Basic Electricity and Electronics for Telecom
This section covers Basic Electricity and Electronics for Telecom. The exam tests your ability to apply electrical laws to exchange power plants, customer premises wiring, and simple electronic circuits, and to interpret component and battery ratings.
Core rules and concepts
- Ohm's law: V = IR. Power: P = VI = I²R = V²/R.
- Series: same current, voltages add, resistances add. Parallel: same voltage, currents add, 1/R = 1/R1 + 1/R2.
- AC in India is 230 V, 50 Hz; RMS values are used for power calculations unless peak is stated.
- Telecom central offices commonly use a nominal -48 V DC plant; the positive rail is earthed and the battery negative is the -48 V rail.
- A lead-acid cell is about 2 V; a 24-cell string gives 48 V. Ah is charge capacity, not current.
- Fuses, MCBs, surge arresters, and earthing protect equipment and people; a fuse must be rated above normal current but below cable capacity.
Worked example
A 48 V DC plant supplies a 240 W load. Current = P/V = 240/48 = 5 A. The round-trip cable resistance is 1.2 Ω, so voltage drop = IR = 5 × 1.2 = 6 V; the load receives 42 V. Cable loss = I²R = 25 × 1.2 = 30 W. The load may still operate, but the drop and heat show why cable gauge and distance matter. A cable with half the resistance would drop only 3 V and waste 15 W.
Traps that cost marks
Using V²/R with the supply voltage when the resistor is not directly across it; ignoring cable or battery internal resistance; treating Ah as amps; mixing peak and RMS; assuming earth is a normal return conductor; and choosing a fuse by load current alone without checking cable rating.