Protection, Earthing and Safety Practices
What is tested
Selection and coordination of fuses, MCBs, MCCBs and RCCBs, earthing systems and their resistance values, and the statutory safety rules a wireman must follow.
Protective devices
A fuse melts on overload; a rewirable fuse is cheap but has a fusing factor around 1.5–2. An MCB gives thermal overload and magnetic short-circuit protection and is resettable; its breaking capacity (6 kA, 10 kA) must exceed the prospective fault current. MCCBs are used from about 63 A upward. An ELCB or RCCB senses earth leakage: 30 mA for personal protection, 100–300 mA for fire protection.
Earthing
IS 3043 governs earthing. Domestic earth resistance is typically a few ohms - around 5 Ω or less for a house and 1 Ω or less for a substation - measured with an earth tester. Plate earth uses a copper or GI plate; pipe earth a 38 mm GI pipe. The earth conductor is green-yellow, must never carry operating current and must never be fused.
Worked example
A faulty appliance puts 230 V on a body of 1000 Ω in dry conditions: I = 0.23 A, i.e. 230 mA, far above a 30 mA RCCB's trip level, so it disconnects in under 30 ms. With a wet body of 500 Ω the current doubles to 460 mA - this is why 30 mA, not 300 mA, protects people.
Statutory points
IE Rule 61 (working space), Rule 77 (clearance above ground), Rule 90 (earth connection), and permit-to-work. Use rubber gloves, insulated tools, discharge capacitors and never work alone on live equipment.
Traps
Using an RCCB as an MCB (it gives no overload protection), using the earth wire as a neutral, and quoting 0 Ω as a practical earth reading.