RRB ALP Electrician Trade — Complete CBT-2 Study Guide

• Ohm's law: V = IR, valid for metallic conductors at constant temperature. Power P = VI = I²R = V²/R (watts). Energy consumed = Power × time, billed commercially in kWh (1 unit = 1 kWh = 3.6×10⁶ Joules). • Series circuit: same current flows through all components, voltages add up (V = V1+V2+V3). Parallel circuit: same voltage across all branches, currents add up (I = I1+I2+I3); equivalent resistance in parallel is always LESS than the smallest individual resistance. • AC fundamentals: frequency in India is 50 Hz; RMS value of a sinusoidal AC = peak value × 0.707; average value over a half-cycle = peak × 0.637; RMS is the value used for all practical power calculations since it represents equivalent DC heating effect. • Three-phase supply: three voltages 120° apart in phase; Star (Y) connection — line voltage = √3 × phase voltage; Delta (Δ) connection — line voltage = phase voltage, but line current = √3 × phase current.

• Induction motor (most common industrial AC motor): rotor never reaches synchronous speed (else no relative motion between stator field and rotor, hence no induced current, hence no torque) — this speed difference is called slip. • Starting methods for induction motors: Direct-On-Line (DOL, for small motors), Star-Delta starter (reduces starting current to about 1/3 by starting in star then switching to delta), Auto-transformer starter (for larger motors, reduces starting voltage/current in controlled steps). • DC motor types: Series motor (very high starting torque, used in traction — must never run unloaded due to dangerous overspeed risk), Shunt motor (near-constant speed under varying load, used in machine tools/fans), Compound motor (blends both characteristics). • Alternator (AC generator) frequency relation: f = PN/120, where P = number of poles, N = rotor speed in rpm — this is why alternators must run at an exact synchronous speed to maintain grid frequency (50 Hz).

• Wiring systems: Casing-capping (wires in PVC casing, older method), Conduit wiring (wires drawn through a metal/PVC pipe — provides good mechanical protection, widely used in commercial/industrial buildings), Concealed wiring (conduit embedded in walls before plastering — standard for modern residential wiring). • Earthing (grounding) provides a low-resistance path for fault current to flow safely into the ground, protecting personnel from electric shock on a faulty appliance's metal body — earth resistance should be as low as practically possible (typically below 5Ω for most installations, per Indian electrical safety codes). • Types of earthing: Plate earthing (a metal plate buried in the ground, connected via a GI wire), Pipe earthing (a galvanized iron pipe driven into the ground, simpler to install, commonly used for domestic/small installations). • Color coding of wires (Indian standard, single-phase): Red/Brown = Phase (Live), Black/Blue = Neutral, Green = Earth — correct color-coded identification prevents dangerous wiring errors during installation and maintenance. • MCB (Miniature Circuit Breaker) automatically trips on overload or short-circuit, protecting the circuit; ELCB/RCCB (Earth Leakage/Residual Current Circuit Breaker) trips on detecting current leakage to earth (e.g. through a person's body), providing shock protection that an MCB alone cannot.

• A transformer works ONLY on AC (requires a continuously changing flux for mutual induction between primary and secondary windings) — it cannot step up or down a pure DC voltage. • Turns ratio: Vs/Vp = Ns/Np = Ip/Is — a step-up transformer increases voltage and proportionally decreases current (and vice versa for step-down), conserving power (ideally). • Transformer losses: Copper loss (I²R heating in windings, varies with load current) and Core/Iron loss (hysteresis + eddy current losses, remains roughly constant regardless of load, present as long as the transformer is energized). • Rectification converts AC to DC: half-wave rectifier (1 diode, higher ripple), full-wave/bridge rectifier (2 or 4 diodes, smoother DC, more efficient) — used in DC power supplies and battery chargers. • Battery basics: Lead-acid battery (used in vehicles/UPS, nominal cell voltage ~2V, typically 6 cells for a 12V battery), specific gravity of electrolyte indicates state of charge — a fully charged lead-acid cell has higher electrolyte specific gravity than a discharged one.