Installation and Protection Devices
What the domain tests
Installation and Protection Devices tests selection and coordination of fuses, MCBs, MCCBs, RCCBs/ELCBs, earthing and surge/lightning protection. You must know what each device does-and does not do.
Core rules
- A fuse melts on overcurrent; an MCB trips on overload or short circuit and can be reset. Its rated current must protect the cable, so the cable’s current rating must be at least the device rating.
- Breaking capacity must exceed the prospective fault current at the point of installation.
- An RCCB detects residual current to earth and protects people; a 30 mA RCCB is used for shock protection. It does not detect overload between phase and neutral.
- Earthing provides a low-resistance path so protective devices operate and touch voltage stays low.
- Discrimination means the device nearest the fault trips first, limiting the outage.
Worked example
A 3 kW single-phase load at 230 V draws 13 A. Select a 16 A MCB and a cable whose corrected current rating is at least 16 A. Add a 30 mA RCCB for socket-outlet shock protection. Reasoning: the MCB handles overload and short circuit; the RCCB handles earth leakage. Replacing the MCB with a 32 A unit “to stop tripping” would leave the cable unprotected.
Traps
Do not use an RCCB as the only overload protection. Do not fit a fuse or MCB rated above the cable capacity. Do not confuse ELCB voltage-operated and RCCB current-operated types. Do not assume earthing eliminates the need for RCD/RCCB protection.