Electrical Fundamentals and Basic Circuit Theory
This domain tests the electrical basics a distribution lineman uses daily: current, voltage, resistance, power, energy, series and parallel circuits, AC single-phase and three-phase relationships, and power factor. Expect numerical questions as well as conceptual ones about why conductors, fuses and transformers are sized the way they are.
Core rules
- Ohm's law: V = IR. Power: P = VI for DC and resistive AC; for AC, P = VI cos φ.
- Balanced three-phase: P = √3 × V_L × I_L × cos φ; V_L = √3 × V_ph in star.
- Series: same current, voltages add, R_total = R1 + R2 + ... Parallel: same voltage, currents add, 1/R_total = 1/R1 + 1/R2 + ...
- Energy in kWh = power in kW × time in hours. A 2 kW load for 3 hours uses 6 kWh.
- Low power factor raises current for the same kW, increasing voltage drop and I²R loss.
Worked example
A 240 V single-phase load draws 3 kW. Find the current and the voltage drop when the phase and neutral conductors each have 0.15 Ω resistance.
I = P/V = 3000/240 = 12.5 A. The current goes out in the phase and back in the neutral, so use the loop resistance: R_loop = 0.15 + 0.15 = 0.30 Ω. Voltage drop = I × R_loop = 12.5 × 0.30 = 3.75 V, which is 3.75/240 × 100 = 1.56%. The load terminal voltage is about 236.25 V.
Traps that cost marks
- Using phase voltage instead of line voltage in the three-phase power formula.
- Forgetting the neutral return path in single-phase voltage-drop calculations.
- Treating kVA as kW; a 100 kVA transformer at 0.8 power factor delivers only 80 kW.
- Adding resistances directly in a parallel circuit.
- Assuming power factor is always unity in AC distribution calculations.