Circuits and Protection
Circuits and Protection
Circuits and Protection tests how a circuit is completed and protected against overload, short circuit and earth leakage, and how protective devices are selected.
Core rules and concepts
- A basic circuit needs a source, conductors, a load, a switch and a protective device. Protection must be in the phase conductor.
- Fuse: melts on overload or short circuit; cheap but needs replacement. MCB: reusable; type B trips at 3–5 times rated current for resistive and lighting loads, type C at 5–10 times for motors, and type D at 10–20 times for high inrush. Breaking capacity must exceed prospective fault current.
- An RCCB or ELCB rated 30 mA protects people against earth leakage; it does not protect against overload or short circuit. An MCB does not protect against small earth leakage. Use both.
- Earthing provides a low-resistance path so fault current trips the device and keeps touch voltage low.
- Discrimination: the downstream device should trip before the upstream one.
Worked example
A 3 kW heater at 230 V draws 13.0 A. A 16 A type B MCB and 2.5 mm² copper cable are selected. The cable's current-carrying capacity in conduit, about 18–20 A, exceeds 16 A; the MCB exceeds the load current but is below the cable capacity. Reasoning: protection must satisfy Ib ≤ In ≤ Iz, where Ib is design current, In the device rating and Iz the cable capacity.
Traps that cost marks
- Putting a fuse or switch in the neutral.
- Treating an RCCB as the only protection against overload.
- Fitting a 32 A MCB on 1.5 mm² cable.
- Choosing a type D MCB for a resistive heater.