Protection, Switchgear, Control and Discrimination
This domain tests selection and coordination of protective devices: circuit breakers, fuses, RCDs, RCBOs, and the switchgear that isolates and controls them. You must match device type, rating and characteristics to the load and to the fault level, and ensure discrimination - the upstream device does not trip before the downstream device for a fault in the downstream section.
Key concepts: overcurrent (overload plus short circuit), breaking capacity (must exceed prospective fault current), tripping curves (B, C, D for MCBs - magnetic trip multiples), RCD types (AC, A, B) and sensitivity (30 mA for personal protection, higher for equipment), and selectivity by current and time. Switchgear must isolate all active conductors and be rated for the load and fault level; isolators need visible isolation and lockability for safe work.
Traps: choosing an MCB with insufficient breaking capacity for the prospective fault current; using a Type AC RCD where DC leakage (e.g. from EV chargers or electronics) requires Type A or B; assuming two devices of the same rating discriminate (they often do not); and forgetting that RCDs do not protect against overload or short circuit - they need overcurrent protection alongside.
Worked example: A sub-board feeds a 20 A circuit from a 63 A main. To discriminate, the upstream device should be time-delayed or of a higher rating and curve so that a downstream fault trips the 20 A device first. Verify both breaking capacities exceed the prospective fault current at their respective points.