Fundamentals of Power and Electrical Safety
What this domain tests
The examiner checks whether you can compute electrical power and energy, tell AC quantities from DC ones, and apply the safety rules that protect a worker and a consumer. Expect short definitions, one small numerical, and a 'what would you do' situation.
Core rules
- DC power: P = V × I (watt). AC power: P = V I cos φ, where cos φ is the power factor.
- Energy: E (kWh) = P (kW) × t (hour). One kWh is one 'unit' on the domestic meter.
- Ohm's law: V = I R, valid for a resistor at constant temperature.
- India supplies 230 V, 50 Hz single phase and 415 V, 50 Hz three phase to consumers.
- Current, not voltage alone, decides shock severity. About 1 mA is felt, 10–15 mA causes 'let-go' failure, and 50 mA and above through the heart can be fatal.
- MCB and fuse clear overloads and short circuits. A 30 mA RCD (ELCB) protects people. Earthing gives fault current a low-resistance path and keeps a metal body near earth potential.
Worked example
A 2000 W geyser runs on 230 V. Find the current, a suitable MCB, and the energy used in 2 hours. I = P / V = 2000 / 230 = 8.7 A. A 10 A MCB sits too close to the running current, so choose the next standard rating, 16 A, with 1.5 mm² copper conductor, which safely carries roughly 10–14 A in conduit. Energy = 2 kW × 2 h = 4 kWh.
Traps that cost marks
Writing kWh as a unit of power; dropping cos φ in AC power sums; claiming an MCB prevents electric shock (only an RCD does that); putting a fuse in the neutral; touching a victim before the supply is switched off.