Protection, Earthing, and Lightning Systems
Protection engineering matches protective devices to the system so faults are cleared quickly and selectively. Know the key devices: fuses, MCBs, MCCBs, and relays (overcurrent, earth-fault, differential). Understand the two requirements of discrimination/selectivity - the device nearest the fault should operate first - and of sensitivity - the device must detect the smallest expected fault.
For earthing, distinguish system earthing (neutral earthing of the supply, e.g., TN-S, TN-C-S, TT, IT systems) from protective earthing (bonding exposed conductive parts). Know that TN systems rely on the metallic return path for fault current, while TT systems rely on earth electrodes and generally require RCD protection because loop impedance is high. Understand touch and step voltage and why equipotential bonding matters.
For lightning, know that a lightning protection system has an air-termination system, down-conductors, and an earth-termination system. The rolling-sphere and mesh methods determine the zone of protection. Surge protective devices (SPDs) divert transient overvoltages.
Worked example: In a TT system, earth-fault loop impedance is high, so the prospective fault current may be too low to trip an MCB magnetically. An RCD (e.g., 30 mA or 100 mA) is used because it responds to residual current rather than loop impedance.
Traps: confusing TN-C-S (combined neutral/earth in the supply, separated in the installation) with TN-S; assuming a TT system can rely on overcurrent devices alone; and mixing up step and touch voltage.