is a bookkeeping method with several conversions, and a single missed base change or a missed squared turns ratio propagates through the whole answer without looking obviously wrong.
What to do instead
Write the base values (S_base, V_base, Z_base = V_base²/S_base) at the top of every calculation and convert each element to the chosen base before combining. For transformer referral, always apply the square of the turns ratio to impedance, and check the final per-unit value is of a plausible order.
Producing or interpreting technical drawings with inconsistent symbols, missing cross-references, or a schematic that omits the protective conductor and terminal numbering, making the drawing unreadable as a wiring document.
Candidates focus on the circuit function and treat drawing conventions as decoration, so standards for symbols, reference designators and terminal identification are not applied consistently.
What to do instead
Use one symbol set consistently (standard graphical symbols for contacts, relays, transformers), label every device with a reference designator and every terminal with a number, and show the PE conductor and its connections. Practise redrawing a small motor-control circuit from a written description and check it against a symbol checklist.
Answering machine questions from nameplate data alone without converting to the correct equivalent circuit, e.g. computing induction-motor slip or torque without first finding synchronous speed from frequency and pole pairs, or mixing line and phase values in a delta-connected machine.
Nameplate and equivalent-circuit parameters are learned separately, and the star/delta relationship between line and phase quantities is the step most often skipped, especially when the connection is not stated explicitly.
What to do instead
Start every machine problem by writing the given quantities and the connection (star or delta), derive n_s = 120f/p and slip s = (n_s − n)/n_s, and convert line to phase values (V_phase = V_line/√3 in star, I_phase = I_line/√3 in delta) before using the equivalent circuit. State the conversion step in the answer.