What is an RCD, MCB and RCBO?

In Plain english: What is an RCD, MCB and RCBO?

If you have had an electrician look at you home electrics then you will undoubtedly heard reference to RCD protection, whether you have it or not. You may even have heard about correctly rated MCBs or the benefits of RCBO… even with some basic knowledge of home electrics this probably meant very little to you.

That’s OK! It is confusing, here is the best explanation you are going to find anywhere(and it’s even written in plain english!):

MCB A.K.A Breaker or Fuse

An MCB stands for Miniature Circuit Breaker. This is the modern equivalent of a fuse. The purpose of an MCB is fire protection (despite the general public's misconception the main reason it is there, is to stop you getting an electric shock):

It’s simple reason is to stop the cable from drawing more current than it should do, overheating and catching fire.

For example: The most a 1mm lighting cable is safely able to take is up to 14 Amps of current anything more than this the cable will overheat and in turn catch fire.

Therefore to stop it from drawing more current/ overheating, we will fit a 6 or 10 amp MCB. So if there is a fault and the cable begins to draw more than the 6 or 10 amps, the MCB will turn off.

Had the cable (only safely able to have up to 14 Amps through it) been protected with a MCB larger than the rating of the cable it would have allowed more energy than it should. For example: a 20A MCB would have allowed an extra 6 amps to flow through the cable before turning off the supply. A 32A would have allowed 18 Amps (over twice the safe rating of the cable!) to flow. This ultimately would have led to the cable overheating and catching fire.

So when people have a tripping fuse and rather than find the cause, they just increase the size of the fuse in order to stop it, they actually were actually creating an extremely dangerous situation and a risk of fire.


RCD stands for Residual Current Device. This is the device that saves you from an electric shock.

The RCD looks for a different type of problem to an MCB (fuse). An RCD is looking for an imbalance between the Live and Neutral of the circuit. The simplest way to explain this is imagine a set of old fashioned scaled, where both sides need to be equal. On one side is the amount of Live current going out and on the other side is the amount of current coming back through the Neutral. The amount of each must be the same in order for the scales to balance.

Therefore if there is more current is going out on the Live than coming back on the Neutral, our scales do not balance. In reality, the scales not balancing means the RCD turning off.

But where does the current go for our scales not to balance? Well the simple answer is to earth, through you or waiting to go through you from a fault on the electrical system. The exact cause will be specific to the nature of the fault in your house at that time. However the mechanics of an RCD tripping is based upon the above, the reason is to protect you from an electric shock and probably save your life.


RCBOs are the hybrid of an RCD and an MCB. It stands for Residual Current Breaker with Overload protection. This device will prevent the cable from drawing more current than it is rated at (like a MCB), and it will trip whenever there is earth leakage like an RCD.

We fit RCBOs as standard in most jobs rather than an MCB/ RCD combination. The benefits of this method are outlined in another post.