Odisha CHSE +2 Vocational EDA (Electrical Domestic Appliances) common mistakes
Odisha CHSE +2 Vocational EDA (Electrical Domestic Appliances) · Understand the traps and how to avoid them.
Writing the formula for series and parallel resistance from memory but then substituting the voltage where resistance belongs, or adding resistances that are in parallel because the circuit 'looks like a line'.
Candidates memorise R = R1 + R2 + ... and 1/R = 1/R1 + 1/R2 as text strings without practising on drawn circuits, so they never build the habit of first identifying which nodes share the same current path. Under exam pressure they pattern-match on layout rather than on current paths.
What to do instead
Before any substitution, redraw the circuit and mark each junction with a letter. Two resistors are in series only if the same current must pass through both with no branch between them; they are in parallel only if both their ends connect to the same two nodes. Practise ten mixed networks and, for each, write the series/parallel decision in one sentence before calculating. Check the answer by sanity: total resistance of a parallel group must be smaller than the smallest branch.
Confusing power, energy and charge in numericals: using P = VI when the question asks for units consumed, or reporting kWh when the answer needs watts, and omitting the time factor entirely.
The three quantities are taught together and all use V, I and t, so candidates grab the first formula that surfaces. Many also never convert watts to kilowatts or minutes to hours, so even a correct formula yields a wrong number.
What to do instead
Adopt a fixed three-line working: (1) list given values with units, (2) convert every unit to W, V, A, h before substituting, (3) state the quantity asked for and its unit before writing the formula. Energy in kWh = (W x h)/1000. Always finish with a unit check: if the answer is in kWh the working must contain a power term multiplied by a time term.
Selecting cable size or fuse rating by copying the rating printed on the appliance only, ignoring derating for ambient temperature, grouping of cables, and the fact that a fuse protects the cable, not the appliance.
Candidates learn 'fuse rating = appliance current' as a shortcut and are never drilled on the protective-device coordination idea. They also forget that a 2.5 mm² copper cable does not carry its free-air rating when run in a conduit with three other circuits.
What to do instead
Learn the sequence: calculate design current Ib, choose cable whose tabulated capacity It (after applying ambient and grouping factors) is at least Ib, then choose the protective device so that Ib <= In <= It. Practise three worked examples where grouping factor changes the answer. Write the inequality chain in every answer; examiners award marks for showing the coordination, not only the final number.
Answering earthing questions by saying 'earth wire is connected to the body' without distinguishing earth wire, neutral and body earthing, and without stating the purpose (limiting touch voltage and tripping the protective device).
Earthing is taught as a wiring diagram rather than as a fault-current path. Candidates can draw the green wire but cannot explain what happens to fault current, so they lose the explanation marks that carry most of the weight.
What to do instead
Prepare a 4-step written explanation you can reproduce: insulation fails, body becomes live, low-resistance earth path carries fault current, current exceeds device rating and disconnects supply within the safe time. Separately memorise the colour code and the difference between pipe earthing and plate earthing (materials, depth, watering arrangement). Rehearse the fault-path sketch until it takes under a minute.
Describing a domestic wiring system (cleat, casing-capping, conduit, batten) only by definition, without giving the specific application, limitation, or the accessory sequence used with it.
These topics look like pure recall, so candidates rote-learn one line per system and stop. Exam questions often ask 'where would you use it and why not elsewhere', which requires comparison, not definition.
What to do instead
Build a comparison table with columns: method, typical location, mechanical protection, moisture suitability, cost, ease of rewiring, typical failure. Fill it from your workshop notes. In the exam, answer the 'why' by naming the physical property (e.g. conduit gives mechanical protection and rewirability, so it suits concealed walls but costs more than cleat).
Explaining the working of a heating appliance by describing the element only, omitting the thermostat, thermal fuse and earth connection, and giving no servicing step for a common complaint such as 'heats but does not cut off'.
Candidates study the heating element because it is visible and simple, and skip the control chain. Fault questions then get a generic answer like 'check the thermostat' with no test method or safety step.
What to do instead
For each heating appliance (iron, heater, geyser, toaster, induction plate) draw a block chain: supply - switch - thermostat - thermal cut-out - element - earth. For each block write one failure symptom and one test (continuity test with supply isolated, resistance reading of element compared with P = V²/R). Practise the answer format: isolate, test, replace, verify, record.
Diagnosing motor-driven appliance faults by replacing parts in sequence without a test, and mixing up symptoms of capacitor failure, brush wear and bearing seizure.
In workshop practice a fan often starts after a part is changed, so candidates learn 'change the capacitor' as a ritual. In the exam they cannot separate the symptoms, so they give a single-cause answer for a multi-cause complaint.
What to do instead
Memorise a symptom table: no hum and no rotation (supply/open winding), hum but no rotation (capacitor or seized bearing), slow speed with weak torque (capacitor), sparking and intermittent running (brushes or commutator), noise and vibration (bearing or blade imbalance). For each, state the confirming test: capacitor capacitance check, winding resistance, brush length, shaft free rotation with power off.
Writing refrigeration answers that confuse the four basic components' roles, e.g. saying the compressor cools the refrigerant or that the condenser absorbs heat from the room, and omitting the expansion device entirely.
Candidates memorise the cycle as a circle of arrows and never attach the heat direction to each component. Because the evaporator and condenser both are coils, they swap them under time pressure.
What to do instead
Anchor each component to one verb and one location: compressor raises pressure and temperature; condenser rejects heat to outside air; expansion device drops pressure and temperature; evaporator absorbs heat from the cabinet. Draw the cycle with arrows for refrigerant flow and a second set of arrows for heat, and label indoor/outdoor explicitly. Then answer a 'why does the cabinet not cool' question by walking the cycle and naming the test at each stage.
Giving estimation answers as a bare lump sum with no itemised measurement, no wastage percentage, and no labour or contingency line, so method marks are lost even when the total is close.
Candidates treat estimation as arithmetic and rush to the total. They have often seen only finished estimates, never the build-up, so they do not know which lines the examiner looks for.
What to do instead
Use a fixed estimate skeleton every time: quantity take-off with units (points, metres, numbers), rate per unit, material subtotal, wastage (typically 5-10 per cent on cable and conduit), labour, contingency, then total. Show the take-off table before the arithmetic. Practise one full estimate for a two-bedroom light-and-socket installation and reuse the same skeleton in the exam.
In practical and viva answers, describing a test without stating the safety isolation step, the instrument setting, or the expected reading, e.g. 'test the insulation with a megger' and stopping there.
Candidates know the name of the test but not the procedure, because in the workshop the instructor sets up the instrument. They therefore cannot state range, voltage, duration or pass criterion, which is exactly what is marked.
What to do instead
For every test memorise a five-part sentence: isolate and prove dead, instrument and range, connections, applied condition (voltage and duration), pass criterion. Example for insulation resistance: isolate, prove dead, 500 V DC megger, between live conductors and earth, read after 60 s, pass if at least 1 MΩ. Build a card of five such sentences (continuity, polarity, insulation, earth resistance, RCD trip time) and recite them before the practical.