Odisha CHSE +2 Vocational EDA (Electrical Domestic Appliances) cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. In AC resistive loads these hold with RMS values.

Series circuit: same current everywhere, R_total = R1 + R2 + …; voltage divides in proportion to resistance.

Parallel circuit: same voltage across branches, 1/R_total = 1/R1 + 1/R2 + …; current divides inversely with resistance.

Single-phase AC: V_rms = V_peak/√2; for 230 V supply V_peak ≈ 325 V. Frequency in India is 50 Hz, period 20 ms.

Inductive reactance X_L = 2πfL, capacitive reactance X_C = 1/(2πfC); impedance Z = √(R² + (X_L − X_C)²).

Power factor = P/S = cos φ for sinusoidal supply; true power in watts, apparent power in VA, reactive power in VAR.

Energy in kWh = watts × hours ÷ 1000; a 1500 W heater run 2 h consumes 3 kWh.

Metering rules: ammeter in series (low resistance), voltmeter in parallel (high resistance), wattmeter current coil in series and pressure coil across supply.

Insulation resistance is measured with a megger at 500 V DC; a healthy 230 V domestic installation should read at least 1 MΩ.

Conductors by conductivity order: silver > copper > aluminium. Copper is standard for wiring; aluminium needs larger cross-section for the same rating.

Copper resistivity ≈ 1.72 × 10⁻⁸ Ω·m at 20 °C; aluminium ≈ 2.83 × 10⁻⁸ Ω·m, roughly 1.6 times that of copper.

Conductor resistance rises with temperature for metals: R2 = R1[1 + α(T2 − T1)], α_Cu ≈ 0.00393 per °C.

Annealed (soft) copper is flexible for wiring; hard-drawn copper is used for overhead lines.

Insulation classes: PVC for general wiring up to 70 °C conductor temperature; XLPE and butyl rubber for higher temperature duty.

Mica, porcelain, bakelite and glass are insulators used where heat resistance matters, as in heater mountings and iron sole plates.

Nichrome (Ni-Cr) is the standard heating element alloy: high resistivity, oxidation resistance, melting point about 1400 °C.

Solder for electrical joints is tin-lead or lead-free tin alloy; flux must be non-corrosive (rosin) to avoid joint failure.

Fuse element materials: tinned copper, silver-plated copper or zinc; the element must melt before the wiring is damaged.

Switch must always be in the phase (live) conductor, never in neutral; this isolates the appliance when switched off.

Socket outlets: 6 A for lighting and small loads, 16 A for appliances such as geysers, air conditioners and heavy kitchen equipment.

Three-pin plug convention: live (phase) to the right, neutral to the left, earth to the top/largest pin.

Earth wire colour code (India): green or green-with-yellow stripe; phase red/brown/yellow/blue depending on phase; neutral black.

A 3-pin plug's earth pin is longer and thicker so it opens the shutter and makes earth before live and neutral connect.

MCB trips on overload (thermal) and short circuit (magnetic); RCCB/ELCB trips on earth leakage current, typically 30 mA for personnel protection.

Fuse rating rule: fuse current slightly above normal load current but below the cable current-carrying capacity.

Earthing resistance for domestic installation should be low, typically below 5 Ω for a rod/pipe earth; measured with an earth tester.

Never use water on an electrical fire; use CO₂ or dry powder extinguishers, and isolate the supply first.

Systems: cleat wiring (temporary, cheap), casing-capping (surface, dry places), batten (wooden/plastic batten), conduit (surface or concealed), and CTS/PVC-sheathed cable wiring.

Conduit wiring is preferred for concealed work in modern houses because cables can be rewired without breaking the wall.

Loop-in system: supply loops from ceiling rose to ceiling rose, with a separate switch drop to each switch; joints are made at the rose, not in the wall.

Wire gauge selection: 1.0 mm² for lighting circuits, 1.5 mm² for 6 A socket circuits, 2.5 mm² and above for 16 A power circuits.

Lighting sub-circuit maximum load is commonly taken as 800 W with no more than 10 points per sub-circuit.

Bending radius of conduit should be at least 6 times the conduit diameter; too sharp a bend damages insulation during pulling.

Wooden battens and casing must be at least 6 mm thick and fixed with countersunk screws; cables should not be run in damp or hot locations without protection.

Total connected load decides the main switch and meter rating; diversity factor reduces the maximum demand estimate below the arithmetic sum.

Protection devices must be rated below the cable's current-carrying capacity and above the appliance's normal load current.

MCB types: Type B trips at 3–5 × In (lighting, resistive), Type C at 5–10 × In (motors, inductive loads), Type D at 10–20 × In.

RCCB 30 mA is for human protection; 100 mA or 300 mA is used for fire protection or as a main incomer.

Fuse and MCB breaking capacity must exceed the prospective short-circuit current at the point of installation.

Short-circuit current I_sc ≈ V/Z_loop; low loop impedance is essential for fast disconnection within the required time.

A 2-way switch circuit uses two two-way switches with a three-wire strapper; a godown (intermediate) switch adds a four-terminal switch for three control points.

A bell push circuit uses a push button in series with the bell and supply; a door chime may need a transformer.

Series test lamp and continuity tester are used for dead-circuit checks; never test for live with a megger.

Load balancing across phases in a three-phase distribution board keeps neutral current low and prevents one phase overloading.

Heating element resistance: R = V²/P. A 1000 W, 230 V element has R ≈ 53 Ω at working temperature.

Nichrome open coils are supported on ceramic/insulator formers; mica or ceramic plates insulate the element from the metal body.

Electric iron: sole plate with embedded element, bimetallic thermostat for temperature control, and a thermal fuse for overheat protection.

Bimetallic strip bends because two metals have different expansion coefficients; it opens the contact when the set temperature is reached.

Electric kettle: element in direct contact with water; a safety cut-out (thermal fuse or steam switch) prevents dry-boil damage.

Immersion heater must be fully submerged; scale build-up raises element temperature and shortens life, so descaling is a service task.

Geyser (storage water heater): thermostat typically set 60–65 °C; a sacrificial anode protects the tank from corrosion.

Oven/room heater elements are often enclosed tubular (sheathed) elements with magnesium oxide insulation between coil and sheath.

Tests for a heating appliance: element continuity (cold resistance), insulation resistance to body (megger), and thermostat continuity through its range.

Ceiling fan uses a single-phase capacitor-start/capacitor-run induction motor; a failed capacitor causes no rotation or slow rotation with hum.

Fan speed control by resistance regulator wastes power as heat; electronic regulators use triac phase control and are more efficient.

Fan capacitor values are typically 2.25–3.15 µF at 400–440 V AC; a bulged or leaky capacitor must be replaced with the same rating.

Mixer/grinder motor is a universal (series) motor with carbon brushes and a commutator; sparking at brushes indicates worn brushes or a rough commutator.

Universal motor speed is high at no load; a mixer should not be run empty because the bearing and coupler wear rapidly.

Washing machine types: semi-automatic (separate wash and spin tubs) and fully automatic (top-load and front-load with a single drum).

Washing machine motor may be a capacitor-start induction motor or a universal motor; the timer/control board sequences fill, wash, drain and spin.

Common washing machine faults: no drain (blocked filter or pump), no spin (lid switch, belt, capacitor), water overflow (pressure switch or inlet valve).

Grinder safety: check the stone for cracks, earth the body, and never exceed the marked wheel/stone speed.

Refrigeration cycle: compressor raises pressure and temperature of vapour, condenser rejects heat, expansion device drops pressure, evaporator absorbs heat.

Refrigerant changes state: liquid absorbs latent heat in the evaporator and vaporises; vapour rejects latent heat in the condenser and liquefies.

Domestic refrigerator uses a hermetic compressor, capillary tube as expansion device, and a forced or natural-draft condenser.

Refrigerants: R-134a for many domestic refrigerators, R-600a (isobutane) in newer units; R-600a is flammable, so no spark sources during service.

Superheat is the temperature of vapour above saturation; frost pattern on the evaporator shows whether the charge is correct.

Air conditioner cooling capacity is rated in tons of refrigeration; 1 ton = 3.517 kW of cooling, roughly 12000 BTU/h.

Split AC has indoor evaporator unit and outdoor condenser unit connected by refrigerant pipes; window AC has both in one casing.

Common faults: no cooling due to low gas charge, blocked capillary, dirty condenser, faulty thermostat, or a compressor that trips on overload.

A refrigerant leak is found with a soap-bubble test or electronic leak detector; recovery and charging must follow the manufacturer's charge weight.

Incandescent lamp: tungsten filament, efficiency about 10–15 lm/W; halogen is a tungsten lamp with halogen gas cycle giving higher life.

Fluorescent tube lamp needs a choke (ballast) to limit current and a starter to preheat the electrodes; electronic ballast replaces both.

CFL contains mercury vapour and a phosphor coating; disposal must avoid mercury contamination.

LED lamp: light-emitting diode driven by a constant-current driver; efficiency typically 80–100 lm/W, polarity matters, and heat sinking is essential.

Luminous efficacy units: lumens per watt (lm/W); higher efficacy means more light for the same power.

Ceiling fan: capacitor-start motor, three-speed winding taps; wobble indicates a bent blade or loose blade screws, not necessarily a motor fault.

Tubelight flicker at switch-on points to starter or choke; a tube that glows at the ends only usually needs a starter or a new tube.

Emergency light and inverter fittings use a battery, charger and inverter circuit; a dead battery or faulty charger is the usual cause of failure.

Small fittings: doorbell, indicator lamp, plug top, socket, and miniature circuit accessories are rated by voltage and current; match rating to the load.

Safe sequence: isolate supply, prove dead with a tester, lock off, then test and repair.

Continuity test with a multimeter or series lamp confirms a complete path; a reading of nearly 0 Ω indicates a good conductor.

Insulation resistance test: megger at 500 V DC between live/neutral joined and earth; below 1 MΩ indicates a fault requiring investigation.

Earth continuity test: measure resistance from the earth pin of each socket to the earth electrode; low value confirms effective earthing.

Polarity test confirms phase in the switch and correct socket wiring; reversed polarity is a common and dangerous installation fault.

Open circuit gives no current and full supply voltage across the break; short circuit gives very low resistance and trips protection; earth fault leaks current to the body or earth.

Motor diagnosis: check winding resistance (should be balanced), capacitor value, brush length, and bearing freedom before condemning the motor.

Heating appliance diagnosis: check element continuity, thermostat contacts, thermal fuse, and insulation to the metal body.

Before replacing a fuse or resetting an MCB, find the cause; repeated blowing means a fault remains in the circuit or appliance.

Estimation: material cost + labour cost + overheads + profit margin = quoted price; add contingency for unforeseen work.

Wire length estimate: measure run length, add 10–15% for slack, bends and termination allowance.

Diversity factor reduces the maximum demand: maximum demand = connected load × diversity factor, used to size the main switch and cable.

A small appliance repair enterprise needs tools, a workbench, spare parts stock, a price list, and a record of each job (job card).

Consumer protection: a written estimate, a warranty statement on repairs, and a receipt protect both customer and technician.

GST registration thresholds and Udyam (MSME) registration are the usual formal steps for a small electrical service business in India.

Pricing a service call should cover travel, diagnosis time and parts; quoting only the part cost leads to loss on repeat visits.

Record keeping: maintain a customer register, purchase invoices, and a warranty register for parts replaced.

Skill and safety certification, plus a first-aid kit and fire extinguisher in the workshop, are practical requirements for a service enterprise.

Workshop practice sequence: read the drawing, select tools, mark and cut, then assemble and test before energising.

Tool use: pliers for gripping and twisting, side cutter for wire, screwdriver matched to screw slot size, and a wire stripper to avoid nicked conductors.

Joint practice: straight joint, married joint, tee joint and western union joint must be mechanically sound, soldered and insulated with tape.

Bending and threading conduit requires a bending spring or bender; keep bends smooth and free of kinks to allow cable pulling.

Board wiring practice: fix components, run wires in right angles, use ferrules and terminal ends, and label circuits clearly.

Testing before power-on: continuity, insulation resistance, polarity and earth continuity, recorded on a test sheet.

Internal assessment marks usually combine attendance, job work, record book, viva and a practical test; the record book must show each job with diagram and result.

Safe workshop habits: one hand rule when testing live, rubber-soled shoes, insulated tools, and no loose clothing near rotating machines.

Report writing: state the fault found, the test readings, the action taken and the final verification result.