Bac F2 Bénin common mistakes
Bac F2 Bénin · Understand the traps and how to avoid them.
Calculating a DC or AC circuit quantity without first converting all units to the coherent SI base (mA left as mA, kΩ left as kΩ, µF left as µF), then reporting a current off by a factor of 1000.
Candidates copy values straight from the statement as written and trust the calculator; the reflex of normalizing to A, V, Ω, F, W is not yet automatic, and results are rarely sanity-checked against the order of magnitude expected from the supply voltage and impedance.
What to do instead
Impose a written conversion line before every substitution (e.g. 'R = 4,7 kΩ = 4,7×10³ Ω'), carry powers of ten explicitly, and finish each numerical question with a one-line plausibility check: compare the computed current with U/Rmax and U/Rmin, or state the expected order of magnitude before computing.
Applying Ohm's law and Kirchhoff's laws to an AC circuit as if it were DC, adding resistances and reactances arithmetically (Z = R + X) instead of vectorially, and ignoring phase angles.
The DC habits from the first weeks of the course dominate, and the impedance triangle is memorized as a formula rather than understood as a geometric sum; candidates also forget that voltages across R and L are not in phase and cannot be added as scalars.
What to do instead
Systematically write the complex impedances (Z_R = R, Z_L = jLω, Z_C = 1/jCω), sum them as complex numbers, and only then take the modulus and argument. Draw the Fresnel/phasor diagram for each circuit before computing, and verify that U² = U_R² + (U_L − U_C)² numerically as a cross-check.
Mixing RMS, peak and mean values in AC calculations - using U_max in power formulas, or using the average value of a sinusoid where the RMS value is required, and forgetting the √2 factor in either direction.
The three values are introduced together and the formulas P = UI cos φ, U_eff = U_max/√2 and U_moy = 2U_max/π are recalled loosely; candidates do not check whether the question gives an amplitude or an effective value, and the wording 'tension 230 V' is assumed to mean the same thing in every context.
What to do instead
At the top of each AC exercise, write a small data table with columns 'given value / nature (max, eff, mean) / converted value'. State explicitly which value the power or meter formula requires, and use moving-coil vs moving-iron instrument behaviour to justify the choice when a measurement is involved.
Treating the transformer as ideal in an exercise that supplies winding resistance, leakage reactance or a short-circuit test, and omitting the turns ratio when referring secondary quantities to the primary.
The ideal-transformer relations (U2/U1 = N2/N1, I1/I2 = N2/N1) are over-learned and applied reflexively; candidates do not read the data carefully enough to notice that a short-circuit test implies series impedances, and they forget that referred impedances scale with the square of the ratio.
What to do instead
Before any transformer calculation, classify the model required: ideal, perfect (with magnetizing branch) or real (with series impedance). If a short-circuit or no-load test is given, draw the equivalent circuit referred to one side, write the referral rules (R2' = R2·m², m = N1/N2) and compute the voltage regulation from the phasor sum rather than by simple subtraction.
Confusing the asynchronous machine's slip, synchronous speed and rotor frequency, and using the wrong torque expression or the wrong equivalent circuit for starting versus rated operation.
Slip is defined by a formula that is easy to invert incorrectly (g = (ns − n)/ns), and candidates substitute rotor speed for synchronous speed in the rotor-frequency relation f_r = g·f_s; the rotor resistance referred to the stator, R2/g, is often replaced by R2 alone.
What to do instead
Always compute ns = 60f/p first and write it on the answer sheet, then compute g from the two speeds and check that 0 < g < 1 in motor operation. Build the rotor branch explicitly with R2/g, and for each question state the operating point (starting: g = 1; rated: g given; synchronous: g = 0) before applying the torque formula.
Choosing and dimensioning a static converter without checking its quadrant of operation and the nature of the load - proposing a single-phase bridge rectifier for a load needing four-quadrant operation, or sizing a chopper's freewheeling diode and capacitor from average current only.
Converters are learned as separate chapters, so the link between the load type (R, RL, back-EMF, motor) and the required topology is weak; candidates also apply average-value formulas to quantities that must be sized on RMS or peak values, such as diode current and smoothing capacitor ripple.
What to do instead
For every converter question, write a three-line specification first: source (AC/DC, value, frequency), load (R, RL, E, motor), and required energy flow (one or two directions, reversible voltage or current). Then select the topology from that specification and size components using the RMS/peak values derived from the conduction intervals, showing the waveform sketch that justifies each interval.
Designing or reading a protection scheme without verifying selectivity and the earthing arrangement - putting a 30 mA differential device on a circuit that also needs overload protection, or assuming a TT installation behaves like a TN one for fault-current calculation.
Protection is studied as a list of device types rather than as a coordination problem; candidates match device names to circuit names instead of checking rated current, breaking capacity, curve and disconnection time against the fault loop impedance and the prescribed maximum touch voltage.
What to do instead
For each circuit, produce a short coordination table: Ib ≤ In ≤ Iz, Icc at the point of installation ≤ breaking capacity, and disconnection time verified against the fault-loop impedance for the declared earthing system (Ia ≤ U0/Zs in TT). Justify the differential device by the required residual current and note that it complements, not replaces, the overcurrent device.
Writing a PLC program or logic equation directly in ladder form without first deriving and simplifying the truth table or Boolean expression, so that the resulting program has redundant rungs or an unintended state for a forbidden combination.
Candidates translate the wording of the specification line by line into contacts, which works for simple cases but fails on interlocking, priority and safety conditions; Karnaugh simplification and the systematic treatment of 'don't care' states are skipped under time pressure.
What to do instead
Impose a fixed method: list inputs and outputs, build the truth table including forbidden states, simplify with Karnaugh or Boolean algebra, and only then draw the ladder, assigning one rung per output with the safety condition as a series contact. Finally simulate mentally each forbidden state to prove the output stays at zero.
Producing a calculation with no symbolic formula, no units in intermediate results, and no labelled final answer, so that a correct numerical result cannot earn full credit and an arithmetic slip cannot be traced.
Candidates work directly on the calculator and write only the final number, treating the script as scratch paper; they were never drilled in the presentation convention of the subject, and under exam time pressure presentation is the first thing abandoned.
What to do instead
Adopt a fixed three-line layout for every calculation: symbolic relation, substitution with units, boxed result with unit and a short sentence of interpretation. Reserve a margin column for unit conversions and checks, and practise this layout on past exercises until it costs no extra time.
Reading a schematic and listing the components without tracing the power and control circuits separately, so the description of operation omits the holding contact, the emergency stop chain or the return path of a coil.
Candidates describe symbols one by one in reading order rather than following the current path; the distinction between the power circuit (motor, contactor poles, protection) and the control circuit (coil, auxiliary contacts, pushbuttons) is not firmly established, and normally-closed safety contacts are misread as breaks in the circuit.
What to do instead
Annotate the drawing before writing: colour or label the power circuit and the control circuit, number the wires, and trace energization step by step from the source through each contact to the coil and back. For each operating phase (initial, start pressed, running, stop, emergency), write the state of every contact and verify the holding contact maintains the coil supply.