Open the distribution board in any modern home and you will see a row of switches with labels like MCB, RCCB or RCBO. They are not interchangeable. Each one protects against a different danger, and understanding the difference could literally save a life.
MCB: protects the wires
A miniature circuit breaker trips when too much current flows. That happens in two ways:
- Overload: too many appliances on one circuit, so the current slowly exceeds the rating. A bimetal strip heats up, bends and trips the breaker.
- Short circuit: live touches neutral, and current spikes instantly. An electromagnet trips the breaker within milliseconds.
An MCB protects cables from overheating and fire. It does not protect a person who touches a live wire, because the current flowing through a body is far too small to trip it.
RCCB: protects people
A residual current circuit breaker constantly compares the current going out through the live wire with the current returning through neutral. In a healthy circuit they are equal. If some current leaks to earth, for example through a person touching a faulty appliance, the two no longer match. The RCCB detects the difference and cuts the power, typically within a fraction of a second.
For protection against shock, RCCBs rated at 30 mA sensitivity are the standard choice. Higher ratings such as 100 mA or 300 mA are used mainly for fire protection.
RCBO: both in one
An RCBO combines an MCB and an RCCB for a single circuit. If one circuit has an earth fault, only that circuit trips, instead of the whole house going dark. It costs more per circuit but gives the best protection and easiest fault-finding.
What a well-protected home board looks like
- A main isolator switch.
- One or more 30 mA RCCBs covering socket circuits, bathrooms and outdoor points, or RCBOs on each circuit.
- Correctly rated MCBs on every circuit, usually smaller ratings for lighting and larger ones for power sockets and heavy appliances.
- Proper earthing, without which shock protection cannot work reliably.
Why does my RCCB keep tripping?
- A genuinely faulty appliance, often a geyser element, washing machine or old extension board.
- Moisture in outdoor sockets or damaged cables.
- Neutral wires from different circuits mixed up in the wiring.
- Many appliances with small normal leakage adding up on one RCCB.
Unplug everything, reset, then plug items back one at a time to find the culprit. Never bypass an RCCB because it trips. It is doing its job.
Frequently asked questions
Is ELCB the same as RCCB?
People often use the names interchangeably. Older voltage-operated ELCBs were a different and less reliable design. Modern installations use current-operated RCCBs.
Do I need an RCCB if I have good earthing?
Yes. Earthing and RCCBs work together. Earthing gives fault current a safe path, and the RCCB disconnects the supply quickly.
How often should I test it?
Press the test button every few months. If it does not trip, call an electrician.