Earthing and Protective Devices
Scope
This domain covers earthing practice to IS 3043 and the selection and coordination of fuses, MCBs, MCCBs, RCCBs and ELCBs.
Core rules
- Earthing systems: plate earth, pipe earth, rod earth and strip earth. Plate and pipe electrodes are set in soil with charcoal and salt to lower resistivity; rod earth is used where space is tight.
- IS 3043 requires earth resistance low enough to operate the protective device. Common targets are ≤1 Ω for substations and ≤5 Ω for domestic installations; values must be measured with an earth tester, not assumed.
- The earth continuity conductor must run from every point to the earth electrode without a fuse or switch in it.
- MCB types: B trips at 3–5 × I_n, C at 5–10 × I_n, D at 10–20 × I_n. RCCB 30 mA protects people; 100 mA or 300 mA protects against fire and earth leakage.
- Discrimination means the device nearest the fault operates first, so ratings and time–current curves must be graded.
Worked example
A 230 V circuit with a 16 A type-C MCB has an earth loop impedance of 2 Ω. Fault current = 230/2 = 115 A, which is only 7.2 × I_n - inside the magnetic band but near its lower edge. If the loop is 1 Ω, the current is 230 A (14 × I_n) and the MCB trips magnetically in milliseconds. Keeping earth resistance low is therefore not a formality; it is what makes the protective device operate within the permitted disconnection time.
Traps that cost marks
Bonding neutral to earth downstream of an RCCB (it will trip), fitting an earth wire with a fuse, and believing an RCCB protects against line-to-neutral contact - it does not, because the current returns through neutral and the residual is zero.