Protection, Earthing and Safety Practices
This domain tests protective device selection, earthing design and the safety discipline expected of a wireman on site.
Protective devices
- Rewirable fuses are cheap but slow and easily oversized; HRC fuses have defined breaking capacity and current-limiting action.
- MCB types: B trips at 3–5 × In, C at 5–10 × In, D at 10–20 × In. Use C for motors, B for lighting.
- MCCBs offer adjustable thermal and magnetic settings; an MPCB combines overload and short-circuit protection for a motor feeder.
- An RCCB at 30 mA, tripping within 30 ms, protects people against earth leakage; it does not protect against overload or short circuit and must be backed by an MCB.
- Thermal overload relays protect motors against sustained overload and single phasing, not against short circuits.
Earthing
- IS 3043 governs earthing design. Plate, pipe and rod electrodes are common, with targets of about 1 Ω for substations and 5–8 Ω for domestic installations, verified by an earth tester.
- Earth conductors must be continuous, mechanically protected and sized in proportion to the phase conductor; the earth wire must never carry normal load current.
- Bond all metalwork: panel body, machine frame, conduit, cable tray and water pipes.
Safety practice
- Isolate, lock out and tag out, prove dead, discharge capacitors, then work.
- PPE: helmet, insulated gloves, safety shoes, goggles, arc-rated clothing for live work.
- For electric shock: switch off, separate the person with dry insulating material, call for help, start CPR if breathing has stopped, and report the incident.
- Use CO2 or dry powder extinguishers on electrical fires; never water.
Worked example
A 30 mA RCCB on a wet-area circuit trips when a heater element leaks 40 mA to earth, disconnecting within 30 ms and limiting shock duration. It cannot clear a 25 A overload on a 6 A circuit - that is the MCB's job.
Traps
- Using the earth wire as a neutral return.
- Assuming an RCCB replaces an MCB.
- Relying on conduit continuity instead of a dedicated earth conductor.
- Resetting a tripped relay without finding the fault.