Electrical Fundamentals and Basic Science
Domain 1 - Electrical Fundamentals and Basic Science. The paper checks whether you can apply Ohm's law, AC and three-phase relationships, energy and magnetism to everyday distribution work such as load calculation, cable sizing and meter reading.
Formulas you must use cold
- Ohm's law V = IR; for resistive DC, P = VI = I²R = V²/R.
- Single-phase AC: P = VI cos φ, Q = VI sin φ, S = VI.
- Three-phase: P = √3 V_L I_L cos φ and S = √3 V_L I_L, where V_L is the line-to-line voltage.
- RMS = 0.707 × peak; form factor 1.11; one 50 Hz cycle lasts 20 ms.
- Resistance rises with temperature: R₂ = R₁[1 + α(T₂ − T₁)], with α ≈ 0.00393 per °C for copper at 20 °C.
- Energy in kWh = kW × hours. A consumer meter records kWh or kVAh, never kW.
Worked example
An 11 kV feeder carries 120 A at 0.85 power factor lagging. S = √3 × 11 000 × 120 = 2 286 kVA. P = 2 286 × 0.85 = 1 943 kW. Since sin φ = 0.527, Q = 2 286 × 0.527 = 1 205 kvar. Because 120 A is the line current, the line-to-line voltage is used directly - dividing 11 000 by √3 as well would count the √3 twice.
Traps
- Mixing line and phase quantities in the three-phase formula.
- Reporting kVA as kW, or dropping power factor entirely.
- Using R₂ = R₁[1 + α(T₂ − T₁)] with the temperatures reversed, which makes hot conductors look better than they are.
- Treating peak values as RMS when checking insulation or rating a cable.