Overcurrent Protection, Grounding, and Bonding
What this domain tests
Selection and coordination of protective devices, breaking capacity and discrimination, RCD application, and the earthing and bonding arrangements that make automatic disconnection of supply work.
Core rules
- I_b ≤ I_n ≤ I_z and I_2 ≤ 1.45 I_z: the device must protect the cable, not only the load.
- Breaking capacity must exceed the prospective fault current at the point of installation.
- MCB bands: Type B trips at 3–5 I_n, Type C at 5–10 I_n, Type D at 10–20 I_n; fuses follow their own time-current curves.
- 30 mA RCD protection is required for socket outlets up to 32 A for general use and for buried cables without earthed metallic protection.
- TN systems use the supply earth (TN-S separate, TN-C-S combined neutral-earth); TT uses a local electrode. Maximum disconnection times are 0.4 s for TN final circuits up to 32 A, 5 s for distribution circuits and 0.2 s for TT final circuits up to 32 A.
- Z_s = Z_e + (R1 + R2) must stay below the device maximum; for TT, R_A ≤ 50/IΔn, so 1667 Ω with a 30 mA RCD.
- Main protective bonding: 10 mm² for PME supplies, 6 mm² otherwise, applied to incoming metallic water, gas, oil and structural steel.
Worked example
A 32 A Type B MCB on a 230 V TN-S final circuit. Instantaneous trip current = 5 × 32 = 160 A, so maximum Z_s = 230/160 = 1.44 Ω. A measured Z_s of 1.1 Ω confirms disconnection within 0.4 s; a measured 1.6 Ω would require a lower-rated device or RCD protection.
Traps
- Confusing bonding with earthing: bonding does not provide a fault current path.
- Comparing measured Z_s with tabulated maxima without the 0.8 temperature adjustment.
- Assuming an RCD excuses an inadequate Z_s on a TN circuit.