National Wiring Rules (IS 10101) and Technical Rules
The framework
IS 10101 (Indian Standard, Code of Practice for Electrical Wiring Installations) is the national wiring code; it is read together with the relevant IS standards for cables, switchgear, earthing and protective devices. Learn the code's structure by topic: protection for safety (shock, thermal, overcurrent, overvoltage), design and selection (load assessment, conductor sizing, voltage drop), installation practice (routing, enclosures, segregation), and inspection and testing. Expect questions that ask which protective measure applies to a given situation rather than pure numbers.
Core technical concepts
- Earthing systems (TN, TT, IT) and why the earth-fault loop impedance must be low enough for the protective device to operate within the disconnection time.
- Overcurrent protection: overload vs short-circuit; discrimination between devices.
- Residual current devices for supplementary/additional protection and for socket outlets; know the rated residual operating current and why 30 mA is used for shock protection.
- Conductor sizing from current-carrying capacity, derating for grouping/temperature, and voltage-drop limits.
- Segregation and identification of conductors, and requirements for enclosures and IP ratings in wet/dusty locations.
- Special locations (bathrooms, swimming pools, construction sites) have stricter rules.
Traps
Mixing up basic protection (insulation, barriers) with fault protection (earthing and automatic disconnection). Confusing an RCD's job (earth-leakage/shock protection) with an MCB's job (overcurrent). Forgetting that derating factors reduce a cable's tabulated capacity. Assuming a larger breaker is 'safer' - it can leave the cable unprotected.
Worked example: a 30 mA RCD does not replace overcurrent protection; a circuit needs both an MCB sized to the cable's derated capacity and the RCD where required.