MCB, RCCB, ELCB: What the Switches in Your DB Box Actually Do

Two completely different jobs

Most people assume every switch in the DB box does roughly the same thing. They do not. There are two different dangers in an electrical system, and different devices handle each one.

Danger one: too much current in a cable. Cable heats up when current flows through it. Push too much current through and it gets hot enough to damage its insulation and eventually to start a fire inside a wall. The device that prevents this is the MCB.

Danger two: current escaping the circuit. Current is supposed to go out on the live wire and come back on the neutral. If some of it leaks out, through damaged insulation, through water, or through a person, it is not coming back. The device that detects this is the RCCB.

An MCB protects your building. An RCCB protects your family. Neither one substitutes for the other, and this is why you need both.

MCB: the small switches

MCB stands for miniature circuit breaker. You have one for each circuit, and the number on it is its current rating in amps.

Common ratings in a Singapore home are 6A or 10A for lighting circuits, 16A or 20A for socket circuits, 20A to 32A for a water heater, and 16A to 20A for an aircon unit.

That number is not arbitrary and it is not a preference. It is chosen to match the cable behind it. A 2.5 square millimetre cable can safely carry a certain current, so it is protected by a breaker that opens before that current is exceeded. The breaker exists to protect the cable, not the appliance.

This is why raising the rating of a breaker to stop nuisance tripping is so dangerous. The tripping is a symptom. Fitting a bigger breaker removes the symptom while leaving the cable in the wall unprotected, so instead of a breaker opening you get a cable overheating. It is the electrical equivalent of putting tape over a warning light.

MCBs respond to two situations. A sustained overload, meaning too many appliances on one circuit, takes seconds to minutes to trip. A dead short, meaning live touching neutral or earth directly, trips almost instantly.

RCCB: the larger one with the test button

RCCB stands for residual current circuit breaker. Its logic is elegantly simple. It continuously compares the current going out on the live against the current returning on the neutral. In a healthy circuit they match exactly. If they differ by more than a small threshold, current is leaking somewhere it should not be, and the device disconnects.

For domestic protection the threshold is typically 30 milliamps, which is 0.03 of an amp. To put that in context, an MCB might be rated at 20 amps, roughly 600 times larger. The RCCB is looking for something far smaller than the MCB will ever notice, because the amount of current that can kill a person is far smaller than the amount that will overheat a cable.

It also acts fast, generally within about 30 milliseconds. That speed is the whole point. It is designed to disconnect before the current passing through a person can stop their heart.

This is why an RCCB trip usually means something more concerning than an MCB trip. It is not saying "too much load." It is saying "current is going somewhere it should not be going."

ELCB: the older term

You will hear ELCB used, particularly by older electricians and in older Singapore flats. It stands for earth leakage circuit breaker.

Strictly, the older voltage operated ELCB is a different and now obsolete device that measured voltage on the earth conductor rather than current imbalance. It had real limitations, including scenarios where it would fail to detect a fault.

In practice, most people in Singapore today use ELCB and RCCB interchangeably to mean the earth leakage protection device in your board. If your flat genuinely has an old voltage operated ELCB, that is a strong reason to consider a board upgrade, because a modern RCCB is meaningfully better protection.

RCBO: both jobs in one device

An RCBO combines both functions in a single unit, one per circuit. Instead of one RCCB covering the whole flat, each circuit gets its own combined overcurrent and earth leakage protection.

The advantage is containment. With a single RCCB covering everything, a fault in the bathroom takes out your lights, your fridge and your router at the same time, and you have very little information about where the problem is. With RCBOs, only the affected circuit goes off, which is both more convenient and much faster to diagnose.

A reasonable middle ground, and increasingly the standard approach, is splitting the flat across two RCCBs so that essential circuits and non essential circuits are separated. If your board is being replaced, this is worth asking about.

The test button, and why you should press it

Every RCCB has a test button. Pressing it deliberately creates a small imbalance to verify the mechanism still works. Do this every few months. It takes ten seconds.

Press it. The device should trip immediately and your power should go off. Switch it back on. Done.

If it does not trip, the device has failed and you are currently unprotected. Everything in your home works normally, so the only way to find out is to press the button, or to have someone get a shock that should have been prevented. Before testing, be aware that your power will cut, so shut down computers and be mindful of anything with a clock or a defrost cycle.

How this maps onto what you experience

One small switch off, rest of the flat fine. An MCB. Overload or a fault on one circuit.

Whole flat dark, one large switch off. The RCCB. Earth leakage somewhere, most commonly a water heater in Singapore homes.

Everything off including the main switch. Either a supply issue outside your flat, or a serious fault.

Nothing tripped but a circuit is dead. Not a protection device operating. Usually a loose connection or a failed component.