Bac F4 Bénin common mistakes
Bac F4 Bénin · Understand the traps and how to avoid them.
Calculating the current, voltage drop or power of a three-phase or single-phase receiver without converting units first (mixing A, mA, kW, W, Ω, mΩ) and writing the raw calculator display as the final answer, without checking whether the order of magnitude is plausible for the installation.
Candidates learn the formulas (P = UIcosφ, P = √3UIcosφ, U = RI) by heart but never practise chaining them with the units used in real technical data sheets. Under exam pressure they jump straight to the numeric keys and skip the conversion step, and the calculator gives a result that looks 'correct' because it has digits.
What to do instead
Impose a three-line written routine before any calculation: (1) list the given quantities with their units, (2) convert everything to SI base units in a dedicated column, (3) only then substitute into the literal formula. Finish by an order-of-magnitude check written on the copy (e.g. 'a 3 kW motor at 400 V draws about 5 A, my result of 4.8 A is consistent'). Train on five exercises per session with deliberately mixed units.
Reproducing a motor or transformer characteristic (torque-speed curve, no-load/load test, efficiency) by copying the shape of a memorised curve, without linking the plotted points to the measured values in the statement, and without naming the axes or units.
The electrical machines course is often revised from summaries and annotated diagrams rather than from measurement tables. Candidates recognise the general shape of the curve but cannot explain why the starting torque is what it is or where the nominal operating point sits, so they draw a plausible-looking curve that contradicts the data given.
What to do instead
For each machine, practise the full chain: measurement table → plotted points → interpretation (starting torque, pull-out torque, slip at nominal load) → one sentence of physical justification. Always label axes with quantity and unit and mark the nominal point. Do at least one complete exercise per machine type (asynchronous motor, transformer, DC machine) where the curve must be justified from the numbers, not from memory.
Drawing or reading a control circuit (power circuit vs control circuit, contactor coil, auxiliary contacts, push-buttons, thermal relay) with the supply and the load in the same line, self-holding contact omitted, or the emergency stop wired in parallel instead of in series.
Candidates memorise the schematic as a picture and reproduce it from visual memory, without reasoning about the function of each component. They do not ask 'what does this wire have to do for the circuit to work?' and therefore cannot detect that their diagram would either never start or never stop.
What to do instead
Learn each diagram by function, not by shape: redraw it from a written functional description ('the start button must be shunted by the auxiliary contact so the coil stays supplied after release'), then verify the drawing against that description line by line. Practise the standard circuits (single start/stop, reversing with interlocking, star-delta starting) until you can draw them from the functional text alone, then simulate start, stop, overload and emergency stop verbally.
On a technical drawing or industrial plan, reading dimensions from the scale of the sheet instead of from the dimension lines, ignoring the scale indication, and confusing the views (plan, elevation, section) or the line types (hidden, centre line, cutting plane).
Candidates look at the drawing as an illustration of the object rather than as a coded document. They have not internalised the standard conventions (line types, projection layout, dimensioning rules) and, under time pressure, they measure with a ruler because it is faster than reading the dimension chain.
What to do instead
Systematically apply a reading grid to every plan: title block and scale first, then the views and their correspondence, then line types, then dimensions. Never measure on the sheet; if a dimension is missing, say so. Train on complete industrial drawings with five questions each (identify the view, give the scale, read three dimensions, identify the material, name the section plane) until the grid becomes automatic.
In applied mathematics, setting up the equation of the problem without defining the unknown, then solving correctly an equation that does not translate the stated situation (wrong percentage base, inverted proportionality, forgotten unit in a derivative or an integral result).
Candidates separate 'maths' from 'technology' and treat word problems as a search for the operation to perform on the numbers they see. They skip the translation step, which is precisely the one the examiner is testing, and they do not check the result against the physical situation.
What to do instead
Enforce a written protocol: 'let x be ...' with the unit, then the relation in literal form, then the numerical application, then a sentence of interpretation with the unit. After solving, substitute the answer back into the original statement and check plausibility. Work through past-paper style problems from the specialty (resistance of a cable, efficiency, energy cost, dilution, exponential decay) using this four-step layout every time.
Applying a physics law (Ohm's law, Joule effect, calorimetry, lens or wave relation) outside its validity conditions, or quoting a result without its unit and without the correct number of significant figures.
Laws are memorised as formulas stripped of their hypotheses (constant temperature, negligible internal resistance, steady state). Candidates also reproduce calculator displays with eight digits, which shows they have not thought about the precision of the given data.
What to do instead
For each law, write on a revision card its conditions of validity alongside the formula, and state them explicitly in the answer ('assuming the resistance is constant and the conductor is at ambient temperature'). Round the final result to the precision of the least precise given data and always write the unit. Practise exercises where the condition is deliberately violated, so you learn to spot it.
Describing a maintenance intervention (diagnosis, preventive visit, repair, lockout) as a list of actions without the safety sequence, without the documents used, and without the traceability element (work order, report, history record).
Candidates revise maintenance as vocabulary (corrective, preventive, predictive) and not as a procedure. They have never written a work order or a diagnosis report, so they describe what they would do physically and forget the lockout, the verification of absence of voltage, the spare parts reference and the feedback to the equipment history.
What to do instead
Learn one standard intervention framework and apply it to every case: request and symptom collection → safety (lockout, verification of absence of voltage) → diagnosis with measuring instruments named → repair with parts references → tests and return to service → written report and update of the history file. Practise by writing one-page reports for typical failures (contactor chatter, motor overheating, tripped differential breaker).
Writing the technical French answer as one undifferentiated block, mixing the analysis of the document, the explanation of the phenomenon and the conclusion, with imprecise vocabulary ('the thing', 'it doesn't work') and no connecting words.
Candidates underestimate the French component of a technical exam and write as they speak, without planning. They also lack the specific lexicon (contactor, armature, insulation, torque) and replace it with vague everyday words, which costs precision marks even when the technical idea is right.
What to do instead
Use a fixed plan for every written production: introduction stating the object and the problem, development in two or three titled parts with one idea per paragraph, conclusion answering the question. Build a personal lexicon sheet of 60 to 80 specialty terms with a definition and one example sentence each, and reuse them in written exercises. Practise rewriting a vague sentence into a precise one (e.g. 'it doesn't work' → 'the contactor coil is no longer supplied because the thermal relay has tripped').
Answering questions on labour law and civic life with general moral statements ('one must respect the rules', 'the employer must be fair') instead of citing the applicable rule, the concerned institution and the concrete consequence for the worker or the company.
These chapters are revised last and superficially, as a list of values to recite. Candidates do not connect the legal notion to a real situation (a contract clause, a dismissal, a workplace accident, a payslip) and therefore cannot produce a precise answer.
What to do instead
For each notion (contract types, trial period, working time, leave, dismissal, social security, workplace accident, staff representation), prepare a three-column table: rule and reference, concrete example, consequence if the rule is not applied. Train on short case studies ('an apprentice is injured on a machine without a guard: who declares what, to whom, within what time?') and answer in the form rule → application to the case → consequence.
Treating the entrepreneurship section as a general essay on 'motivation and the qualities of a leader' without a quantified or structured element (estimated costs, break-even, market, legal form, sources of financing), and without linking the project to the technical specialty.
Candidates believe this part rewards enthusiasm and general knowledge, so they write long paragraphs with no figures. They have never built even a simple business model and do not know the legal forms or the financing channels available to a young technician.
What to do instead
Prepare a one-page project canvas that you can adapt to any prompt: need and target customer, product or service linked to the specialty, estimated start-up costs and monthly charges, expected revenue and break-even calculation, legal form and registration steps, financing sources, main risks. Practise filling it with realistic figures for a small electrotechnical or electronics maintenance workshop, then write a 20-line summary using the canvas as a skeleton.