Basic Electrical Principles and Measurements
Basic Electrical Principles and Measurements
Basic Electrical Principles and Measurements tests the laws, units and instruments that underpin every domestic appliance job. Expect numerical questions on Ohm's law, power, energy and series/parallel circuits, plus instrument-connection and safety questions.
Core rules and concepts
- Ohm's law: V = IR. Power: P = VI = I²R = V²/R. Energy in kWh = watts × hours ÷ 1000.
- Series: one path, same current, voltages add, R_total = R1 + R2. Parallel: same voltage, currents add, 1/R_total = 1/R1 + 1/R2.
- Indian single-phase domestic supply is 230 V rms, 50 Hz; peak value is 230 × √2 ≈ 325 V. Three-phase line voltage is 415 V.
- An ammeter goes in series and has very low resistance; a voltmeter goes in parallel and has very high resistance. An energy meter integrates power over time. A megger measures insulation resistance and must never be used on a live circuit. A clamp meter reads current without breaking the circuit.
Worked example
A 2 kW, 230 V geyser runs for 2 hours. Current I = P/V = 2000/230 = 8.70 A. Element resistance R = V/I = 230/8.70 = 26.4 Ω (or R = V²/P = 26.45 Ω). Energy = 2 kW × 2 h = 4 kWh. At ₹5 per kWh the cost is ₹20. Reasoning: use rms supply voltage for a resistive load; power is the rate of energy use, so multiply by time and convert watts to kilowatts.
Traps that cost marks
- Using 325 V instead of 230 V in power calculations for a resistive appliance.
- Connecting an ammeter across a supply, which causes a short circuit.
- Confusing power (kW) with energy (kWh).
- Treating a megger as an ordinary ohmmeter on live equipment.