RBSE SS Voc Electrical and Elect cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. Units: volt, ampere, ohm, watt.

Series circuit: same current through all elements; R_total = R1 + R2 + …; voltage divides in proportion to resistance.

Parallel circuit: same voltage across all branches; 1/R_total = 1/R1 + 1/R2 + …; two resistors: R_total = R1R2/(R1+R2).

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

Resistance of a conductor: R = ρL/A, so resistance rises with length and resistivity and falls with cross-sectional area.

Temperature coefficient: for copper R rises with temperature (positive α ≈ 0.0039/K); for carbon and semiconductors R falls.

Energy: 1 kWh = 1000 W × 3600 s = 3.6 MJ; billed units on a domestic meter are kWh.

AC values: RMS = peak/√2 for a sine wave; form factor 1.11, peak factor 1.414; f = 1/T in hertz.

Earthing limits touch voltage: earth resistance should be low (commonly ≤1 Ω for substations, ≤5–8 Ω for domestic electrodes) so fault current trips the protective device fast.

Protective device selection: MCB/fuse rated above design load current but below cable current-carrying capacity; MCB types B, C, D differ in magnetic trip multiples (3–5, 5–10, 10–20 × In).

RCD/ELCB residual current ratings: 30 mA for socket circuits giving personal protection, 100–300 mA for fire/equipment protection; test button and trip time must be checked.

Cable colour convention (IEC): brown/red line, blue/black neutral, green-yellow earth; never use earth conductor as a live or neutral.

Domestic circuits are run as sub-circuits from a distribution board: lighting, sockets, and fixed appliances on separate overcurrent devices.

Wire gauge rule: 1.0–1.5 mm² lighting, 2.5 mm² socket outlets, 4–6 mm² for water heaters and air conditioners, subject to local code.

Shock severity depends on current path and duration: about 30 mA can cause respiratory distress, 50–100 mA ventricular fibrillation; body resistance falls when skin is wet.

First aid for electric shock: isolate supply, do not touch the casualty until isolated, then assess breathing and call emergency services.

Transformer EMF equation: E = 4.44 f N Φm; turns ratio V1/V2 = N1/N2 = I2/I1 for an ideal transformer.

Transformer losses: copper loss varies with load squared (I²R), core loss (hysteresis + eddy current) is nearly constant at rated voltage and frequency.

DC machine EMF: E = (PΦNZ)/(60A); torque T = 0.159 PΦIa Z/A; shunt motor speed falls slightly with load, series motor must never run unloaded.

Three-phase induction motor synchronous speed Ns = 120f/P; slip s = (Ns − Nr)/Ns, typically 2–5% at full load.

Star-delta starting reduces phase voltage by 1/√3 and starting torque to one third; it is used for squirrel-cage motors on light starting loads.

Single-phase induction motors are not self-starting; split-phase, capacitor-start, and shaded-pole designs create a phase-displaced auxiliary flux.

Three-phase power: P = √3 VL IL cos φ; for balanced star connection VL = √3 Vph and IL = Iph; in delta VL = Vph and IL = √3 Iph.

Motor protection: overload relay set to motor full-load current, short-circuit protection by fuse/MCB, and no-load test used to check bearing and winding condition.

Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; gold ±5%, silver ±10%.

Capacitor charge Q = CV; energy E = ½CV²; series and parallel rules are the reverse of resistors; impedance Xc = 1/(2πfC).

Inductor impedance Xl = 2πfL; energy stored = ½LI²; current through an inductor cannot change instantaneously.

Diode: forward drop about 0.7 V (silicon) or 0.3 V (germanium); reverse breakdown occurs at the rated peak inverse voltage.

BJT: Ic = βIb; Vbe ≈ 0.7 V for conduction; operating regions cutoff, active (amplifier), saturation (switch).

Zener diode works in reverse breakdown as a voltage reference; LED forward drop 1.8–3.3 V depending on colour, needs a series resistor.

MOSFET/IGBT are voltage-controlled switches; MOSFET gate is insulated, so gate charge must be supplied and static discharge avoided.

Power supply blocks: transformer, rectifier, filter capacitor, regulator; ripple frequency is 2f for full-wave rectification.

Half-wave rectifier: output ripple frequency = f; full-wave and bridge: ripple = 2f; average DC of a full-wave rectifier ≈ 0.637 Vpeak.

Bridge rectifier uses four diodes and needs no centre-tapped transformer; two diodes conduct per half cycle, each carrying the load current.

Common-emitter amplifier: voltage gain ≈ −Rc/re, output inverted, input/output impedance moderate; emitter bypass raises gain.

Op-amp ideal rules: inputs draw no current and the feedback holds V+ = V−; inverting gain = −Rf/Rin, non-inverting gain = 1 + Rf/Rin.

Oscillator needs positive feedback with loop gain ≥1 and 360° (or 0°) phase shift; Wien bridge frequency f = 1/(2πRC).

Logic gates: AND outputs 1 only if all inputs 1; OR outputs 1 if any input 1; NAND and NOR are universal gates.

Boolean identities: A + A' = 1, A·A' = 0, De Morgan: (A·B)' = A' + B' and (A + B)' = A'·B'.

Combinational building blocks: half adder sum = A⊕B, carry = A·B; full adder adds a carry-in; flip-flops store one bit and are clocked.

Ammeter connects in series and should have very low resistance; voltmeter connects in parallel and should have very high resistance.

Moving-coil instruments read average value and suit DC; moving-iron instruments read RMS and suit AC; a rectifier-type meter scales to RMS for sine waves.

Wheatstone bridge balance condition R1/R2 = R3/R4; used for medium resistance; Kelvin bridge for low resistance, megger for insulation resistance.

Megger insulation test: apply 500 V DC for a 440 V system; acceptable insulation is typically ≥1 MΩ, higher is better.

Digital multimeter checks: continuity (buzzer) for open circuits, diode test for junction drop, voltage ranges selected above the expected value.

Oscilloscope: probe ×10 divides signal and raises input impedance; timebase sets horizontal scale, volts/div sets vertical; trigger stabilises the waveform.

Clamp meter measures current by magnetic field around a conductor and works on AC (and Hall-type DC); it cannot measure two conductors of opposite current together.

Fault finding sequence: verify supply, inspect for visible damage, test continuity and insulation, then isolate to the faulty section by substitution or half-split.

Soldering: iron tip 300–350 °C for tin-lead solder, 350–380 °C for lead-free; heat the joint then apply solder, not the iron tip.

Good solder joint is shiny, concave, and wets both surfaces; a dull rounded ball indicates a cold joint and must be reworked.

Flux removes oxide; rosin flux is used for electronics, acid or zinc chloride flux for sheet metal work and must be cleaned off PCBs.

PCB assembly order: lowest profile first (resistors, diodes), then capacitors, transistors, then connectors and heat-sensitive parts last.

Desoldering uses a pump or braid; excessive heat lifts copper pads, so limit contact time and use the correct tip size.

Hand tools: use the correct screwdriver type and size to avoid cam-out; never use pliers as a hammer; insulated tools for live work.

Cable preparation: strip only the required length, avoid nicking strands, twist and tin stranded wire before terminal clamping.

Maintainability: label conductors and terminals, route wiring in looms, keep spare fuses and diagrams, record modifications in a log.

Costing a job: total cost = material + labour + overhead + contingency, then add profit margin; quote must state scope, exclusions, and validity period.

Electrical contracting compliance: licence/permit requirements, inspection certificates, and compliance with the national wiring code are preconditions for payment.

Workplace safety duties: risk assessment before work, PPE (insulated gloves, safety footwear, helmet), lockout-tagout, and reporting of incidents.

Communication in the trade: confirm the customer's requirement in writing, explain test results in plain language, and record readings on a test certificate.

Self-employment basics: register the business, keep separate business accounts, issue GST/tax invoices where applicable, and maintain insurance cover.

Pricing rule of thumb: charge-out rate must cover idle time, travel, tool replacement, and training, not only the hours on site.

Estimating material: use measured lengths plus 5–10% wastage; check stock and lead time before committing to a delivery date.

Customer relations and repeat work depend on punctuality, a clean site, and a written warranty on the work performed.

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. Units: volt, ampere, ohm, watt.

Series circuit: same current through all elements; R_total = R1 + R2 + …; voltage divides in proportion to resistance.

Parallel circuit: same voltage across all branches; 1/R_total = 1/R1 + 1/R2 + …; two resistors: R_total = R1R2/(R1+R2).

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

Resistance of a conductor: R = ρL/A, so resistance rises with length and resistivity and falls with cross-sectional area.

Temperature coefficient: for copper R rises with temperature (positive α ≈ 0.0039/K); for carbon and semiconductors R falls.

Energy: 1 kWh = 1000 W × 3600 s = 3.6 MJ; billed units on a domestic meter are kWh.

AC values: RMS = peak/√2 for a sine wave; form factor 1.11, peak factor 1.414; f = 1/T in hertz.