Electrical Safety, Earthing and Protective Devices
Electrical Safety, Earthing and Protective Devices
This domain tests the physiology of electric shock, protective earthing, residual-current protection, fuses and circuit breakers, and the safety rules governing electrical work in India.
Core rules
- Shock severity depends on current, path and duration. About 1 mA is perceptible; 10–30 mA can prevent letting go; 50 mA and above across the heart can be fatal.
- Body current I = V/R_body; dry skin may be tens of kΩ, wet skin under 1 kΩ, so 230 V can drive a lethal current.
- Earthing: pipe or plate electrode plus earth continuity conductor; earth resistance is measured by the fall-of-potential method. IS 3043 requires low values, commonly ≤1 Ω for large substations.
- MCB protects against overload and short circuit; a 30 mA RCCB protects people against earth leakage but not overload; an MCCB handles larger industrial currents.
- A fuse must be in the phase conductor, never the neutral.
- Safety sequence: isolate, lock out/tag out, prove dead, discharge, use PPE, work under permit by a competent person.
- IP ratings show protection against dust and water ingress.
Worked example
A person touches a 230 V live part while standing on wet ground; body resistance is 1 kΩ. I = 230/1000 = 0.23 A = 230 mA, far above the fatal threshold and well above a 30 mA RCCB trip level. The RCCB should disconnect within tens of milliseconds; earthing and insulation remain the first line of defence.
Traps that cost marks
- Claiming an RCCB protects against overload, or that it removes the need for earthing.
- Placing a switch or fuse in the neutral.
- Assuming a 30 mA RCCB prevents all shock; it limits duration, not the initial current.
- Quoting earth resistance without stating the measurement method or electrode.