Basic electricity, electrical measuring instruments and control devices
What this domain tests
Ohm's law, power and power factor, single- and three-phase supply, insulation and earth testing, and the selection and function of MCB, RCCB/ELCB, overload relay, thermostat, pressure switch and solenoid valve.
Core rules
- V = IR; P = VI = I²R; single phase P = VI cos phi; three phase P = √3 x V x I x cos phi.
- Indian supply: 230 V single phase, 415 V three phase, 50 Hz.
- Insulation resistance with a 500 V megger: new wiring should read several MΩ; below about 1 MΩ is suspect. Never megger a live circuit, a capacitor, or a compressor with the PCB still connected.
- Earth electrode resistance is measured with an earth tester; the earth conductor is never fused or switched.
- An MCB protects the cable against overload and short circuit; an RCCB of 30 mA protects people against earth leakage. They are not substitutes for each other.
- Current is measured in series, voltage in parallel. A clamp meter must encircle one conductor only, or the currents cancel and it reads zero.
Worked example
A 1.5 TR split unit draws 1.6 kW at 230 V with power factor 0.9. I = P / (V cos phi) = 1600 / (230 x 0.9) = 7.7 A. A 16 A MCB with 2.5 mm² copper is selected because the cable rating (about 21–24 A) exceeds the MCB rating, so the MCB protects the cable. Locked-rotor current of 5–6 times full load, about 40–46 A, is cleared by the MCB short-time element without nuisance tripping on start.
Traps
Leaving the probes in the current socket when measuring volts; meggering a circuit with electronics attached; thinking an ELCB trips on overload; ignoring power factor when sizing cable; clamping a two-core cable and concluding the load is dead.