Basic Electrical Principles and Safety
This domain tests the foundation of electrical work: current, voltage, resistance, power, series and parallel circuits, AC and DC, and the safety rules that prevent shock, fire and equipment damage. The exam expects correct units, correct formula selection and safe isolation practice.
Core rules
- Ohm's law: V = IR. Power: P = VI = I²R = V²/R. Use consistent units (V, A, Ω, W).
- In series, current is common and resistances add. In parallel, voltage is common and 1/R_total = 1/R1 + 1/R2.
- Domestic supply in India is nominally 230 V, 50 Hz. Live is brown or red, neutral is blue or black, and protective earth is green-yellow.
- Treat every conductor as live until it is isolated, tested and proved dead. Use lockout/tagout, insulated tools and PPE.
- Earthing gives fault current a low-resistance path and lets protective devices trip. An RCCB protects people against earth leakage; an MCB protects cables against overload and short circuit.
Worked example
A 2,000 W heater runs on 230 V. I = P/V = 2000/230 = 8.70 A. A 10 A MCB is adequate for the load, while a 6 A MCB would trip in normal use. The heater resistance is R = V²/P = 52,900/2000 = 26.45 Ω.
Traps
Do not assume neutral is safe to touch. Do not confuse an MCB with an RCCB. Do not add parallel resistances directly. Do not ignore the difference between measured voltage and rated voltage when checking a load.