RRB JE Electrical Engineering — Complete CBT-2 Study Guide

• Generation voltage is typically 11-25 kV at the power plant, stepped up to transmission levels (132/220/400/765 kV in India) to reduce I²R transmission losses over long distances, then stepped down at substations for distribution (33/11 kV) and finally to utilization voltage (230/440 V). • Transmission line parameters: resistance (causes I²R loss and voltage drop), inductance (dominant in overhead lines, causes reactive power absorption), capacitance (dominant in cables, causes Ferranti effect — receiving-end voltage exceeding sending-end voltage on lightly-loaded long lines). • Types of faults: Symmetrical (three-phase, all phases equally affected, least frequent but most severe) vs Unsymmetrical (Line-to-Ground — most common, Line-to-Line, Double-Line-to-Ground). • Power factor: cosφ, ratio of real power (kW) to apparent power (kVA); low power factor increases current for the same real power delivered, increasing I²R losses — corrected using capacitor banks connected in parallel with the inductive load. • Circuit breakers interrupt fault current under load; types include Oil, Air-blast, SF6 (sulphur hexafluoride, excellent arc-quenching, widely used in modern high-voltage switchgear), and Vacuum breakers (used at medium voltage, no external quenching medium needed).

• DC Generator: EMF equation E = (ΦZNP)/(60A), where Φ = flux/pole, Z = total armature conductors, N = speed (rpm), P = number of poles, A = number of parallel paths — increasing speed or flux directly increases generated EMF. • DC Motor types: Series (high starting torque, speed drops sharply with load, used for traction/cranes — never run unloaded, risk of dangerous overspeed), Shunt (near-constant speed regardless of load, used for lathes/fans), Compound (combines both characteristics). • Transformer: works only on AC (needs changing flux for mutual induction); turns ratio Vs/Vp = Ns/Np = Ip/Is; an ideal transformer has no losses, real transformers have copper loss (I²R, varies with load) and core/iron loss (hysteresis + eddy current, constant regardless of load). • Three-phase Induction Motor: most common industrial motor; slip s = (Ns − N)/Ns, where Ns = synchronous speed = 120f/P; the rotor never reaches synchronous speed (else no relative motion, no induced EMF, no torque) — this is why it's called an "asynchronous" motor. • Synchronous machines run exactly at synchronous speed Ns = 120f/P regardless of load (unlike induction motors) — used as generators (alternators) in power plants and as synchronous motors/condensers for power-factor correction.

• Kirchhoff's Current Law (KCL): sum of currents entering a node equals sum leaving (charge conservation). Kirchhoff's Voltage Law (KVL): sum of EMFs and IR drops around any closed loop is zero (energy conservation). • Network theorems: Thevenin's theorem reduces any linear network (as seen from two terminals) to a single voltage source in series with a single resistance; Norton's theorem gives the equivalent current-source-with-parallel-resistance form — Thevenin and Norton equivalents are interconvertible (Isc = Vth/Rth). • Superposition theorem: in a linear circuit with multiple sources, the response (current/voltage) at any point equals the sum of responses from each source acting alone (all other independent sources set to zero — voltage sources shorted, current sources opened). • AC circuit basics: in a purely resistive circuit, voltage and current are in phase; in a purely inductive circuit, current lags voltage by 90°; in a purely capacitive circuit, current leads voltage by 90° (mnemonic: "ELI the ICE man" — in an inductor (L), E leads I; in a capacitor (C), I leads E). • Resonance in RLC circuits occurs when inductive reactance equals capacitive reactance (XL = XC), at which the circuit behaves purely resistively and impedance is minimum (series) or maximum (parallel).

• Ammeters are connected in SERIES (must have very low internal resistance, ideally zero, so as not to disturb circuit current); Voltmeters are connected in PARALLEL (must have very high internal resistance, ideally infinite, so as not to draw current from the circuit). • Wheatstone bridge: used to measure unknown resistance precisely by balancing a bridge circuit (null-deflection method) — at balance, product of opposite arms are equal: P×R = Q×S, giving the highest accuracy for medium resistance values. • Megger (insulation resistance tester) measures insulation resistance of cables/windings using a high DC test voltage — critical for verifying safe insulation before energizing equipment after maintenance. • Energy meter (electricity meter): traditional induction-disc meters measure energy consumption (kWh) via a rotating aluminium disc whose speed is proportional to power — modern meters use electronic/digital measurement instead. • CT (Current Transformer) steps DOWN high current to a safely measurable low value for meters/relays — CT secondary must NEVER be left open-circuited while primary is energized (produces dangerously high voltage spikes); PT (Potential Transformer) steps down high voltage similarly and its secondary should never be short-circuited.