Karnataka SSLC Elements of Electrical Engineering IV - JTS (KSEAB) common mistakes
Karnataka SSLC Elements of Electrical Engineering IV - JTS (KSEAB) · Understand the traps and how to avoid them.
When computing total resistance, candidates add parallel branches as if they were in series (R = R1 + R2) or apply the product-over-sum shortcut to three or more resistors without reducing them first.
The series formula is learned first and is more intuitive; the reciprocal form of the parallel rule is easily forgotten under time pressure, and product-over-sum is only valid for exactly two resistors.
What to do instead
Before calculating, redraw the circuit and mark each junction. For two branches, use product over sum; for three or more, always use 1/RT = 1/R1 + 1/R2 + 1/R3 and invert at the end. Check the result: the equivalent parallel resistance must be smaller than the smallest branch.
Candidates apply Ohm's law and Kirchhoff's laws to a circuit without first fixing a sign convention, producing answers with reversed current directions or negative voltage drops that they then discard.
Textbook problems rarely state a convention explicitly, so students adopt inconsistent assumptions between the loop equation and the branch current.
What to do instead
State the convention in writing at the top of the working: choose a loop direction (say clockwise) and mark every assumed branch current with an arrow. A voltage rise in the direction of travel is positive, a drop negative. Keep the algebra; a negative result simply means the real current flows opposite to the assumed arrow.
Students compute the magnetic flux density inside a solenoid or a magnetic circuit with the wrong length, or they add reluctance values in parallel when the flux path is series.
The analogy between reluctance and resistance is taught but the geometry of the magnetic path is not sketched, so the mean path length and the air-gap length get confused.
What to do instead
Always sketch the flux path and label its mean length l and cross-section A. Use B = μ0μrNI/l with l the mean path length, and for a series magnetic circuit add reluctances S = l/(μ0μrA) directly, including the air gap with μr = 1. Treat parallel paths like parallel resistors.
In AC single-phase problems, candidates add rms voltages or currents arithmetically across a series R-L circuit instead of adding them as phasors, and they use Z = R + XL without the square root.
DC habits carry over, and the impedance triangle is memorised as a formula rather than understood as a right triangle.
What to do instead
Draw the impedance triangle for every series circuit: Z = √(R² + XL²) for R-L, and the phase angle from tan φ = XL/R. For voltages, use V = √(VR² + VL²) or phasor addition. Sanity check: the supply voltage must exceed each individual component voltage in a series circuit, never equal their plain sum.
Candidates compute line and phase quantities in a three-phase system without checking whether the load is star or delta, and they mix up the √3 factors for voltage and current.
The two sets of relations look symmetrical and are often memorised as a jumble; the distinction that line current equals phase current in star but line voltage equals phase voltage in delta is easily inverted.
What to do instead
Write the four relations on the rough sheet before starting: star VL = √3 Vph, IL = Iph; delta VL = Vph, IL = √3 Iph. Then compute total power as P = √3 VL IL cos φ for a balanced load, which is valid for both connections and serves as a cross-check.
Students confuse transformer EMF per turn, turns ratio and the effect of loading, for example claiming that the secondary voltage falls because the primary current increases, or applying the turns ratio to power.
The ideal-transformer relations are memorised as a block without the underlying flux-linkage reasoning, so the direction of each ratio is guessed.
What to do instead
Anchor on two facts: EMF per turn is the same in both windings (E1/N1 = E2/N2), and an ideal transformer conserves apparent power (V1I1 = V2I2). A step-down transformer steps voltage down and current up; the ratio never applies to power. For a real transformer, attribute secondary droop to regulation and leakage impedance, not to the primary current directly.
When answering on DC machines, candidates state that back EMF opposes the supply in a motor but then use the applied voltage alone to find armature current, ignoring the back EMF.
The back-EMF term is a single symbol in the formula and is dropped when the problem gives only terminal voltage and resistance.
What to do instead
Always write the armature circuit equation first: V = Eb + IaRa for a motor, so Ia = (V − Eb)/Ra. Compute Eb from Eb = (PφNZ)/(60A) when speed and flux are given. Check that Eb is slightly less than V for a motor and slightly greater for a generator.
Candidates select fuse or MCB ratings by matching the load current exactly, or they size the earth-continuity conductor and the protective device independently of the cable rating.
Protection coordination is not intuitive; students treat the rating as a property of the appliance rather than of the circuit.
What to do instead
Apply the coordination rule in order: design current ≤ protective device rating ≤ cable current-carrying capacity, and check the device disconnects within the required time. Choose the next standard rating above the design current, and remember that the earth conductor must be sized so that fault current trips the device quickly.
Students read an analog wattmeter or energy meter scale without accounting for the multiplying factor, or they compute energy cost using power in watts directly against time in hours without converting to kilowatts.
Scale reading and unit conversion are treated as trivial and skipped in worked practice, so the factor is forgotten in the exam.
What to do instead
For every meter reading, record three things: the pointer reading, the full-scale value and the multiplying factor (full-scale value divided by full-scale divisions). For tariffs, convert to kWh first (W × h ÷ 1000), then multiply by the per-unit rate and add fixed charges. State the units in every step.
On safety and energy-conservation questions, candidates give generic answers such as 'use energy-efficient appliances' or 'follow safety rules' instead of naming the specific protective measure, earthing type or conservation technique asked for.
These topics are revised by reading rather than by practising written answers, so recall is vague and the candidate pads with generalities.
What to do instead
Build a one-page list of named items and learn them as terms: earthing (pipe, plate, rod), ELCB and its trip current, MCB types B/C/D, insulation resistance test values, and conservation measures such as power-factor correction, LED retrofits, and load scheduling. In the exam, name the device or method and state its function in one line.