Protection, Earthing and Electrical Safety
Protection questions ask you to prove that a fault will be cleared before anyone is endangered. The exam links device selection (MCB curve, fuse, RCD), the earthing system (TN-S, TN-C-S, TT, IT), earth-fault loop impedance and touch voltage into one chain of reasoning, then adds safe-working practice and IP ratings.
Core rules
- MCB magnetic ranges: type B 3–5 In, type C 5–10 In, type D 10–20 In. The device must also have a short-circuit breaking capacity at least equal to the prospective fault current at its terminals.
- Fuses: gG for general protection, aM for motor circuits; check the discrimination ratio against downstream devices (typically 1.6:1 for fuses).
- Earthing systems: TN-S separate neutral and earth, TN-C-S combined to the origin (typical EAC supply), TT installation electrode with RCD protection, IT isolated or impedance-earthed with insulation monitoring.
- Earth-fault loop impedance Zs = Ze + R1 + R2, compared with the tabulated maximum for the device and disconnection time.
- Equipotential bonding keeps touch voltage down; a 30 mA RCD limits the earth-fault energy where Zs cannot be achieved economically (mandatory in TT).
- RCD types: AC for sinusoidal residual current, A for pulsating DC, B for smooth DC (needed downstream of some drives).
- IP code: first digit solids, second digit water. Safe isolation: identify, isolate, lock, prove dead on the tester, then prove the tester.
Worked example
A 400/230 V TN-C-S installation has Ze = 0.35 Ω. A 32 A type B MCB feeds 25 m of 2.5 mm² line with 1.5 mm² cpc, giving R1 + R2 = (7.41 + 12.10) mΩ/m × 25 = 0.49 Ω. Then Zs = 0.35 + 0.49 = 0.84 Ω, comfortably below the 1.44 Ω maximum for a 32 A type B device at 0.4 s. Prospective earth-fault current is 230/0.84 = 274 A, well above the 5 × 32 = 160 A needed for instantaneous magnetic operation, so disconnection occurs in under 0.1 s and touch voltage stays within safe limits.
Traps that cost marks
- Taking the maximum Zs from the table without confirming the conductor is at its operating temperature, or applying the wrong disconnection time.
- Assuming an RCD clears line-to-neutral short circuits or replaces overcurrent protection.
- Using a type AC RCD downstream of equipment with DC leakage.
- Forgetting that TT systems rely on RCDs because loop impedance alone cannot give disconnection.
- Skipping the prove-dead step, or re-using a tester that has not been proved on a known source.