Protective Devices
Protective devices must clear a fault before anyone is harmed and must not operate on healthy load. The exam tests device characteristics, correct selection against the cable and the load, breaking capacity, selectivity, and the difference between overcurrent protection and residual current protection.
Device families
- MCB to BS EN 60898-1: type B trips magnetically at 3 to 5 times In, type C at 5 to 10 times, type D at 10 to 20 times. Type B suits resistive and lighting loads, type C suits inductive and motor loads with inrush.
- Fuses to BS 88, BS 1361 and BS 3036 have different time-current curves and need the appropriate factor when applied to cable capacity.
- RCCBs and RCBOs to BS EN 61008 and BS EN 61009. Type AC responds to alternating residual current only; type A also to pulsating d.c.; type F and type B cover particular d.c. and high-frequency components, for example from inverter drives and some EV chargers.
- MCCBs and switch-disconnectors for larger supplies; contactors and thermal overloads for motor control; SPDs for transient overvoltage; AFDDs for series and parallel arc detection.
Selection rules
In must be at least Ib and must not exceed the derated cable capacity. Breaking capacity must exceed the prospective fault current at the point of installation, commonly 6 kA at a domestic origin but verified rather than assumed. Selectivity between devices in series needs the manufacturer's data, or the commonly used ratio of about 1.6 to 1 for MCBs of the same family.
Worked example
A 12 kW range at 230 V. Ib = 12000 / 230 = 52.2 A, so a 50 A device is too small and a 63 A type B MCB is chosen. 16 sq mm twin and earth clipped direct has a tabulated capacity of 85 A, comfortably above 63 A, and the voltage drop is checked over the actual run. Maximum Zs for a 63 A type B device at 0.4 s is 0.72 ohm; the measured Ze of 0.35 plus R1 + R2 of 0.30 gives 0.65 ohm, which passes. The measured prospective fault current of 2.1 kA is below the 6 kA rating of the device. The RCD protecting the socket outlets is not used as the sole means of fault protection for this fixed appliance.
Traps
Relying on a 30 mA RCD as the only fault protection; using a type AC RCD on a circuit with d.c. leakage; ignoring selectivity; mixing manufacturers' devices in one board; assuming a 6 kA board suits a commercial origin; rating the device above the cable capacity to stop nuisance tripping.