Maharashtra HSC Electrical Maintenance (Bifocal) cheat sheet

Ohm's law: V = IR; power P = VI = I²R = V²/R. In AC circuits use RMS values, so P = V_rms·I_rms·cos φ for single phase.

Series: R_total = R1 + R2 + …; current common, voltage divides. Parallel: 1/R_total = 1/R1 + 1/R2 + …; voltage common, current divides.

Kirchhoff's laws: KCL - algebraic sum of currents at a node is zero; KVL - algebraic sum of EMFs and drops around a closed loop is zero.

AC quantities: f = 1/T; ω = 2πf; X_L = 2πfL (opposes current, leads voltage by 90°); X_C = 1/(2πfC) (lags by 90°).

Impedance Z = √(R² + (X_L − X_C)²); power factor cos φ = R/Z; φ = tan⁻¹((X_L − X_C)/R). Resonance at f = 1/(2π√(LC)).

Three-phase: line voltage V_L = √3 × phase voltage in star; line current I_L = √3 × phase current in delta; total power P = √3 V_L I_L cos φ.

Magnetic relations: F = BIl sin θ (force on conductor); e = Blv (generated EMF); B = μ0μr H; μ0 = 4π × 10⁻⁷ H/m.

Faraday/Lenz: induced EMF e = −N dΦ/dt; direction of induced current opposes the change producing it (Lenz's law).

Energy: 1 kWh = 3.6 × 10⁶ J; heating effect H = I²Rt joules; 1 HP = 746 W.

Domestic supply in India: 230 V single phase, 50 Hz; three-phase 415 V. Distribution uses TN-S/TN-C-S or TT arrangements; live brown, neutral blue, earth green-yellow (old: red/black/green).

Lighting sub-circuit wire: 1.0–1.5 mm² copper; power sub-circuit: 2.5 mm²; main incoming cable sized by load current and voltage drop.

Voltage drop must not exceed about 3% of supply for a final sub-circuit (IS 732 guidance); drop = (2 × ρ × L × I)/A for single phase, √3 factor for three phase.

Loop/radial rules: each lighting circuit limited to 800 W / 10 points; power circuit 3 kW / 2 points typical for domestic wiring design.

Conduit fill: total conductor cross-section ≤ 40% of conduit internal area; bends between draw boxes limited so cables can be drawn without damage.

Types: cleat, casing-capping (wood/PVC), batten, CTS, concealed conduit, surface conduit, and busbar trunking for heavy industry.

Two-way switching: two 2-way switches (COM–L1–L2) plus strappers; intermediate switch (4-terminal) inserted in strapper for 3-point control.

Wiring must follow IS 732; height of switchboards ~1.2–1.5 m, socket outlets 1.2 m; all metalwork bonded to earth.

Cable designation: conductor material, insulation (PVC, XLPE, rubber), armour (steel wire for vertical, steel tape for horizontal), and sheath.

Copper conductivity ≈ 58 MS/m, resistivity ≈ 0.0172 Ω·mm²/m at 20 °C; aluminium ≈ 35 MS/m and 0.028 Ω·mm²/m - aluminium needs ~1.6× the copper area.

Current rating depends on cross-section, insulation temperature rating (70 °C PVC, 90 °C XLPE), ambient temperature, grouping and installation method.

Cable core colours: single phase live brown, neutral blue, protective earth green-yellow; three-phase L1 brown, L2 black, L3 grey.

Armouring provides mechanical protection and an earth continuity path but must be bonded; it is not a substitute for the earth conductor.

Accessories: MCB, MCCB, ELCB/RCCB, isolator, plug-top fuse, ceiling rose, batten holder, industrial socket (IEC 60309), cable gland and lug.

Bending radius: typically 8–12 × overall cable diameter for armoured cables; never below manufacturer data to avoid insulation damage.

Cable jointing: use heat-shrink or taped joint with staggered cores; insulation resistance after jointing tested with a 500 V or 1000 V megger.

Transformer EMF equation: E = 4.44 f N Φ_max; turns ratio V1/V2 = N1/N2 = I2/I1 (ideal).

Transformer losses: copper loss ∝ I² (varies with load), iron loss constant (hysteresis + eddy current); maximum efficiency when copper loss = iron loss.

Induction motor slip s = (N_s − N)/N_s; N_s = 120f/P. Rotor frequency = s·f. Torque ∝ s·V² near normal operation.

Starting: DOL for small motors, star-delta for motors designed for delta run (reduces starting current and torque to 1/3), autotransformer and soft starter for large loads.

DC motor types: shunt (nearly constant speed), series (high starting torque, never start unloaded), compound. Back EMF E_b = V − I_aR_a; speed N ∝ E_b/Φ.

DC generator EMF: E = (PΦNZ)/(60A); for lap A = P, for wave A = 2.

Single-phase induction motors are not self-starting; split-phase, capacitor-start and capacitor-run provide the phase displacement.

Cooling and insulation classes: A 105 °C, E 120 °C, B 130 °C, F 155 °C, H 180 °C; motors are rated by temperature rise above ambient.

Fuse: melts on overcurrent, I²t characteristic, no adjustable setting, must be replaced; HRC fuses are current-limiting and chosen for breaking capacity.

MCB: thermal (overload) + magnetic (short circuit) trip; Type B 3–5×In, C 5–10×In, D 10–20×In. MCCB is adjustable and used at higher ratings.

RCCB detects residual current to earth (30 mA for personnel, 100/300 mA for fire/equipment); it does not protect against overload or line-to-neutral shock.

RCBO = MCB + RCCB in one device; ELCB (voltage-operated) requires a good earth and is now largely obsolete.

Contactor and overload relay: contactor handles frequent switching; thermal overload relay protects against sustained overload, not short circuit - a fuse/MCB must back it up.

DOL starter control circuit: start button energises the coil, auxiliary NO contact seals it in, stop button breaks the circuit; overload NC contact in series.

Star-delta timer sequence: main + star contactors close, then after set time star opens, delta closes; interlock prevents star and delta closing together.

Earth-fault protection: earth leakage relay, restricted earth fault for transformer/generator windings, and directional overcurrent for ring networks.

Planned preventive maintenance: schedule cleaning, tightening of terminals, insulation resistance, earth resistance, contact resistance (micro-ohm meter) and thermography at defined intervals.

Insulation resistance test: 500 V DC megger for LV motors and cables; 1 MΩ minimum for LV motors, 100 MΩ for new HV, and correct for temperature (halve per 10 °C rise).

Polarisation index (PI) = R10 min / R1 min; PI ≥ 2 indicates healthy insulation, PI < 1.5 indicates moisture or degradation.

Motor troubleshooting: no start → supply, fuse, overload, winding open circuit; overheating → overload, ventilation, voltage imbalance, bearing friction, single phasing.

Single phasing causes severe imbalance, high current in remaining phases and overheating; protect with phase-failure relay or bimetal overload.

Bearing failure: vibration, noise, temperature rise; check alignment, lubrication and belt tension; use vibration analyser and IR thermometer as routine tools.

Terminal tightening torque and cleanliness of contacts prevent hot spots; loose joints show discolouration, higher resistance and heat under load.

Records: log all tests, spares replaced, and failure analysis (5-why / root cause) to prevent recurrence; use a machine history card per motor.

Motor selection factors: load torque-speed characteristic, duty cycle (S1–S9), starting current limits, environment (IP rating), and efficiency class (IE2/IE3/IE4).

Load types: constant torque (conveyors, positive displacement pumps), variable torque (centrifugal fans/pumps, torque ∝ N², power ∝ N³), constant power (machine tools).

Star-delta and soft starters limit inrush; VFD gives variable speed and energy saving on variable-torque loads.

Crane and hoist duty: high starting torque, frequent starts, use slip-ring or inverter-controlled induction motors with mechanical brake.

Lighting loads: LED drivers have high inrush, so circuits need appropriate MCB type; use contactors rated for capacitive/inductive inrush.

Power factor correction: capacitor bank kVAR = P (tan φ1 − tan φ2); improves PF, reduces current and losses; avoid overcorrection leading to leading PF.

Welding transformers and furnaces are intermittent high-current loads; separate feeders and adequate short-circuit rating are required.

Motor protection coordination: fuse/MCCB for short circuit, overload relay for thermal, earth-fault relay for leakage; discrimination must be checked by manufacturer tables.

VFD principle: rectifier → DC link capacitor → IGBT inverter with PWM; output frequency and voltage vary to control speed, V/f constant for constant torque below base speed.

VFD parameters: base frequency, maximum frequency, acceleration/deceleration ramp, current limit, carrier frequency, and motor data for auto-tuning.

VFD cable must be shielded and earthed at both ends; long motor cables require output reactors or dv/dt filters to protect motor insulation.

PLC scan cycle: input scan → program execution → output update; ladder, function block, structured text are IEC 61131-3 languages.

Digital input 24 V DC sinking/sourcing; analogue signals 4–20 mA (live zero) or 0–10 V; 4–20 mA allows broken-wire detection.

PID control: proportional, integral, derivative; used for pressure, flow, temperature loops; tuning methods Ziegler-Nichols and manual trial.

HMI and SCADA: HMI local operator interface, SCADA for plant-wide monitoring; communication protocols Modbus RTU/TCP, Profibus, Ethernet/IP.

Safety: emergency stop must be hardwired and independent of PLC; safety relays and category-rated circuits per ISO 13849.

IS 3043 covers earthing design; earth resistance for domestic installation should be ≤ 1 Ω (practically ≤ 5 Ω), substation ≤ 1 Ω, and measured with fall-of-potential method.

Earthing types: plate (60 × 60 cm), pipe (38 mm dia, 2.5 m long), rod, strip and mesh; earth pit with alternate layers of charcoal and salt.

IE Rules 1956 (now CEA Regulations 2010): Rule 61 - earthing required; Rule 44 - safety of supply; Rule 45 - precautions against supply voltage; Rule 46 - supply to be of proper capacity.

Permit-to-work system: isolation, lockout/tagout, proving dead with a tester, discharging capacitors, earthing the conductor before working.

First aid for electric shock: switch off supply, do not touch the person, use insulated material, start CPR if breathing stopped, call medical help.

PPE: insulating gloves, mat, helmet, safety shoes, arc-rated clothing; test instruments must be rated for the voltage and category (CAT III/IV).

Fire classes: A solid, B liquid, C gas, D metal; CO2 for electrical fires, never water on live equipment.

Residual current device 30 mA is the standard personnel protection; earth leakage current must not exceed the trip threshold under normal conditions.

Estimation: calculate connected load, apply diversity factor, choose cable size from current rating and voltage drop, then select protective device and earth size.

Diversity factor = sum of individual maximum demands / maximum demand on the system; demand factor = maximum demand / connected load.

Earth conductor size: for phase conductor up to 16 mm², earth equal to phase; 16–35 mm², earth 16 mm²; above 35 mm², half the phase conductor.

Instruments: ammeter in series, voltmeter in parallel, wattmeter (current coil series, potential coil parallel), energy meter in circuit with load.

Megger for insulation resistance, earth tester for earth resistance, multimeter for voltage/current/resistance, clamp meter for load current without breaking circuit.

Extension of range: shunt for ammeter, multiplier for voltmeter; CT and PT for AC high current/voltage measurement.

Digital meters have high input impedance; analogue meters have lower accuracy class but show trends; accuracy classes 0.5, 1.0, 1.5, 2.5.

Test sequence for a new installation: visual inspection, continuity, insulation resistance, polarity, earth resistance, then functional test and energisation.

Wire stripping and termination: strip only the required length, do not nick the conductor, use correct ferrule/crimp, and tighten terminal to specified torque.

Conduit work: cut square, ream burrs, bend with a bending spring or machine to a radius ≥ 2.5 × conduit diameter, and fix saddles at ≤ 1 m spacing.

Soldering: clean surfaces, apply flux, heat the joint and apply solder to the joint (not the bit), avoid a dry joint; use heat-shrink over the joint.

Crimping tools are matched to lug size; use a pull test on sample terminations; do not solder a crimp lug.

Panel wiring: route wires in ducts, use ferrules matching the drawing, separate power and control wiring, and label both ends.

Ladder and working at height: use a safety belt, ensure the ladder is at 75° and secured; never work alone on live or high-level work.

Motor terminal connection: star or delta determined by the nameplate and supply voltage; verify before energising with a continuity test.

Lockout/tagout: isolate, lock, tag, prove dead, and keep the key with the person working; only that person removes the lock.

Transformer tests: open-circuit test gives iron loss and magnetising branch; short-circuit test gives copper loss and equivalent impedance.

Transformer cooling: ONAN, ONAF, OFAF, OFWF; oil temperature and winding temperature indicators trip or alarm at set values.

Induction motor speed control: pole changing, stator voltage, rotor resistance (slip-ring), and VFD; VFD is the modern method for squirrel-cage motors.

Induction motor tests: no-load test gives iron + friction loss, blocked-rotor test gives copper loss and equivalent circuit; ratio of starting to full-load current typically 5–8.

DC motor starting: a starter (3-point or 4-point) limits armature current; never start a DC motor without field current.

DC motor speed control: armature voltage control below base speed, field flux control above base speed; field weakening reduces torque.

Synchronous motor runs at N_s = 120f/P and can supply leading reactive power for power factor correction.

Motor nameplate: kW/HP, voltage, current, frequency, speed, insulation class, IP rating, duty, connection (star/delta), power factor.

Circuit breaker arc quenching: air, oil, SF6, vacuum; vacuum and SF6 dominate modern MV/HV switchgear.

Isolator cannot break load current; it is used only for isolation after the breaker opens. A load-break switch can break rated load current.

CT and PT: CT secondary 5 A or 1 A, never open-circuit a live CT; PT secondary 110 V, never short-circuit a live PT.

Protection relays: overcurrent (50/51), earth fault (50N/51N), differential (87), distance (21), under/over voltage (27/59), Buchholz for transformers.

Buchholz relay detects gas accumulation and oil surge in oil-filled transformers; it is mounted between the tank and conservator.

Short-circuit current calculation: I_sc = V/(√3·Z_total); protective device breaking capacity must exceed prospective short-circuit current.

Discrimination: time-graded or current-graded settings ensure only the device nearest the fault trips.

Switchgear ratings: rated voltage, rated current, rated short-circuit breaking current, short-time withstand current, and IP degree of protection.

Thermography finds loose joints and overloaded cables by temperature rise; compare phase temperatures - a 10 °C rise above the others indicates a problem.

Motor current imbalance limit: 10% between phases; voltage imbalance limit: 5%; higher imbalance causes derating and overheating.

Insulation resistance decreases with moisture, dirt and temperature; record at a standard temperature (usually 40 °C) for comparison.

Vibration analysis: unbalance shows 1× running speed, misalignment 2×, bearing defects show high-frequency peaks; correct alignment reduces failures.

Contactor maintenance: inspect contacts for pitting, replace if worn beyond the manufacturer limit, check coil voltage and mechanical interlock.

Cable fault location: Murray loop and Varley loop for low-resistance faults, time-domain reflectometer for open/short, and high-voltage test for high-resistance faults.

Preventive maintenance intervals: motors - annually or per duty; transformers - oil test annually, Buchholz and breather checks monthly.

Root cause analysis: distinguish overload, single phasing, insulation failure, bearing failure and supply voltage problems from the failure pattern.

Pumping: centrifugal pump power ∝ N³; throttling wastes energy, VFD saves energy at part load.

Fan laws: flow ∝ N, pressure ∝ N², power ∝ N³; damper control wastes energy compared with speed control.

Conveyor drives: constant torque, high starting torque, use VFD with torque boost or slip-ring motor; braking may be regenerative or mechanical.

Compressors: reciprocating needs high starting torque; screw compressors use star-delta or soft starter; unload/load control affects power factor.

Crane duty: S4/S5 duty cycle, high starting torque, frequent starts; motor rated for the duty, not continuous.

Machine tools: constant power above base speed, speed control by VFD or DC drive; spindle motors need high-speed capability.

Power factor correction: capacitor bank kVAR = P(tan φ1 − tan φ2); automatic PF controller switches steps to avoid overcorrection.

Harmonics: non-linear loads (VFD, LED, UPS) inject harmonics; use detuned reactors or active filters to keep THD within IEEE 519 limits.

VFD V/f control: keep V/f constant below base speed to maintain flux; above base speed field weakening reduces torque.

VFD braking: dynamic braking resistor dissipates energy; regenerative braking returns energy to the supply; DC injection braking stops the motor but is not for frequent use.

VFD protection: overcurrent, overvoltage, undervoltage, earth fault, overheat, stall prevention; read the fault code before resetting.

PLC input types: sinking (NPN) and sourcing (PNP) - match sensor type to input module; 24 V DC is standard.

Analogue 4–20 mA: 4 mA = minimum, 20 mA = maximum; below 4 mA indicates broken wire or fault.

PID tuning: increase P until oscillation, then reduce; add I to remove offset; add D to reduce overshoot - used for pressure and temperature loops.

Communication: Modbus RTU over RS-485, Modbus TCP, Profibus DP, Profinet, Ethernet/IP; baud rate and parity must match on all nodes.

Safety functions: STO (safe torque off) in the drive, safety relay, guard interlock, and emergency stop wired to the safety circuit, not the PLC.

IE Rules 1956 (CEA Regulations 2010): Rule 61 - earthing; Rule 44 - safety of supply; Rule 45 - precautions against supply voltage; Rule 46 - supply of proper capacity; Rule 47 - testing of installations.

Earthing: plate 60 × 60 cm, pipe 38 mm dia × 2.5 m; earth pit filled with alternate layers of charcoal and salt; earth resistance ≤ 1 Ω for substations, ≤ 5 Ω for domestic.

Earth resistance measurement: fall-of-potential (3-pole) method with two spikes; earth tester reads resistance; measure in dry season for worst case.

Types of earthing: pipe, plate, rod, strip, mesh; TN-S, TN-C-S, TT, IT systems; in TT the earth electrode is local and RCD is mandatory.

Permit-to-work: isolate, lock, tag, prove dead with a tester, discharge capacitors, earth the conductor, then work.

First aid for electric shock: switch off supply, do not touch the person, use insulated material, start CPR if breathing stopped, call for medical help.

PPE and instruments: insulating gloves, mat, helmet, safety shoes, arc-rated clothing; test instruments rated CAT III/IV for the voltage.

Fire: CO2 for electrical fires, never water on live equipment; fire classes A (solid), B (liquid), C (gas), D (metal).

Estimation: connected load, diversity factor, cable size from current rating and voltage drop, protective device, earth size, and bill of materials.

Diversity factor = sum of individual maximum demands / maximum demand on the system; demand factor = maximum demand / connected load.

Earth conductor: phase ≤ 16 mm² → earth equal to phase; 16–35 mm² → 16 mm²; > 35 mm² → half the phase conductor.

Instruments: ammeter in series, voltmeter in parallel, wattmeter current coil in series and potential coil in parallel, energy meter with load.

Megger for insulation resistance, earth tester for earth resistance, multimeter for voltage/current/resistance, clamp meter for load current without breaking the circuit.

Range extension: shunt for ammeter, multiplier for voltmeter; CT and PT for AC high current/voltage measurement.

Accuracy classes: 0.5, 1.0, 1.5, 2.5; digital meters have high input impedance; analogue meters show trends.

Test sequence: visual inspection, continuity, insulation resistance, polarity, earth resistance, functional test, then energisation.

Wire stripping: strip only the required length, do not nick the conductor, use the correct ferrule/crimp, and tighten to the specified torque.

Conduit work: cut square, ream burrs, bend with a bending spring or machine to a radius ≥ 2.5 × diameter, fix saddles at ≤ 1 m spacing.

Soldering: clean surfaces, apply flux, heat the joint and apply solder to the joint (not the bit), avoid a dry joint, and cover with heat-shrink.

Crimping: use a crimping tool matched to the lug size; do a pull test on sample terminations; never solder a crimp lug.

Panel wiring: route wires in ducts, use ferrules matching the drawing, separate power and control wiring, and label both ends.

Ladder safety: 75° angle, secured, safety belt, never work alone on live or high-level work.

Motor terminal connection: star or delta per nameplate and supply voltage; verify with a continuity test before energising.

Lockout/tagout: isolate, lock, tag, prove dead, keep the key with the person working; only that person removes the lock.

Ohm's law: V = IR; power P = VI = I²R = V²/R; in AC use RMS values and P = V_rms I_rms cos φ for single phase.

Series: R_total = R1 + R2 + …; parallel: 1/R_total = 1/R1 + 1/R2 + …; series current common, parallel voltage common.

Kirchhoff: KCL - algebraic sum of currents at a node is zero; KVL - algebraic sum of EMFs and drops around a closed loop is zero.

AC: f = 1/T; ω = 2πf; X_L = 2πfL leads by 90°; X_C = 1/(2πfC) lags by 90°; resonance at f = 1/(2π√(LC)).

Impedance Z = √(R² + (X_L − X_C)²); power factor cos φ = R/Z; φ = tan⁻¹((X_L − X_C)/R).

Three-phase: star V_L = √3 V_ph; delta I_L = √3 I_ph; total power P = √3 V_L I_L cos φ.

Magnetic: F = BIl sin θ; e = Blv; B = μ0μrH; μ0 = 4π × 10⁻⁷ H/m.

Faraday/Lenz: e = −N dΦ/dt; induced current opposes the change producing it.

Energy: 1 kWh = 3.6 × 10⁶ J; H = I²Rt; 1 HP = 746 W.

Domestic supply: 230 V single phase, 50 Hz; three-phase 415 V; TN-S/TN-C-S or TT arrangements; live brown, neutral blue, earth green-yellow.

Lighting sub-circuit 1.0–1.5 mm² copper; power sub-circuit 2.5 mm²; main cable sized by load current and voltage drop.

Voltage drop ≤ 3% of supply for a final sub-circuit (IS 732); drop = 2ρLI/A single phase, √3 factor for three phase.

Domestic design limits: lighting circuit 800 W / 10 points; power circuit 3 kW / 2 points typical.

Conduit fill: total conductor cross-section ≤ 40% of conduit internal area; bends between draw boxes limited to allow drawing.

Types: cleat, casing-capping, batten, CTS, concealed conduit, surface conduit, busbar trunking.

Two-way switching: two 2-way switches (COM–L1–L2) with strappers; intermediate switch (4-terminal) for 3-point control.

Heights: switchboards ~1.2–1.5 m, socket outlets 1.2 m; all metalwork bonded to earth.

Designation: conductor material, insulation (PVC, XLPE, rubber), armour (steel wire for vertical, steel tape for horizontal), sheath.

Copper 58 MS/m, 0.0172 Ω·mm²/m; aluminium 35 MS/m, 0.028 Ω·mm²/m; aluminium needs ~1.6× the copper area.

Current rating depends on cross-section, insulation rating (70 °C PVC, 90 °C XLPE), ambient temperature, grouping, installation method.

Core colours: single phase live brown, neutral blue, earth green-yellow; three-phase L1 brown, L2 black, L3 grey.

Armour gives mechanical protection and an earth path but must be bonded; it is not a substitute for the earth conductor.

Accessories: MCB, MCCB, RCCB, isolator, plug-top fuse, ceiling rose, batten holder, IEC 60309 socket, cable gland, lug.

Bending radius: typically 8–12 × overall diameter for armoured cable; never below manufacturer data.

Jointing: heat-shrink or taped joint with staggered cores; insulation resistance after jointing tested with a 500 V or 1000 V megger.

Transformer EMF: E = 4.44 f N Φ_max; turns ratio V1/V2 = N1/N2 = I2/I1 (ideal).

Copper loss ∝ I² varies with load; iron loss constant; maximum efficiency when copper loss = iron loss.

Induction motor slip s = (N_s − N)/N_s; N_s = 120f/P; rotor frequency = s·f; torque ∝ s·V² near normal operation.

Starting: DOL for small motors; star-delta for delta-rated motors (1/3 current and torque); autotransformer and soft starter for large loads.

DC motor: shunt nearly constant speed, series high starting torque (never start unloaded); back EMF E_b = V − I_aR_a; N ∝ E_b/Φ.

DC generator EMF: E = PΦNZ/(60A); lap A = P, wave A = 2.

Single-phase induction motors are not self-starting; split-phase, capacitor-start and capacitor-run provide phase displacement.

Insulation classes: A 105 °C, E 120 °C, B 130 °C, F 155 °C, H 180 °C; motors rated by temperature rise above ambient.

Fuse: melts on overcurrent, I²t characteristic, no adjustable setting, must be replaced; HRC fuses are current-limiting with high breaking capacity.

MCB: thermal (overload) + magnetic (short circuit) trip; Type B 3–5×In, C 5–10×In, D 10–20×In; MCCB is adjustable for higher ratings.

RCCB detects residual current to earth (30 mA personnel, 100/300 mA fire/equipment); it does not protect against overload or line-to-neutral shock.

RCBO = MCB + RCCB in one device; ELCB (voltage-operated) requires a good earth and is largely obsolete.

Contactor handles frequent switching; thermal overload relay protects against sustained overload, not short circuit - a fuse/MCB must back it up.

DOL control: start button energises coil, auxiliary NO contact seals in, stop button breaks circuit; overload NC contact in series.

Star-delta timer: main + star close, then star opens and delta closes; interlock prevents star and delta closing together.

Earth-fault protection: earth leakage relay, restricted earth fault for transformer/generator windings, directional overcurrent for ring networks.

Planned preventive maintenance: cleaning, tightening terminals, insulation resistance, earth resistance, contact resistance (micro-ohm meter) and thermography at defined intervals.

Insulation resistance: 500 V DC megger for LV motors and cables; 1 MΩ minimum for LV motors, 100 MΩ for new HV; halve per 10 °C rise.

Polarisation index PI = R10 min / R1 min; PI ≥ 2 healthy, PI < 1.5 indicates moisture or degradation.

Motor troubleshooting: no start → supply, fuse, overload, open winding; overheating → overload, ventilation, voltage imbalance, bearing friction, single phasing.

Single phasing causes severe imbalance, high current in remaining phases and overheating; protect with phase-failure relay or bimetal overload.

Bearing failure: vibration, noise, temperature rise; check alignment, lubrication and belt tension; use vibration analyser and IR thermometer.

Loose joints show discolouration, higher resistance and heat under load; tighten to specified torque.

Records: log tests, spares replaced and failure analysis (5-why / root cause); keep a machine history card per motor.

Motor selection: load torque-speed characteristic, duty cycle (S1–S9), starting current limits, environment (IP rating), efficiency class (IE2/IE3/IE4).

Load types: constant torque (conveyors, positive displacement pumps), variable torque (centrifugal fans/pumps, torque ∝ N², power ∝ N³), constant power (machine tools).

Star-delta and soft starters limit inrush; VFD gives variable speed and energy saving on variable-torque loads.

Crane and hoist duty: high starting torque, frequent starts; slip-ring or inverter-controlled induction motors with mechanical brake.

LED drivers have high inrush; circuits need appropriate MCB type and contactors rated for capacitive/inductive inrush.

Power factor correction: capacitor bank kVAR = P(tan φ1 − tan φ2); avoid overcorrection leading to leading PF.

Welding transformers and furnaces are intermittent high-current loads; separate feeders and adequate short-circuit rating.

Motor protection coordination: fuse/MCCB for short circuit, overload relay for thermal, earth-fault relay for leakage; check discrimination by manufacturer tables.

VFD: rectifier → DC link capacitor → IGBT inverter with PWM; output frequency and voltage vary; V/f constant for constant torque below base speed.

VFD parameters: base frequency, maximum frequency, acceleration/deceleration ramp, current limit, carrier frequency, motor data for auto-tuning.

VFD cable must be shielded and earthed at both ends; long motor cables require output reactors or dv/dt filters.

PLC scan cycle: input scan → program execution → output update; IEC 61131-3 languages include ladder, function block, structured text.

Digital input 24 V DC sinking/sourcing; analogue 4–20 mA (live zero) or 0–10 V; 4–20 mA allows broken-wire detection.

PID: proportional, integral, derivative for pressure, flow, temperature loops; tuning by Ziegler-Nichols or manual trial.

HMI local operator interface, SCADA plant-wide monitoring; protocols Modbus RTU/TCP, Profibus, Ethernet/IP.

Safety: emergency stop hardwired and independent of PLC; safety relays and category-rated circuits per ISO 13849.

IS 3043 covers earthing design; earth resistance ≤ 1 Ω for substations, ≤ 5 Ω for domestic; measured by fall-of-potential method.

Earthing types: plate 60 × 60 cm, pipe 38 mm dia × 2.5 m, rod, strip, mesh; earth pit with alternate layers of charcoal and salt.

IE Rules 1956 (CEA Regulations 2010): Rule 61 earthing; Rule 44 safety of supply; Rule 45 precautions against supply voltage; Rule 46 supply of proper capacity; Rule 47 testing of installations.

Permit-to-work: isolation, lockout/tagout, proving dead with a tester, discharging capacitors, earthing the conductor before working.

First aid for electric shock: switch off supply, do not touch the person, use insulated material, start CPR if breathing stopped, call medical help.

PPE: insulating gloves, mat, helmet, safety shoes, arc-rated clothing; test instruments rated for voltage and CAT III/IV.

Fire classes: A solid, B liquid, C gas, D metal; CO2 for electrical fires, never water on live equipment.

Residual current device 30 mA is the standard personnel protection; earth leakage current must not exceed the trip threshold under normal conditions.

Estimation: connected load, diversity factor, cable size from current rating and voltage drop, protective device, earth size, bill of materials.

Diversity factor = sum of individual maximum demands / maximum demand on the system; demand factor = maximum demand / connected load.

Earth conductor: phase ≤ 16 mm² → earth equal to phase; 16–35 mm² → 16 mm²; > 35 mm² → half the phase conductor.

Instruments: ammeter in series, voltmeter in parallel, wattmeter current coil in series and potential coil in parallel, energy meter with load.

Megger for insulation resistance, earth tester for earth resistance, multimeter for voltage/current/resistance, clamp meter for load current without breaking circuit.

Range extension: shunt for ammeter, multiplier for voltmeter; CT and PT for AC high current/voltage measurement.

Digital meters have high input impedance; analogue meters show trends; accuracy classes 0.5, 1.0, 1.5, 2.5.

Test sequence: visual inspection, continuity, insulation resistance, polarity, earth resistance, functional test, then energisation.

Wire stripping: strip only the required length, do not nick the conductor, use the correct ferrule/crimp, and tighten to the specified torque.

Conduit work: cut square, ream burrs, bend with a bending spring or machine to a radius ≥ 2.5 × diameter, fix saddles at ≤ 1 m spacing.

Soldering: clean surfaces, apply flux, heat the joint and apply solder to the joint (not the bit), avoid a dry joint, cover with heat-shrink.

Crimping tools are matched to lug size; do a pull test on sample terminations; do not solder a crimp lug.

Panel wiring: route wires in ducts, use ferrules matching the drawing, separate power and control wiring, label both ends.

Ladder safety: 75° angle, secured, safety belt, never work alone on live or high-level work.

Motor terminal connection: star or delta per nameplate and supply voltage; verify with a continuity test before energising.

Lockout/tagout: isolate, lock, tag, prove dead, keep the key with the person working; only that person removes the lock.

Cable designation: conductor material, insulation (PVC, XLPE, rubber), armour (steel wire for vertical, steel tape for horizontal), sheath.

Copper 58 MS/m, 0.0172 Ω·mm²/m; aluminium 35 MS/m, 0.028 Ω·mm²/m; aluminium needs ~1.6× the copper area.

Current rating depends on cross-section, insulation rating (70 °C PVC, 90 °C XLPE), ambient temperature, grouping, installation method.

Core colours: single phase live brown, neutral blue, earth green-yellow; three-phase L1 brown, L2 black, L3 grey.

Armour gives mechanical protection and an earth path but must be bonded; it is not a substitute for the earth conductor.

Accessories: MCB, MCCB, RCCB, isolator, plug-top fuse, ceiling rose, batten holder, IEC 60309 socket, cable gland, lug.

Bending radius: typically 8–12 × overall diameter for armoured cable; never below manufacturer data.

Jointing: heat-shrink or taped joint with staggered cores; insulation resistance after jointing tested with a 500 V or 1000 V megger.

Fuse: melts on overcurrent, I²t characteristic, no adjustable setting, must be replaced; HRC fuses are current-limiting with high breaking capacity.

MCB: thermal (overload) + magnetic (short circuit) trip; Type B 3–5×In, C 5–10×In, D 10–20×In; MCCB is adjustable for higher ratings.

RCCB detects residual current to earth (30 mA personnel, 100/300 mA fire/equipment); it does not protect against overload or line-to-neutral shock.

RCBO = MCB + RCCB in one device; ELCB (voltage-operated) requires a good earth and is largely obsolete.

Contactor handles frequent switching; thermal overload relay protects against sustained overload, not short circuit; a fuse/MCB must back it up.

DOL control: start button energises coil, auxiliary NO contact seals in, stop button breaks circuit; overload NC contact in series.

Star-delta timer: main + star close, then star opens and delta closes; interlock prevents star and delta closing together.

Earth-fault protection: earth leakage relay, restricted earth fault for transformer/generator windings, directional overcurrent for ring networks.