Type of consumer unit

What you should know when having your consumer unit changed.

We currently fit one of two types of domestic consumer unit. One is called the split load RCD board the other is the RCBO board. See the summary at the bottom of this page for a brief explanation of the difference. Or read on for a little more detail.

So what is the difference?

Each circuit needs to be protected. It needs to be protected from having too much power drawn down the wires. This protection is often in the form of an MCB or micro circuit breaker. This is the trip switch that will trip if too much power is drawn down the cable. If too much power is drawn down the cable the cable will get hot and potentially over heat, melt and catch fire. So the MCB is very important. The MCB Protects the cable.

A typical MCB

On a split load board there will be a number of circuits protected by MCBs which are grouped together under an RCD. A residual current device.

What does an RCD do?

In summary the RCD is used to protect the human. If there is a fault on the circuit or in an appliance connected to the circuit the RCD will trip in milliseconds, preventing the possibility of an electric shock.

A typical RCD

An RCD will always have a test button, which by the way should be pressed every 6 months.

Why is it called a split board?

The consumer unit will have two groupings of MCBs with an RCD.

This is an example of a Split load RCD board. It has a main switch on the far right. Two RCDs with a blue test button and each RCD has 4 MCBs protected circuits.

What is an RCBO board?

Another device that can be used to protect a circuit is called an RCBO. This does both the job of the MCB and the RCD. It protects the circuit form having too much power drawn down the cable and also looks for faults on the circuit.

This is an example of an RCBO board. Each circuit has and RCBO doing the combined job of the MCB and RCD. See the yellow test buttons on the RCBOs. This particular board also has surge protection installed. I will write an explanation for the SPD (Surge protection device) shortly.

So what is the benefit of one board over the other?

On a split load board where circuits are group to one RCD, you can get the situation where the is a fault on one of the circuits which will trip the RCD and then all the circuit on the half of the board are now turned off.

With an RCBO board, only the circuit with the fault will trip. All the other circuits are unaffected.

An example where this is particularly useful is if you have a fault on a lighting circuit which is on the same RCD as the kitchen appliances, including the freezer, then all the kitchen appliances will be affected by the RCD tripping due the fault on a lighting circuit. Until the fault is diagnosed the RCD will not reset and the appliances will remain without power.

So in summary.

RCBOs are designed to protect individual circuits from both overload (drawing to much power in layman’s terms) and earth faults (potential for electric shock) whereas RCDS only provide earth fault protection and normally protect multiple circuits with the overload protection provided by other means, typically miniature circuit breakers, Under fault conditions the advantage of having RCBOs installed are you will only loose the individual circuit where the fault is present instead of multiple circuits which would be typically protected by MCBs and an RCD. The only disadvantage of RCBOs when compared to RCDs and MCBs is the increased cost to purchase as opposed to RCDs . However the cost of RCBOs is coming down.