Maharashtra SSC Electrical Appliances (MSBSHSE) cheat sheet

Ohm's law: V = I × R; rearrange as I = V/R and R = V/I. Current rises with voltage and falls as resistance rises.

Power: P = V × I = I²R = V²/R, in watts. Energy: W = P × t, in joules when t is in seconds; 1 kWh = 3.6 × 10⁶ J.

Series circuit: same current through all elements, R_total = R₁ + R₂ + …, voltages add.

Parallel circuit: same voltage across all branches, 1/R_total = 1/R₁ + 1/R₂ + …, branch currents add.

For two resistors in parallel: R = (R₁ × R₂)/(R₁ + R₂).

Resistance of a conductor: R = ρL/A - proportional to length and resistivity, inversely proportional to cross-sectional area.

Temperature coefficient: metals (copper, tungsten) increase resistance when hot; carbon and most electrolytes decrease it.

AC supply: 230 V domestic in India at 50 Hz; peak value = RMS × √2 ≈ 325 V.

Heating effect: heat produced H = I²Rt joules, the basis of every resistive heating appliance.

Conductors: copper, aluminium, silver (best conductivity), brass, nichrome for heating elements.

Insulators: PVC, rubber, porcelain, bakelite, mica, glass, ceramic. Mica is used where high heat and insulation are both needed.

Nichrome (Ni-Cr alloy) has high resistivity, high melting point and resists oxidation at red heat - standard heater element.

Copper has low resistivity and good ductility; aluminium is lighter and cheaper but needs larger cross-section and careful jointing.

Magnetic materials: soft iron for cores (easy magnetisation and demagnetisation), steel for permanent magnets.

Fuse wire is a low-melting-point alloy (tin-lead) chosen so it melts before the circuit wiring overheats.

Carbon brushes in universal motors conduct current to the rotating commutator while wearing gradually.

Solder is a tin-lead or lead-free alloy with a melting point well below that of copper conductors.

Insulated tools rated 1000 V are used for live working; verify insulation before each use.

Test lamp and series test board are used for continuity checks; a neon tester or multimeter confirms supply presence.

Lockout/tagout: isolate the supply, remove the fuse or trip the MCB, tag the point, and verify dead before touching conductors.

First aid for electric shock: switch off supply or separate the victim with dry insulating material, then call for help and start CPR if breathing has stopped.

Never work alone on live circuits; stand on dry insulating mat and avoid contact with earthed metalwork.

Fire on electrical equipment: use CO₂ or dry powder extinguisher, never water or foam.

Standard PPE: rubber-soled shoes, insulated gloves, goggles for drilling, helmet near overhead lines.

Wire strippers are sized to the conductor gauge; cutting pliers or knives nicking the copper reduce current capacity and cause hot spots.

Cleat wiring: cheapest, on wooden battens, suitable only for temporary dry installations.

CTS (batten) wiring: porcelain cleats, dry indoor locations, low cost but exposed.

Conduit wiring: PVC or steel conduit, surface or concealed; concealed steel conduit gives mechanical and earth-continuity protection.

Casing and capping wiring: PVC casing with cap over the wires, used on walls in dry areas.

Single-phase domestic supply uses three wires: phase (red/brown), neutral (black/blue) and earth (green or green-yellow).

Standard domestic circuit ratings: 5 A lighting circuit, 15 A/16 A power circuit; kitchen and geyser loads on dedicated circuits.

Wire gauge selection follows load current and permissible voltage drop; 1.0 mm² for lighting, 1.5–2.5 mm² for socket circuits, larger for sub-mains.

Every circuit must have a separate neutral; neutrals of different circuits must never be commoned.

A one-way switch controls a lamp from one point; a two-way switch (three terminals, common plus two ways) controls it from two points.

Intermediate switch is used between two two-way switches for control from three or more points.

Socket outlets: 6 A for lighting and small loads, 16 A for power appliances; the earth pin is longer and thicker to make first and break last.

Ceiling rose connects the pendant lamp; loop-in system takes the supply to the ceiling rose rather than to the switch.

Regulator (dimmer) is connected in series with the lamp; electronic regulators use triac phase control.

MCB is connected in the phase conductor only; it protects against overload and short circuit, not against earth leakage.

Switch must always be in the phase conductor, never in the neutral.

Batten holder, angle holder and pendant holder are selected by mounting position; holder rating must match lamp wattage.

An electric iron has a nichrome element wound on a mica sheet, a soleplate, a thermostat and an indicating lamp in parallel with the element.

Electric kettle and immersion heater use a sheathed (tubular) nichrome element embedded in magnesium oxide inside a metal tube.

Electric fan: single-phase induction motor, capacitor start/run, speed varied by tapping the auxiliary winding or by an electronic regulator.

Mixer grinder: universal motor (series-wound, with brushes and commutator) giving high speed and high starting torque.

Refrigerator: vapour-compression cycle - compressor, condenser, capillary/expansion device, evaporator; thermostat and overload protector control the compressor.

Automatic electric iron uses a bimetallic strip thermostat; the pilot lamp glows only when the element is energised.

Water heater (geyser) uses an immersion element and a capillary or rod thermostat; thermal cut-out is a backup against thermostat failure.

Induction motor appliances (fan, washing machine, pump) have no brushes; universal-motor appliances (mixer, vacuum cleaner) do.

Bimetallic thermostat: two dissimilar metals with different expansion coefficients bend on heating and open the contacts at the set temperature.

Thermal cut-out (fuse) is one-shot protection; it opens permanently if the thermostat fails and the appliance overheats.

Heat transfer in appliances: conduction through the soleplate, convection in ovens and geysers, radiation from toasters and room heaters.

Energy for heating: kWh = (wattage × hours)/1000; a 2000 W geyser run 1 hour consumes 2 kWh.

Element resistance from rating: R = V²/P; a 1000 W, 230 V element is about 53 Ω.

A heater element that reads open circuit with a multimeter is broken; a partial short or earth fault shows low insulation resistance.

Capillary thermostat in a geyser senses water temperature through a phial and tube, and switches the contactor or element supply.

Simmerstat (energy regulator) switches the element on and off in a duty cycle to give continuous heat control on hotplates.

Universal (series) motor: works on AC or DC, brushes and commutator, high starting torque, used in mixers, drills and vacuum cleaners.

Single-phase induction motor is not self-starting; a capacitor or split auxiliary winding provides the rotating field.

Capacitor-start motor: large electrolytic capacitor in the start winding, switched out by a centrifugal switch near full speed.

Permanent-split capacitor motor: capacitor stays in circuit, giving quieter running and better power factor - used in ceiling fans.

Shaded-pole motor is cheapest and least efficient, used in small fans and turntables.

Speed of an induction motor: Ns = 120f/P synchronous; actual speed is lower by the slip.

Common motor faults: worn carbon brushes, dirty or burnt commutator, open start winding, failed capacitor, seized bearings.

A failed run capacitor in a ceiling fan shows as the fan humming but not turning, or turning only when pushed.

Fuse: melts on excess current, one-shot, must be replaced with the same rating; never rewire a fuse with thicker wire.

MCB: resettable thermal-magnetic device, protects against overload and short circuit; rated in amperes with a B, C or D trip curve.

ELCB (voltage-operated) senses earth leakage voltage; RCCB (current-operated) senses imbalance between phase and neutral currents.

RCCB trip threshold for human protection is 30 mA; 100 mA or 300 mA devices are for fire or equipment protection.

MCB protects wiring and equipment; RCCB protects people. A complete circuit needs both.

Fuse and MCB are placed in the phase conductor; a two-pole device may break both phase and neutral.

Overload protector in appliances (thermal relay or bimetal in the motor winding) opens the circuit when winding temperature rises.

Discrimination: the downstream device should trip before the upstream one, so ratings are graded from load to supply.

Purpose of earthing: provide a low-resistance path so fault current trips the protective device and the metal body stays at safe potential.

Plate earthing uses a copper or galvanised iron plate buried vertically with charcoal and salt; pipe earthing uses a GI pipe.

Earth wire is green or green-yellow and is connected to the metal body of appliances; it must never carry working current.

Earth continuity test: measure resistance from the appliance body to the earth pin; it should be near zero (typically below 1 Ω).

Earth electrode resistance should generally be below 5 Ω for domestic installations, measured by an earth tester.

Types: pipe, plate, rod and strip earthing; chemical earthing uses a backfill compound to keep resistance stable.

Double insulation (Class II appliance) has no earth terminal and relies on two layers of insulation.

A broken earth wire is detected by an earth continuity test, not by switching the appliance on.

Dead testing sequence: isolate, verify dead, then measure continuity and insulation resistance before re-energising.

Multimeter functions: voltage (parallel to supply), current (series in circuit), resistance and continuity (circuit dead).

Insulation resistance is measured with a megger at 500 V DC; a good appliance reads above 1 MΩ to earth.

Open circuit fault: no continuity, appliance dead; short circuit: very low resistance, fuse blows or MCB trips immediately.

Earth fault: low insulation resistance from live parts to the body, causing RCCB tripping or a shock.

Lamp does not glow: check supply, fuse/MCB, switch continuity, holder contact and lamp filament in that order.

Heater does not heat but the lamp glows: element open circuit - replace element, not the thermostat.

Fan hums but does not rotate: check capacitor, start winding and bearings before replacing the motor.

Efficiency = useful output energy / input energy × 100%; losses appear as heat, sound and vibration.

BEE star rating: 1 star is least efficient, 5 stars most efficient; the label states annual energy consumption in kWh.

LED lamps give roughly 80–100 lm/W against about 10–15 lm/W for incandescent lamps, so the same light costs far less to run.

Replace old filament lamps with LED, use electronic regulators and set geysers/ACs with timers and thermostats.

Power factor: low power factor increases current for the same useful power; capacitors in fans and fluorescent fittings improve it.

Standby (phantom) load: switch off at the socket or use a master switch to avoid continuous small consumption.

Consumer rights: ISI/BIS mark, BEE label, warranty card and cash memo; a faulty appliance within warranty is repaired or replaced by the dealer or manufacturer.

Energy audit of a home: list appliance wattage × daily hours to find the largest consumers, usually geyser, AC and refrigerator.