Engineering Discipline: Civil, Electrical, Mechanical and Electronics & Instrumentation
The Engineering Discipline paper is the technical core of the THDC JE selection and is attempted in the branch you apply for-Civil, Electrical, Mechanical, or Electronics & Instrumentation. Questions stay at diploma/degree fundamentals: definitions, units, simple design checks and standard values, not detailed consultancy design.
Core rules to revise
- Write every formula with its units; convert mm, cm, kPa and hours to SI before substituting.
- Civil: stress σ = P/A (N/mm² = MPa), strain = ΔL/L, and M20 concrete means 20 N/mm² characteristic cube strength at 28 days.
- Electrical: V = IR; series resistances add, parallel conductances add; P = VI = I²R; in three-phase, P = √3 V_L I_L cos φ.
- Mechanical: power = work/time = mgh/t for lifting; efficiency = output/input × 100%.
- E&I: a 4–20 mA loop uses 4 mA for live zero; accuracy is error as % of full-scale span, not reading.
Worked example
An electric motor lifts 500 kg of water through 20 m in 25 s. Input power is 5 kW and g = 10 m/s². Find output power and efficiency. Output work = mgh = 500 × 10 × 20 = 100,000 J. Output power = 100,000/25 = 4,000 W = 4 kW. Efficiency = 4/5 × 100 = 80%. The reasoning: identify output (useful lifting) versus input (electrical), and keep seconds.
Traps
Using diameter as radius in A = πr²; adding parallel resistors directly; reporting gauge pressure as absolute; assuming 1 MPa = 10 N/mm² (it is 1 N/mm²); forgetting √3 and power factor in three-phase calculations; and quoting concrete strength at 7 days instead of 28.