Punjab PSEB Matric NSQF Power flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Ohm's law

For a conductor at constant temperature, V = I × R. Voltage in volts equals current in amperes multiplied by resistance in ohms, allowing any one unknown to be calculated from the other two.

Single-phase AC power

P = V × I × cos φ, measured in watts. Cos φ is the power factor, the ratio of real power to apparent power; a purely resistive load has cos φ equal to one.

Three-phase power formula

For a balanced three-phase load, P = √3 × line voltage × line current × cos φ. The √3 factor arises because line and phase quantities differ by that ratio in star or delta connections.

Star connection relation

In a star-connected load, line voltage equals √3 times phase voltage while line current equals phase current. The neutral point is the common junction of the three phase windings.

Delta connection relation

In a delta-connected load, line voltage equals phase voltage while line current equals √3 times phase current. Each phase winding is connected directly between two supply lines.

Energy in kWh

Electrical energy consumed equals power in kilowatts multiplied by time in hours. A 1,500 W heater running four hours uses 6 kWh, the unit charged on domestic bills.

Power factor correction

Capacitors connected in parallel with an inductive load supply reactive power locally, reducing the current drawn from the supply and raising cos φ toward unity. Capacitor rating is in kvar.

Transformer turns ratio

Primary voltage divided by secondary voltage equals primary turns divided by secondary turns. Current transforms inversely, so a step-down voltage transformer steps current up.

Conductor resistance formula

R = ρL/A: resistance rises with length and resistivity and falls as cross-sectional area increases. This explains why long thin cables drop more voltage than short thick ones.

Indian LV supply standards

Domestic single-phase supply in India is 230 V at 50 Hz; industrial three-phase supply is 415 V at 50 Hz. Equipment nameplates must match these values before connection.

Fuse rating selection rule

The protective device rating must be above the load's normal full-load current yet below the cable's current-carrying capacity, ensuring the cable is disconnected before overheating damages insulation.

Induction motor slip

Slip equals (synchronous speed minus rotor speed) divided by synchronous speed. Synchronous speed is 120 × frequency ÷ number of poles, giving 1500 rpm for a 4-pole 50 Hz motor.

Earthing purpose

Earthing connects exposed metalwork to the general mass of earth, giving fault current a low-resistance path so protection operates quickly and touch voltage stays at a safe level.

MCB versus fuse

An MCB trips thermally on overload and magnetically on short circuit, and can be reset. A fuse melts a element and must be replaced, but is cheaper and gives high breaking capacity.

Continuity test procedure

With supply isolated, set the multimeter to continuity or resistance. A sound conductor reads near zero ohms; a broken one reads open circuit. Confirm the meter on a known good link first.

Insulation resistance test

Apply 500 V DC from an insulation tester between conductors and between conductor and earth on a dead circuit. A healthy low-voltage installation reads at least one megohm.

Earth resistance measurement

Use the fall-of-potential method with the tester's spikes driven in a straight line at specified spacings. Record the ohm value and compare with the installation's specified limit.

Cable colour code India

Flexible cords use brown for phase, blue for neutral and green-yellow for earth. Three-phase wiring uses red, yellow and blue for phases with black as neutral.

Three-pin plug wiring

Earth goes to the top longest pin, phase to the right terminal and neutral to the left when viewed from the front. The earth lead is made longest inside the plug so it breaks last.

Reversing a 3-phase motor

Interchange any two of the three supply phase connections at the motor terminals. This reverses the rotating magnetic field direction and therefore the shaft rotation.

Reversing a DC motor

Reverse either the armature connections or the field connections, never both. Reversing both leaves the torque direction unchanged because the two effects cancel each other.

Ammeter connection rule

An ammeter is connected in series with the load, so the circuit must be broken to insert it. Its internal resistance is very low to avoid altering the current being measured.

Voltmeter connection rule

A voltmeter is connected in parallel across the component or supply being measured. Its internal resistance is very high so it draws negligible current from the circuit.

Lockout tagout steps

Switch off, isolate the supply, apply a lock and warning tag, prove the circuit dead with a tester, discharge stored energy, then earth conductors before work begins.

Proving dead requirement

After isolation, test the actual conductors with a voltage tester, not the switch position. Test before and after on a known live source to confirm the tester itself works.

Megger use precaution

Never use an insulation tester on a live circuit or on equipment sensitive to high voltage. Discharge the circuit after testing because the tester charges capacitance.

Cable size selection factors

Cross-section is chosen from load current, allowable voltage drop, ambient temperature, grouping and installation method. Voltage drop on a long run often decides the size, not current alone.

Voltage drop calculation

For a single-phase run, drop ≈ (2 × ρ × L × I) / A. The factor two accounts for the outgoing and return path; keep total drop within the permitted percentage.

Insulation tester selection

A 500 V DC tester suits low-voltage installations up to 1000 V. Higher-voltage equipment needs a tester of correspondingly higher test voltage, per the equipment rating.

Series and parallel lamps

Lamps in series share the supply voltage and one failure breaks the circuit. Lamps in parallel each receive full supply voltage and continue working independently of the others.

Soldering a joint

Clean and tin the iron tip, heat the joint, then apply solder to the work not the iron. A good joint is bright and smooth; a dull grainy surface indicates a cold joint.

Energy meter constant

The meter constant states revolutions per kWh. Counting revolutions over a timed interval lets you verify the connected load against the meter's registered consumption.