Electrical Protection and Earthing
Core concepts
Understand the purpose and operation of circuit breakers, fuses, residual current devices (RCDs) and surge protective devices. Know the difference between overcurrent protection (overload and short circuit) and earth fault protection. Understand the multiple earthed neutral (MEN) system, the protective earth conductor, equipotential bonding, and the role of the main earth and earth electrode. Know the types of earthing systems and the requirements for protective earthing of exposed conductive parts.
Traps in questions
Questions often confuse an RCD with a circuit breaker: an RCD does not protect against overload or short circuit, and a circuit breaker does not protect people from earth leakage. Candidates also forget that RCDs must be tested and tagged with a test date. Another trap is whether the MEN link is installed at the switchboard or at the point of supply, and whether an earth electrode is required. Be careful with the difference between protective earthing and functional earthing.
Worked example
A final subcircuit supplying socket outlets requires RCD protection. The RCD must operate within its rated residual current and trip time. To verify, use an RCD tester to confirm the trip time at the rated residual current and at half the rated current (the RCD should not trip at half). Record the results and the test date on the switchboard label.
Rule of thumb
Protection must operate before the fault current can damage the installation or endanger persons. Never rely on an RCD as the sole means of protection against overload.