Assam HS Skill Electrical Mechanics and Electrician cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R, with V in volts, I in amperes, R in ohms, P in watts.

Series circuit: same current through every element, R_total = R1 + R2 + …, voltage divides in proportion to resistance.

Parallel circuit: same voltage across every branch, 1/R_total = 1/R1 + 1/R2 + …, current divides inversely with resistance.

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

AC sine wave: RMS = peak ÷ √2 ≈ 0.707 × peak; average of a full sine cycle is zero; form factor = RMS ÷ average ≈ 1.11.

Single-phase AC power: P = V I cos φ (watts), Q = V I sin φ (VAR), S = V I (VA); three-phase P = √3 V_L I_L cos φ.

Inductive reactance X_L = 2πfL (lags current by 90°); capacitive reactance X_C = 1/(2πfC) (leads by 90°); impedance Z = √(R² + (X_L − X_C)²).

Conductor resistance R = ρL/A: rises with length and temperature (positive coefficient for copper), falls with cross-sectional area.

Magnetic fundamentals: flux Φ in webers, flux density B = Φ/A in tesla, and induced EMF e = −N dΦ/dt (Faraday's law with Lenz's law direction).

Indian cable colour convention: phase red (or brown), neutral black (or blue), earth green (or green-yellow); a three-phase run uses red, yellow, blue for phases.

Copper conductor current density for domestic wiring is taken near 4–6 A/mm² in trunking; 1.5 mm² serves lighting circuits, 2.5 mm² serves 5 A/15 A socket outlets.

Standard domestic lighting sub-circuit: maximum 10 points, 800 W; power sub-circuit: maximum 2 points, 1000 W per point on a 15 A circuit.

Good joint requirements: mechanically tight, low resistance, electrically continuous, insulated to full circuit voltage, and solderable without excess bulk.

Western Union joint: twist conductors, bind with a third wire, solder; used for solid conductors in junction boxes; must be taped or sleeved after soldering.

Married (Britannia) joint: used for stranded conductors; strands are fanned, interlocked, bound and soldered, giving high mechanical strength.

Bending radius of conduit must be at least 6–10 times the conduit diameter; conduit fill should not exceed about 40% of the internal area.

Insulation resistance test: 500 V DC megger, minimum acceptable 1 MΩ for a low-voltage installation; readings below that indicate moisture or damaged insulation.

PVC conduit is used in damp situations and on surface runs; metal conduit needs continuity bonding and both ends bushed to protect conductor insulation.

Transformer EMF equation: E = 4.44 f N Φ_max; voltage ratio equals turns ratio V1/V2 = N1/N2, and current ratio is inverse I1/I2 = N2/N1.

Ideal transformer: input VA equals output VA; losses are iron (hysteresis + eddy current) and copper (I²R), with maximum efficiency when copper loss equals iron loss.

DC motor back EMF E_b = V − I_a R_a; torque T ∝ Φ I_a, and speed N ∝ (V − I_a R_a)/Φ, so field weakening raises speed.

DC shunt motor is a constant-speed machine; series motor has high starting torque and must never be started unloaded; compound motor combines both characteristics.

Three-phase induction motor: synchronous speed N_s = 120f/P; slip s = (N_s − N)/N_s, typically 2–5% at full load; rotor frequency = s × f.

Star-delta starter reduces starting voltage per phase to 1/√3, cutting starting torque and line current to one-third of direct-on-line values.

Single-phase induction motors are not self-starting; capacitor-start, capacitor-run and shaded-pole designs create the rotating field needed for starting.

Alternator frequency f = PN/120; EMF per phase E = 4.44 f Φ T × k_d × k_p, where k_d and k_p are distribution and pitch factors.

Transformer cooling codes: ONAN for oil-immersed natural cooling, ONAF with fans, AN for dry-type; oil temperature rise limits are typically 40–50 °C over ambient.

PMMC instrument: deflection ∝ current, measures DC only, linear scale, used with a rectifier or shunt for AC and high-current ranges.

Moving iron instrument: measures AC and DC, reads RMS, scale is non-linear and cramped at the low end; cheap and robust for panel use.

Dynamometer wattmeter: fixed current coil in series with load, moving pressure coil in parallel; reading P = V I cos φ; reversal of one coil causes backward deflection.

Energy meter: 1 kWh = revolutions ÷ meter constant (rev/kWh); creeping is prevented by a brake magnet and a shading band on the shunt magnet.

Wheatstone bridge suits medium resistance measurement; Kelvin double bridge suits very low resistance; megger (insulation tester) applies 500 V or 1000 V DC.

Multimeter on ohms range must be used with circuit dead and isolated; a low reading across a capacitor indicates leakage, not capacitance.

Instrument accuracy classes: 0.5 and 1.0 for precision/panel work, 1.5 and 2.5 for general switchboard instruments; error % = (true − indicated)/true × 100.

Ammeter is connected in series and has very low resistance; voltmeter is in parallel and has very high resistance; CT and PT extend AC ranges safely.

Digital clamp meter measures current by transformer action around one conductor only; clamping both live and neutral together reads near zero.

Earthing resistance limits: domestic installation typically ≤ 1 Ω to 5 Ω depending on supply; substation earth grid often ≤ 1 Ω; IS practice requires measurement by fall-of-potential method.

Plate earth electrode: copper plate at least 60 cm × 60 cm × 3 mm buried vertically below ground level with charcoal and salt, connected by 8 SWG GI wire or equivalent.

Pipe earthing: GI pipe of 38 mm diameter and 2.5 m length; length increases in dry soil; water funnel is provided for periodic watering.

MCB types: B trips at 3–5 × In, C at 5–10 × In, D at 10–20 × In; selection depends on inrush current of the load.

ELCB (voltage-operated) senses earth leakage voltage; RCCB (current-operated) senses residual current imbalance; 30 mA RCCB is used for personal protection.

Fuse rating rule: fuse current should be above normal load current but below the cable's current-carrying capacity; HRC fuses clear faults faster than rewirable fuses.

First aid for electric shock: isolate supply, do not touch the victim until isolated, then start CPR if breathing has stopped and call medical help.

Fire classes: Class C is electrical; use CO2 or dry powder, never water or foam on live electrical equipment.

Lockout/tagout and proving dead with a tester before work on any circuit are mandatory; test the tester on a known live source first.

Single-line diagram shows generators, transformers, busbars and protective devices as symbols with ratings; it omits individual wiring runs.

Wiring plan symbols: a circle with a cross is a ceiling rose/lamp point, a socket symbol with three pins is a 6 A or 16 A outlet, and a fan symbol is a ceiling fan point.

Estimation sequence: take off quantities from the drawing, apply wastage (5–10% cable, 3–5% accessories), price at current market rate, then add labour and overheads.

Labour cost is often estimated as a percentage (10–20%) of material cost, or by scheduled rates per point; profit margin is added separately.

Cable length estimate = route length × number of runs + 10% for slack at boxes and boards; conduit length follows the same route with its own wastage.

Earthing estimate includes electrode, charcoal/salt, earth wire run and chamber; earthing wire size is chosen from the phase conductor size table.

A bill of quantities lists item description, unit (point, metre, number), quantity and rate; the abstract totals each item and adds contingencies.

Load estimate: sum connected loads, apply diversity factor (0.5–0.8 for domestic), then select main switch and cable by derated current capacity.

Diode: forward biased above ~0.7 V (silicon) or ~0.3 V (germanium); reverse biased it blocks until breakdown; used in rectifiers and freewheeling paths.

Half-wave rectifier output is 0.318 V_m average; full-wave and bridge give 0.636 V_m; ripple frequency is f for half-wave and 2f for full-wave.

SCR conducts only when anode positive and gate pulsed; it latches on until anode current falls below holding current; used in controlled rectifiers and AC phase control.

TRIAC is a bidirectional SCR used for AC dimming and fan regulators; DIAC is a bidirectional trigger diode used to fire the TRIAC symmetrically.

BJT: NPN conducts when base-emitter is forward biased; used as a switch in relay and contactor drivers; I_C = β I_B in the active region.

Relay and contactor logic: NO contacts close when coil energised, NC open; interlocking prevents two contactors from closing together in forward-reverse starters.

PLC basics: inputs are sensors and switches, outputs drive contactors and solenoids; ladder logic uses contacts in series for AND and parallel for OR.

VFD controls induction motor speed by varying supply frequency; speed ≈ 120f/P, so 25 Hz on a 4-pole motor gives about 750 rpm synchronous.

Optocoupler isolates control circuitry from power circuitry; used at PLC inputs and in solid-state relay circuits to prevent ground loops and surges.

Electric iron: element open circuit, thermostat contacts pitted, or bimetallic strip out of calibration are the common faults; check continuity of the element first.

Electric kettle: element failure or scale build-up gives slow heating; a faulty cord or plug gives no heating at all; test with a series lamp or multimeter.

Ceiling fan: capacitor failure causes no rotation or humming; winding short or open causes blown fuse; bearing noise indicates worn bearings needing replacement.

Mixer/grinder: carbon brushes worn, armature commutator dirty, or field winding open; universal motors spark if brushes are short or commutator is out of round.

Refrigerator: compressor not starting may be relay/PTC starter, thermostat open, or winding failure; check overload protector and capacitor before replacing compressor.

Air conditioner: low cooling from dirty filter, low gas charge, blocked condenser, or faulty capacitor on the compressor; measure running current against nameplate.

Washing machine: no spin from lid switch, belt slip, or capacitor; drain failure from blocked filter or faulty drain pump; motor winding test by resistance.

Microwave oven: high-voltage capacitor and transformer can retain lethal charge; discharge before servicing; magnetron failure gives no heating with normal fan and lamp.

Water heater/geyser: element failure or thermostat trip; check earthing and RCCB before returning to service; scale shortens element life.

Bench tools: hacksaw blade 18 or 24 TPI for general metal, file cut chosen by finish required, and a try square checks squareness to 90°.

Wire stripping: strip only enough insulation to enter the terminal; nicked conductor strands reduce current capacity and are a rejection point in practical exams.

Soldering: clean bit, tin the bit, heat the joint and apply solder to the joint not the bit; a bright smooth fillet is a good joint; a dull grainy surface is a cold joint.

Megger test procedure: isolate circuit, disconnect loads and electronics, connect megger between live and earth, crank to rated speed, read after one minute.

Multimeter continuity test: confirm the meter reads zero when probes touch; a reading of a few ohms across a switch contact indicates pitting or looseness.

Motor winding test: measure resistance between phases (should be balanced), and insulation resistance between winding and body (should exceed 1 MΩ).

Viva questions often ask the difference between MCB and fuse, why earthing is needed, and why the neutral is earthed at the transformer only.

Continuity of earth conductor: measure from the earth pin of the socket to the earth electrode; a high reading means a loose or corroded connection.

Safe isolation sequence: identify circuit, switch off, lock and tag, prove tester, test for dead on all conductors, then begin work.