Punjab PSEB Matric NSQF Power cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. In single-phase AC, P = V × I × cos φ, where cos φ is the power factor.

Three-phase power: P = √3 × V_L × I_L × cos φ, with V_L the line voltage and I_L the line current. For a balanced star load V_L = √3 × V_phase and I_L = I_phase; for a delta load V_L = V_phase and I_L = √3 × I_phase.

Energy is billed in kWh: kWh = (watts × hours) / 1000. A 2 kW load run 5 hours daily for 30 days consumes 2 × 5 × 30 = 300 kWh.

Standard Indian low-voltage supply is 230 V single-phase, 50 Hz, and 415 V three-phase, 50 Hz. The relationship 415 ≈ √3 × 240 reflects the star connection of the distribution transformer secondary.

Power factor correction: a capacitor bank of Q_c = P (tan φ1 − tan φ2) var raises cos φ from cos φ1 to cos φ2. Capacitors are rated in kvar and connected in parallel with the inductive load.

Transformer basics: turns ratio N1/N2 = V1/V2 = I2/I1. A step-down distribution transformer has more primary turns; kVA rating is independent of load power factor.

Conductor resistance: R = ρL/A, so doubling length doubles resistance and doubling cross-section halves it. Resistivity ρ for copper ≈ 1.72 × 10⁻⁸ Ω·m at 20 °C.

Protection coordination rule: the fuse or MCB rating must exceed normal full-load current but lie below the cable's current-carrying capacity, so the cable is protected before it overheats.

Continuity test: isolate supply, set the multimeter to resistance/continuity, and confirm near-zero ohms across a known-good path and open circuit (OL) across a break.

Insulation resistance test: use a 500 V DC insulation tester (megger) on a de-energised circuit; a healthy LV installation reads ≥ 1 MΩ, and readings below that require investigation before energising.

Earth resistance is measured with a fall-of-potential (three-spike) tester; IS 3043 guidance targets a low value, commonly ≤ 1 Ω for substations and a few ohms for small installations - record the actual reading.

Cable colour code for Indian flexible cords: live/phase brown, neutral blue, earth green-yellow. In old 3-phase wiring red, yellow and blue are phases with black neutral.

A 3-pin plug wiring rule: earth to the top/longest pin, phase to the right-hand terminal as viewed from the front, neutral to the left; the earth conductor must be the last to break if the cord is pulled.

Motor direction reversal: on a three-phase induction motor, interchange any two of the three supply phases at the terminals. On a DC series motor, interchange the armature or field connections, not both.

Multimeter current measurement requires breaking the circuit and connecting the meter in series; voltage is measured in parallel with the load. Never measure resistance on a live circuit.

Lockout/tagout sequence: switch off, isolate, lock and tag, prove dead with a tester on the actual conductors, discharge capacitors, and earth the conductors before touching them.