RRB Technician AC Mechanic Syllabus

The RRB Technician AC Mechanic CBT 2 discipline section tests technical knowledge at the ITI and Diploma level across all major topic areas of the AC Mechanic trade. It carries 100 questions and 100 marks within the 150-question CBT 2 paper. The CBT 2 score is the sole basis for the final Technician merit list.

Quick Facts: Technician AC Mechanic

Detailed AC Mechanic Syllabus

Questions in the AC Mechanic section test trade theory, applied calculations, materials knowledge, tools and equipment, process steps, quality standards, and safety regulations at the NCVT ITI level. The following topic clusters are covered:

1. Refrigeration Fundamentals and Thermodynamics

•        Thermodynamic Properties: pressure (gauge, absolute, differential, vacuum); temperature (Celsius, Fahrenheit, Kelvin - conversion); specific volume; density; enthalpy; entropy

•        Laws of Thermodynamics applied to refrigeration: First Law (energy balance across components); Second Law (heat flows from hot to cold spontaneously; refrigerant requires work input); Carnot COP = TL/(TH-TL)

•        Unit of Refrigeration: 1 Ton of Refrigeration (TR) = 3.517 kW = 12,000 BTU/hr = 50 kcal/min; refrigerating effect (RE); COP (Coefficient of Performance) = RE / Work input

•        Vapour Compression Refrigeration Cycle (VCRC): ideal cycle on P-H diagram (Mollier chart for refrigerants); compression (isentropic); condensation (isobaric - heat rejected); expansion (isenthalpic through expansion device); evaporation (isobaric - heat absorbed); COP calculation from P-H diagram; effect of sub-cooling (increases RE, increases COP) and superheating at suction (increases compressor work, effect on COP depends on refrigerant)

•        Vapour Absorption Cycle (VAR): generator; condenser; expansion valve; evaporator; absorber; solution pump; working pair (LiBr-water for chiller; NH3-water for refrigeration below 0 degrees C); COP typically 0.6 to 0.8 (less than VCC); advantage - uses heat energy; no compressor (only solution pump - very low electrical input)

•        Air Cycle Refrigeration: Bell-Coleman cycle; working; COP; applications (aircraft cabin cooling; food refrigeration in transport)

2. Refrigerants

•        Classification: primary refrigerants (halocarbon - CFC: R11, R12, R113; HCFC: R22, R123; HFC: R134a, R404A, R407C, R410A, R32, R507; HFO: R1234yf, R1234ze); natural refrigerants (R717 ammonia; R744 CO2; R600a isobutane; R290 propane; R718 water); secondary refrigerants (brines, glycols)

•        Properties: boiling point at 1 atm (R134a = -26.1 degrees C; R32 = -51.7 degrees C; R410A = -51.6 degrees C; NH3 = -33.3 degrees C; CO2 = -78.5 degrees C at 1 atm; sublimates); latent heat (R717 has very high latent heat of 1368 kJ/kg at 0 degrees C - high volumetric refrigerating effect; R134a 197 kJ/kg; R410A 220 kJ/kg at 0 degrees C); ODP (Ozone Depletion Potential - R22: 0.05; R134a: 0; R32: 0); GWP (Global Warming Potential - R134a: 1430; R32: 675; R1234yf: 4; R744: 1); ASHRAE 34 safety classification (A1 - low toxicity, non-flammable: R134a; A2L - low toxicity, mildly flammable: R32, R1234yf; B1 - high toxicity, non-flammable: NH3)

•        Refrigerant Selection Criteria: boiling point; ODP; GWP; toxicity; flammability; compatibility with lubricants and materials; COP; volumetric capacity; availability and cost

•        Phase-Out and Alternatives: Montreal Protocol (1987) - phase-out schedule for CFCs (Phase-out completed) and HCFCs (R22 phase-out in India by 2030); Kigali Amendment (2016) - phase-down of HFCs; alternatives: HFOs (R1234yf for automotive AC; R1234ze for centrifugal chillers); natural refrigerants (R290 for room AC; R600a for domestic refrigeration; R744 for supermarket refrigeration; R717 for industrial)

•        Refrigerant Handling: recovery (evacuating refrigerant from system into recovery cylinder before repair); recycling (cleaning recovered refrigerant for reuse in the same system); reclaiming (returning to original purity standards by reprocessing); charging (weight-based using electronic charging scale; sight glass indication; superheat and subcooling method); leak detection (electronic halide detector; UV dye injection; soap bubble for large leaks); cylinder colour coding (Indian Standard) and handling safety

3. Compressors

•        Reciprocating Compressor: hermetic (fully sealed - motor and compressor in same shell; for domestic refrigerators and small commercial systems); semi-hermetic (serviceable - bolted head; motor accessible; for medium commercial systems); open type (belt or direct drive; motor external; for industrial applications); single and multi-cylinder; construction (piston, cylinder, crankshaft, con-rod, reed valves, valve plate); volumetric efficiency factors (clearance ratio, refrigerant leakage, pressure drop, heat transfer, superheating of suction gas); compression ratio and its effect on volumetric efficiency; discharge temperature; performance characteristics (P-Q curve; power-capacity relationship)

•        Scroll Compressor: working principle (fixed and orbiting scroll; spiral wraps; eccentric shaft; progressive reduction in gas volume); advantages (fewer moving parts, quieter, higher efficiency, oil-free possible, compact); discharge port; flooded vs dry compressor; scroll bearing; applications (residential AC, commercial chillers); defrost compatibility

•        Rotary Compressor: rolling piston type (rotor eccentrically mounted; blade separates suction and discharge); vane type (multiple vanes; centrifugal force); working; oil lubrication; applications in small room ACs

•        Screw Compressor: twin screw (male and female rotors; helical profile; asymmetric rotor profile for efficiency; Vi - volume index/built-in volume ratio; capacity modulation using slide valve); single screw; oil flooding (lubrication and sealing); oil separator; applications in large chillers and industrial refrigeration; part-load efficiency

•        Centrifugal Compressor (Turbo-Compressor): dynamic compression (velocity-to-pressure conversion); impeller; diffuser; volute; surging (unstable operation at low flow); choke (maximum flow limit); capacity control (inlet guide vanes IGV; speed variation); multi-stage (for high pressure ratio); oil-free magnetic bearing versions; applications in large chiller plants (>200 TR)

4. Condensers, Evaporators and Expansion Devices

•        Condensers: air-cooled (forced convection with fan - most common in residential and light commercial AC; fins and tubes construction; air flow direction; condenser subcooling; hot gas bypass at high ambient temperature; dirty coil effects); water-cooled (shell and tube - refrigerant in shell, water in tubes; advantages for large systems; water quality and scaling; shell and coil; double pipe); evaporative condenser (combined cooling by evaporation of water and convection; most efficient; applied to ammonia industrial plants); condensing temperature; condenser duty = compressor power + refrigerating capacity

•        Evaporators: dry expansion (DX) coil with TXV - vapour refrigerant exits fully dry; flooded evaporator - evaporator always has liquid refrigerant; advantages of flooded (better heat transfer; lower superheat); fin-and-tube coil (residential and commercial AC - aluminum fins on copper tubes; face area; fin spacing; number of rows; circuitry); shell and tube (chiller evaporator - refrigerant in shell, chilled water in tubes; LMTD; U value); plate heat exchanger (brazed plate for small chillers; high efficiency; compact); frost accumulation and defrost requirement

•        Defrost Methods: electric defrost (resistance heaters embedded in coil - heating wire type; radiant heater; most energy intensive); hot gas defrost (redirecting discharge gas through evaporator - fastest; cycle reversal required; used in supermarket refrigeration and industrial systems); air defrost (for above-0 degrees C applications; fans blow ambient air over coil during off-cycle); water defrost (spraying water over coil - fast but requires drainage; used in blast freezers)

•        Expansion Devices: Capillary Tube (small diameter tube 0.5-2mm; 0.5-5m long; pressure drop by friction; fixed restriction - cannot compensate for load variation; used in hermetic systems like domestic refrigerators, small window ACs; advantages: simple, cheap, no moving parts; limitations: critical charge required; flooding at part load); Thermostatic Expansion Valve (TXV/TEV - sensing bulb at evaporator outlet measures superheat; feeler bulb generates pressure in actuating element; spring sets static superheat 4-8K; external equaliser for multi-pass or long evaporators; types: internally and externally equalized; advantages: regulates superheat regardless of load; disadvantage: complex, requires close supervision; hunting); Electronic Expansion Valve (EEV - stepper motor or solenoid driven; electronic controller; fast response; precise control; used in VRF and modern chillers); Automatic Expansion Valve (AXV - constant low-side pressure; used in constant load systems); Float Valves (high-side and low-side - for flooded evaporators)

5. Air Conditioning Systems

•        Psychrometrics: dry-bulb temperature (DBT); wet-bulb temperature (WBT); dew point temperature (DPT); specific humidity (omega = kg of moisture per kg of dry air = 0.622 Pv / (Pa-Pv)); relative humidity (phi = actual moisture / maximum moisture at same DBT = Pv / Psat(DBT)); enthalpy of moist air h = 1.006 DBT + omega(2501 + 1.86 DBT) kJ/kg of dry air; specific volume; psychrometric chart; psychrometric processes: sensible cooling (horizontal left; DBT decreases; DPT unchanged); sensible heating (horizontal right; DBT increases; DPT unchanged); humidification (DPT and WBT rise); dehumidification (moisture removed; DPT decreases; if also cooled then DBT also drops); adiabatic saturation (along WBT line); mixing of two air streams

•        Comfort Air Conditioning: comfort conditions (summer 22-26 degrees C DBT; 50-60% RH; winter 21-23 degrees C DBT; 40-55% RH); ASHRAE 55 standard; PMV (Predicted Mean Vote); PPD; thermal comfort factors (DBT, RH, radiation, air velocity, clothing, activity)

•        Window Air Conditioner: single room unit; hermetic compressor; R32 or R410A refrigerant; capillary tube expansion; cross-flow indoor blower; axial outdoor fan; thermostat; filter; modes (cool, fan, dry, auto); EER (Energy Efficiency Ratio = cooling capacity in Btu/hr / power input in watts); BEE star rating; installation; maintenance (filter cleaning; coil cleaning; drain cleaning; gas top-up - rare if no leak)

•        Split Air Conditioner: wall-mounted indoor unit (DC brushless fan motor; cross-flow blower; evaporator coil; air filter; drain pan); outdoor unit (hermetic or scroll compressor; air-cooled condenser; axial fan; service valves; electrical components); connecting pipes (liquid line - copper tube 6.35mm or 9.52mm; suction line - copper tube 12.7mm to 15.88mm; insulated; flaring and brazing); inverter technology (variable speed compressor using VFD; BLDC or PMSM compressor motor; saves 30-50% energy vs fixed speed); installation (mounting; pipe run; gas charging using superheat/subcooling method); fault diagnosis (no cooling - check refrigerant, compressor, blower; poor cooling - dirty filter, low refrigerant, TXV hunting; water leaking - blocked drain; noise - blower imbalance, refrigerant migration, loose parts)

•        Cassette and Ceiling Concealed AC: 4-way cassette (radial air distribution; suspended ceiling installation; fresh air intake option; condensate pump); ceiling concealed duct type (AHU connected to ductwork; for large spaces; damper control for zone control)

•        VRF/VRV Systems: Variable Refrigerant Flow; one outdoor unit connected to multiple indoor units (up to 64); inverter scroll or rotary compressor; electronic expansion valve for each indoor unit; three-pipe heat recovery systems (simultaneous heating and cooling; one condensing circuit serves both cooling and heating indoor units); refrigerant pipe sizing (complex; manufacturer software used); advantages (individual zone control; high efficiency at part load; flexibility)

•        Chilled Water AC Systems: central chiller (air-cooled or water-cooled; scroll, screw, or centrifugal compressor; capacity in TR or kW); chilled water distribution (primary-secondary pumping; variable primary flow; chilled water temperature 6-12 degrees C); cooling tower (evaporative cooling; fill; drift eliminator; fan types; approach and range; water quality - TDS, blowdown, biocide treatment, Legionella risk); Air Handling Unit (AHU - coil; fan; filter MERV or G4-F9 class; humidifier; bypass damper; supply air and return air connections); Fan Coil Unit (FCU - local cooling or heating; 2-pipe and 4-pipe systems; blow coil and duct type; Grundfos/Wilo fan motor; drain pan)

6. Railway Coach Air Conditioning (RMPU)

•        RMPU (Roof Mounted Package Unit): self-contained AC unit mounted on roof of railway coach; two independent refrigeration circuits for redundancy; R407C or R134a refrigerant (Indian Railways transitioning to R134a phase-out replacement); capacity 3 kW to 7.5 kW per circuit; hermetic scroll compressors; air-cooled condenser using outdoor ambient air; evaporator on coach interior side; independent blower fans for each circuit

•        Power Supply for Railway Coach AC: 415V AC 3-phase 50 Hz from locomotive HEP (Head End Power) via power cable along train; EOG (End-On-Generation) power car on older trains; battery backup for control circuit; RMPU rated for 415V +/- 15%; protection against under and over-voltage, phase failure, phase reversal

•        Control System: microprocessor-based controller; temperature set point; thermostat; fan speed control; fault display; auto restart after power restoration; event logging; SCADA monitoring from guard van; BIS standards for railway HVAC

•        RMPU Maintenance: periodic inspection schedule (monthly, quarterly, annual); coil cleaning (dirty filter indication; coil cleaning with water jet and cleaner); refrigerant pressure checks (suction 3-5 bar; discharge 15-18 bar for R134a at typical conditions); compressor current check; fan motor bearing check; evaporator drain check (blockage causes water dripping in coach); condenser coil check (fouling reduces heat rejection; clean with low-pressure water or brush)

•        Fault Diagnosis in RMPU: HP (High Pressure) trip - causes: dirty condenser coil; low ambient temp in winter; overcharge; non-condensable gases (air in system); HP switch setting incorrect; LP (Low Pressure) trip - causes: undercharge (refrigerant leak); dirty evaporator; blocked air filter; expansion valve stuck closed; compressor not starting - causes: HP or LP trip; motor overload; thermal overload; phase failure; starting capacitor failure (scroll compressors); no cooling despite running - causes: low refrigerant; stuck expansion valve open; condenser fouled; compressor reduced capacity; water dripping in coach - blocked drain; icing on evaporator (low refrigerant or dirty filter)

•        Under-Slung AC Unit: mounted under the coach frame (older coaches); similar refrigeration circuit; exposed to dust and water; maintenance challenges; being replaced by RMPU in new LHB coaches

7. Tools, Instruments and Safety

•        Refrigeration Tools: manifold gauge set (high-side and low-side gauges and hoses; refrigerant-specific; 4-valve type for charging and recovery); vacuum pump (single-stage and two-stage; recommended vacuum level 500 microns or 0.67 mbar before charging); electronic refrigerant leak detector (heated diode type for halocarbon; specific for each refrigerant type); flaring tool (imperial and metric sizes; 45-degree flare for refrigeration; proper flare angle and surface finish for leak-free joint); tube cutter (rotary; ratchet type; sharp cutting wheel to prevent burr); tube bender (spring; gear type; mandrel type - minimum radius = 3 to 5 x tube diameter); swaging tool (for brazed joints); charging cylinder (for gravimetric charging); refrigerant recovery machine; nitrogen pressure test kit (manifold, regulator, nitrogen cylinder - for system pressure test before charging; leak test at 1.5 x maximum working pressure)

•        Electrical and Electronic Tools: clamp meter (measuring AC current without breaking circuit; Fluke 322, 376); digital multimeter (DMM - measuring voltage, current, resistance, diode, capacitance, frequency); insulation resistance tester (megger - 250V, 500V, 1000V types; testing motor winding, cable, contactor); oscilloscope (waveform analysis); power analyser (measuring power factor, THD, true RMS); temperature gun (infrared non-contact thermometer for measuring pipe surface temperature; coil temperature; compressor discharge temperature)

•        Safety: refrigerant cylinder safety (never heat cylinder; keep upright; no direct sunlight; store below 50 degrees C; check hydrostatic test date; use pressure reducing valve); electrical safety (LOTO - Lockout-Tagout before any electrical work; insulated tools; PPE - Class 0 insulating gloves for up to 1000V); refrigerant exposure first aid (eye - flush with water 15 minutes; skin - remove contaminated clothing, wash with water; inhalation - move to fresh air; CPR if necessary; seek medical attention); asphyxiation risk in confined spaces (CO2 and NH3 at high concentration; oxygen monitoring required); NH3 safety (toxic; highly pungent; maximum 25 ppm for 8-hour exposure; 300 ppm - immediately dangerous; PPE required: full-face respirator with NH3 cartridge; SCBA for high concentration; emergency shower and eye wash station)

Common Technical Section (50 Questions)

In addition to the 100-mark trade section, all Technician candidates must answer the following 50-mark common section in CBT 2:

Physics and Chemistry (20 Questions - Class 12 / Diploma Level)

•        Physics (Class 12 / Diploma Level): Units and Measurement; Motion (kinematics; Newton's Laws); Work, Energy and Power; Properties of Matter (elasticity, viscosity, surface tension); Heat and Thermodynamics (specific heat, latent heat, laws of thermodynamics, Carnot cycle); Sound (wave motion, Doppler effect, resonance); Light (laws of reflection and refraction; lenses and mirrors; Young's double slit; diffraction; polarisation); Electrostatics (Coulomb's Law, capacitance, dielectrics); Current Electricity (Ohm's Law, Kirchhoff's Laws, Wheatstone bridge, power); Magnetic Effects of Current (Biot-Savart, Ampere's Law, force on conductor); Electromagnetic Induction (Faraday and Lenz); AC Circuits (RMS, LCR, resonance, power factor); Semiconductor Devices (p-n junction, transistor basics, logic gates); Modern Physics (photoelectric effect, radioactivity, nuclear reactions)

•        Chemistry (Class 12 / Diploma Level): Atomic Structure (Bohr model, quantum numbers, electronic configuration); Periodic Table (periodicity, s, p, d, f blocks, periodic trends); Chemical Bonding (ionic, covalent, metallic, VSEPR, hybridisation); States of Matter (gas laws - Boyle's, Charles's, Avogadro's; ideal and real gases); Thermodynamics (enthalpy, Hess's Law, entropy, Gibbs free energy); Chemical Equilibrium (Le Chatelier's principle, Kp, Kc); Electrochemistry (electrode potential, Nernst equation, electrolysis, Faraday's Laws, corrosion); Chemical Kinetics (rate law, order, activation energy, Arrhenius equation); p-Block Elements (Groups 15 to 18 - properties, compounds, oxoacids); d-Block Elements (transition metals, oxidation states, colour); Organic Chemistry (reactions SN1, SN2, E1, E2; IUPAC nomenclature; functional groups; biomolecules); Polymers and Chemistry in Everyday Life

Basics of Computer Applications (15 Questions)

•        Computer Applications Basics (Class 10 / ITI Level): Computer components (CPU, RAM, ROM, storage - HDD, SSD; I/O devices; ports); Operating system functions; MS Office (Word - formatting, tables, find-replace; Excel - formulae SUM, AVERAGE, IF, VLOOKUP, charts; PowerPoint - slides, transitions); Internet (browser, search engine, URL, HTTP, HTTPS, email, online safety); File management (folders, file types, copy, move, delete, zip); Network basics (LAN, WAN, Wi-Fi, IP address, router, modem); Cybersecurity (password best practices, phishing, antivirus, firewalls); Binary, octal, hexadecimal number systems and conversion; Logic gates (AND, OR, NOT, NAND, NOR, XOR) - truth tables and Boolean expressions

Environment and Pollution Control (15 Questions)

•        Environmental and Pollution Control (Class 10 / ITI Level): Types of pollution - Air (pollutants: PM2.5, PM10, SOx, NOx, CO, VOCs; sources; health effects; vehicular emission norms BS6; AQI); Water (BOD, COD, DO; sources of water pollution; effects; treatment methods - primary, secondary, tertiary); Soil (pesticides, heavy metals, plastic; remediation); Noise (dB scale; CPCB limits; control measures); Solid Waste Management (classification; 3R principle; composting; sanitary landfill; e-waste; plastic waste rules; biomedical waste); Renewable Energy (solar PV and thermal; wind; hydro; biomass; geothermal; hydrogen; India's renewable targets); Climate Change (greenhouse gases; global warming; Paris Agreement; India's NDC); Environmental Legislation (EPA 1986; Air Act 1981; Water Act 1974; Forest Conservation Act; Wildlife Protection Act; National Green Tribunal); Indian Railways Green Initiatives (bio-toilets; solar energy on trains and stations; LED lighting; rainwater harvesting; ISO 14001; plastic-free stations)

Preparation Strategy

1. NCVT ITI AC Mechanic Trade Theory is the Primary Source

The NCVT-prescribed ITI textbooks for the AC Mechanic trade (published by DGT or state ITI boards) are the most accurate and exam-aligned source. Read every chapter systematically, noting definitions, formulae, IS codes, process steps, safety rules, and tool specifications.

2. Focus on Factual, Numerical, and Standards-Based Content

MCQs in the trade section frequently test specific technical values, IS code provisions, standard formulations, and process parameters. Memorise standard values (voltage levels, wire gauges, material properties, temperature limits, safety clearances) for reliable marks.

3. Practise from SSC JE and RRB Technician Question Banks

Solve previous years' RRB Technician and SSC JE AC Mechanic question banks for difficulty calibration and exposure to typical question formats. Previous paper analysis reveals the most frequently tested topics.

Frequently Asked Questions

Q1. Is the AC Mechanic Technician syllabus the same as the JE AC Mechanic syllabus?

The topics overlap but the Technician paper is at ITI level while the JE paper is at Diploma/Degree level. JE questions are more analytical and deeper; Technician questions are more applied and practical at the trade level.

Q2. Are IS codes tested in the Technician trade section?

Yes. Questions on relevant IS codes, safety standards, and BIS specifications for the trade appear regularly. Prepare a list of key IS codes applicable to your trade.