Maharashtra HSC Electrical Maintenance (Bifocal) flashcards

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

Ohm's Law

For a linear conductor at constant temperature, V = IR. Power is P = VI = I²R = V²/R. In AC circuits use RMS values and multiply by cos φ for real power.

Series and Parallel Resistance

Series resistances add: R_total = R1 + R2 + … . For parallel, 1/R_total = 1/R1 + 1/R2 + … . Series current is common; parallel voltage is common.

Three-Phase Power Formula

Total three-phase power P = √3 × V_L × I_L × cos φ, where V_L and I_L are line values. In star, V_L = √3 V_ph; in delta, I_L = √3 I_ph.

Faraday and Lenz Law

Induced EMF e = −N dΦ/dt. Lenz's law states the induced current flows in a direction that opposes the change in flux producing it, explaining self-induction.

Two-Way Switch Wiring

Two 2-way switches with COM–L1–L2 terminals interconnected by two strapper wires control one lamp from two positions. An intermediate 4-terminal switch extends control to three or more positions.

Voltage Drop in Cables

Voltage drop = 2ρLI/A for single-phase two-wire circuits and √3ρLI/A for three-phase. IS 732 guidance keeps final sub-circuit drop within about 3% of supply voltage.

Conduit Fill Rule

The total cross-sectional area of all conductors in a conduit must not exceed 40% of the conduit internal area, allowing cables to be drawn without damage.

Cable Core Colour Code

Single-phase live is brown, neutral blue, protective earth green-yellow. Three-phase conductors are brown (L1), black (L2), grey (L3). Old wiring used red, black and green.

Armoured Cable Construction

Steel wire armour suits vertical runs; steel tape armour suits horizontal runs. Armour gives mechanical protection and an earth path but must be bonded and does not replace the earth conductor.

Transformer EMF Equation

E = 4.44 f N Φ_max, where f is frequency, N turns and Φ_max peak flux. The 4.44 factor arises from the sinusoidal waveform RMS value.

Induction Motor Slip

Slip s = (N_s − N)/N_s, where N_s = 120f/P. Rotor frequency equals s·f. At full load typical slip for a squirrel-cage motor is 2–5%.

Star-Delta Starting

The motor starts in star, reducing phase voltage to 1/√3, so starting current and torque fall to one-third. After a set time, the timer switches to delta for normal running.

DC Motor Back EMF

Back EMF E_b = V − I_aR_a. Armature current I_a = (V − E_b)/R_a. Speed N ∝ E_b/Φ, so field weakening raises speed above base speed.

RCCB Function

A residual current circuit breaker trips when the vector sum of live and neutral currents exceeds its sensitivity, typically 30 mA for personnel. It does not detect overload or line-to-neutral shock.

MCB Trip Types

Type B trips at 3–5 times rated current, Type C at 5–10 times, Type D at 10–20 times. The magnetic trip covers short circuits; the thermal element covers overload.

DOL Starter Control Circuit

The start push button energises the contactor coil; an auxiliary normally-open contact seals the circuit. The stop button and overload normally-closed contact are in series with the coil.

Insulation Resistance Testing

Measure with a 500 V DC megger for LV equipment. Minimum 1 MΩ for LV motors and 100 MΩ for new HV windings. Insulation resistance roughly halves for every 10 °C rise.

Polarisation Index

PI = R10 min / R1 min from insulation resistance readings. PI of 2 or more indicates healthy insulation; below 1.5 suggests moisture, dirt or degradation requiring investigation.

Single Phasing Effects

Loss of one supply phase leaves the motor running on two phases with severe current imbalance and overheating. A phase-failure relay or bimetal overload relay should trip the motor quickly.

Variable Torque Load

For centrifugal fans and pumps, torque is proportional to speed squared and power to speed cubed. Reducing speed by 20% cuts power to about 51%, giving large energy savings.

Power Factor Correction

Capacitor bank kVAR = P(tan φ1 − tan φ2) to raise power factor from cos φ1 to cos φ2. It reduces line current and losses; overcorrection causes leading power factor and voltage rise.

VFD Operating Principle

A rectifier converts AC to DC, a DC link capacitor smooths it, and an IGBT inverter produces PWM output of variable frequency and voltage. V/f is held constant below base speed.

PLC Scan Cycle

The PLC reads all inputs, executes the program logic, then updates all outputs. Scan time matters for fast processes; IEC 61131-3 defines ladder, function block and structured text languages.

4-20 mA Analogue Signal

4 mA represents the minimum value and 20 mA the maximum, with live zero allowing broken-wire detection. A reading below 4 mA indicates an open circuit or sensor fault.

Earth Resistance Measurement

The fall-of-potential method uses an earth tester with two auxiliary spikes. Substation earth resistance should be 1 Ω or less; domestic installations typically 5 Ω or less.

Permit-To-Work System

Isolate the supply, lock and tag the isolator, prove dead with a tester, discharge capacitors and earth the conductor. Only the person who applied the lock removes it.

First Aid for Electric Shock

Switch off the supply before touching the casualty. Use insulated material to separate them from the conductor, start CPR if breathing has stopped, and call medical help immediately.

Diversity Factor

Diversity factor = sum of individual maximum demands / maximum demand on the system. Demand factor = maximum demand / connected load. Both are used to size cables and protective devices.

Earth Conductor Sizing

For phase conductors up to 16 mm², the earth conductor equals the phase size. For 16–35 mm², use 16 mm². Above 35 mm², use half the phase conductor cross-section.

Megger Use

A megger applies DC test voltage to measure insulation resistance between conductors and earth. Always isolate and discharge the circuit before testing and reconnect earth afterwards.

Star-Delta Motor Connection

Six-terminal motors connect in star or delta according to the nameplate and supply voltage. Verify with a continuity test before energising; wrong connection causes overheating or failure to start.