Vocational Trade Theory (Power)
Vocational Trade Theory (Power)
The PSEB Matric NSQF Power trade theory paper checks whether you can explain the electrical principles behind power work and apply them to numbers and circuits. Expect questions on units, Ohm's law, AC/DC supply, protective devices, earthing, transformers, motors, and generation-to-distribution terms.
Core rules and concepts
- Current (A), voltage (V) and resistance (Ω); Ohm's law V = IR. Power P = VI = I²R = V²/R.
- India's nominal low-voltage supply is 230 V single-phase, 400 V three-phase, 50 Hz. On AC inductive loads, P = VI cos φ.
- AC changes direction periodically and is transformed by mutual induction; DC from cells does not transform.
- Earthing connects exposed metalwork to earth. It gives fault current a low-resistance path so the fuse or MCB operates.
- A fuse or MCB protects the circuit, not the person. An RCCB/ELCB with 30 mA sensitivity is used for shock protection.
- A transformer transfers energy by mutual induction and is rated in kVA; it does not change frequency. A motor converts electrical energy to mechanical energy.
- High-voltage transmission lowers current for the same power, reducing I²R line loss.
Worked example
A 230 V heater draws 5 A for 2 hours. P = VI = 230 × 5 = 1150 W. Resistance R = V/I = 230/5 = 46 Ω. Energy = 1.15 kW × 2 h = 2.3 kWh. A 6 A MCB is suitable because 5 A is below its rating; a 4 A fuse would be too small and would operate in normal use.
Traps that cost marks
Do not write kWh when the question asks for power in kW. Do not use P = VI on an inductive AC load without cos φ. The neutral is a current-carrying conductor, not a safe touch point. A fuse placed only in the neutral leaves the phase live. Never measure resistance on a live circuit.