Protection, Earthing and Bonding
What you must know
This domain covers overcurrent protection (fuses and circuit breakers), residual current devices, and the earthing and bonding arrangements that make automatic disconnection of supply work. Understand the difference between earthing (connecting exposed conductive parts to the earth) and bonding (connecting conductive parts together to equalise potential). Know the role of the main earthing terminal, circuit protective conductors, and main protective bonding of incoming metallic services. Understand RCD types, rated residual current, and why 30 mA is used for additional protection. Know the concept of fault loop impedance and disconnection times.
Traps
Examiners test whether you know that an RCD does not replace overcurrent protection, and that bonding is not the same as earthing. A frequent trap is asking whether a supplementary bond is required in a bathroom - the answer depends on the specific regulation and the presence of extraneous conductive parts. Another trap is disconnection time: it depends on circuit type and voltage, not on a single universal figure.
Worked example
A 30 mA RCD protecting a socket circuit operates when the residual current reaches 30 mA. For a 230 V supply, the touch voltage at 30 mA through a 1 kΩ body is 30 V, which is why 30 mA is chosen for additional protection. If the question asks for the maximum disconnection time for a final circuit, check the circuit classification before answering.
Rule of thumb
Protection works as a system: overcurrent device, earthing, bonding and, where required, RCD must all be correct together.