Protection, Earthing and Bonding
Core concepts
Know the purpose of protective devices: overload protection, short-circuit protection, and additional protection by RCD. Understand disconnection times for TN and TT systems, the role of the main earthing terminal, circuit protective conductors, and main and supplementary bonding. Grasp the difference between earthing (a fault path) and bonding (equalising potential). Be familiar with device characteristics (B, C, D curves), discrimination, and the principle that the protective device must operate within the required disconnection time at the prospective fault current.
Traps in questions
A frequent trap is confusing earthing with bonding, or assuming an RCD replaces the need for a CPC. Others include ignoring disconnection times for TT systems (which often rely on RCDs) and forgetting that discrimination between upstream and downstream devices must be checked. Watch for questions that mix overload protection with fault protection, and for device curve selection where high inrush loads need type C or D.
Worked example / rule of thumb
For a TN system with a 0.4 s disconnection requirement on a final circuit, the earth fault loop impedance Zs must be low enough that the device trips within that time. Using the device's maximum Zs from its data, confirm measured Zs is below it. Rule of thumb: if Zs is too high for the breaker, an RCD may provide the required disconnection, but the CPC must still be present and continuous.