Protection, Earthing and Fault Diagnosis
Protection, Earthing and Fault Diagnosis tests how circuits are protected against overload, short circuit and earth fault, how earthing systems achieve disconnection, and how faults are traced systematically. Questions cover device types and ratings, RCDs, TN-S, TN-C-S and TT systems, bonding, loop impedance and test limits.
Core rules
- Overcurrent has two forms: overload (small excess current over time) and short circuit (very large current). The device must protect the cable: Ib less than or equal to In less than or equal to Iz, and I2 less than or equal to 1.45 Iz.
- MCB types by tripping range: B 3 to 5 times In, C 5 to 10 times In, D 10 to 20 times In. Choose for the inrush current of the load.
- An RCD compares live and neutral current and trips on residual current. It gives earth-fault and supplementary protection but does not replace overcurrent protection. 30 mA is used for socket outlets and increased-risk locations.
- Earthing systems: TN-S has a separate protective earth from the supply, TN-C-S has a combined PEN split at the origin, TT uses a local earth electrode and requires RCD protection.
- Maximum Zs ensures disconnection: 0.4 s for final circuits up to 32 A at 230 V, 5 s for distribution circuits.
- Main protective bonding connects incoming metal water, gas and structural metal to the main earthing terminal. Bonding is not a substitute for earthing.
- Fault finding: gather information, confirm the symptom, divide the circuit and test in sections, use the half-split method, then record findings.
Worked example: Zs check
A 16 A Type B MCB protects a socket radial. Maximum Zs from the standard table is 2.73 ohm. Measured earth loop impedance at the origin (Ze) is 0.35 ohm and the circuit's R1+R2 is 0.85 ohm, so Zs = 1.20 ohm. Since 1.20 is below 2.73, disconnection is achieved within 0.4 s. If the measured value were 3.5 ohm, options include shortening the run, increasing the circuit protective conductor size, or fitting a 30 mA RCD so that the RCD provides the disconnection.
Traps
- Comparing only Ze with the maximum Zs instead of the full loop impedance.
- Expecting an RCD to give overload protection.
- Confusing main protective bonding with earthing.
- Ignoring parallel earth paths that make a measured Zs look better than it is.
- Replacing a device with a higher rating to stop nuisance tripping instead of finding the cause.