MCB, RCCB, RCBO or AFDD: which protects against what?
An MCB protects the wiring from overload and short circuit. An RCCB protects people from electric shock by detecting earth leakage. An RCBO combines both in one device. An AFDD detects the arcing in damaged wiring that causes fires but never draws enough current to trip an MCB.
They protect different things, so most installations need several
This is the most common source of confusion in domestic and small commercial installations, and it leads to boards that appear well protected but are not. Each device answers a different failure, and one cannot substitute for another.
| Device | Detects | Protects |
|---|---|---|
| MCB | Overload and short circuit | The cable and the installation from overheating |
| RCCB | Residual (earth leakage) current, typically 30 mA | People, from electric shock |
| RCBO | Both of the above, in one device; check the model for module width | Cable and people, on a single circuit |
| AFDD | Series and parallel arcing faults | The building, from electrical fire |
| SPD | Transient overvoltage from lightning or switching | Connected equipment |
Why an MCB cannot catch an arcing fault
Picture a screw terminal that has worked slightly loose, or a cable nicked by a nail during construction. Current arcs across the tiny gap. The arc is intensely hot - hot enough to ignite timber, insulation or dust - but the current flowing through it may only be a few amps, because the arc itself has resistance.
To a 16 A MCB, a few amps is a perfectly normal load. It will never trip. To a 30 mA RCCB, a series arc within the circuit produces no leakage to earth at all, so it will never trip either. The fault is invisible to both devices, and it will keep arcing until something catches fire. This is precisely the gap an arc fault detection device exists to close - it analyses the current waveform for the electrical signature of an arc rather than its magnitude.
Type AC or Type A residual current device?
A Type AC RCCB detects only alternating residual current. A Type A also detects pulsating DC. That distinction has become important, because electronic equipment - LED drivers, variable speed drives, EV chargers, solar inverters, computer power supplies - can produce DC components in a fault current that will blind a Type AC device entirely.
In any modern installation with meaningful electronic load, Type A is the right default. Type A costs roughly twice as much, which is why Type AC is still specified by habit, but a device that cannot see the fault is not a saving.
Where AFDDs earn their cost
AFDDs are not yet mandatory in India, and they are not cheap. They make most sense where the consequence of a fire is severe or evacuation is slow: hospitals, schools, hotels, hostels, care homes, heritage buildings, data centres, and anywhere with timber structure or stored combustibles.
They also make particular sense in refurbishment, where the existing wiring is of unknown age and condition - which is exactly the situation where arcing faults are most likely and least visible.
Sources and further reading
Published by Electrolite Systems. Last updated
Last reviewed 3 September 2026. This guide explains how the decision is made; it is not a substitute for a design calculation for your specific installation.