Consumer Unit

The consumer unit or fuse board as it is also know is often overlooked. For most people it is tucked away in a cupboard somewhere and is ignored until it trips. So what is it for? The answer is safety. The following sections look at why it is there:


Fundamentally, you could wire everything on one circuit. We don’t because it would be prohibitively expensive and inconvenient to run heavy copper cables all around the house. So the consumer unit breaks down the supply into smaller “final circuits”. Typically these are:

  • sockets
  • lights
  • cooker
  • shower
  • Immersion heater

In this way the supply to the house are broken down into groups. Each group can now be wired with smaller cables suitable to how much current is expected to be used. It has another advantage in that is a fault develops only one group is affected and can be switched off without losing the entire electrical supply.

Fault Protection – Overcurrent

A Consumer unit will have micro circuit breeakers (MCBs), fuses or fuse wire to protect the circuit from too much current. The modern MCB has the advantage that it is more sensitive than a fuse and can be easily reset.

Too much current often arises for different reasons:

  1. A Fault : such as cutting through the cable with a lawnmower. In this case the simultaneous contact of the blade with the earth conductor or the live contacting the ground causes a huge draw of current. A MCB will trip very quickly.
  2. Overloading : plugging in many electric fires on a cold winters day may draw more current than the circuit was designed to cope with. The MCB will not trip immediately, as the circuits are designed to allow the use of more current than the rating on the MCB (fuse).

In the first case the MCB often protects people from being exposed to electricity for a long period of time. In the second case the MCB protects the cables from permanent damage and potentially a fire starting.

Additional Protection – Residual Current Devices

In 1991 the sixteenth edition of the wiring regulations required all socket outlets to be protected by a Residual Current Device (RCD). The seventeenth edition has extended their use to provide additional protection to people.

An RCD rated at 30mA is designed specifically to save life. 30mA is the current that a healthy person can withstand. Like the phrase “we counted them out and we counted them back in” employed in the war about pilots, An RCD does the same thing with current. If less comes back than goes out something has gone wrong and the RCD trips.

A consumer unit installed to the seventeenth edition will in general have at least two RCD devices.

Must I change my Consumer Unit

The answer is NO. Although, it is adviseable to have the additional protection offered by a modern consumer unit.

When alterations are made to your electrical installation, a new consumer unit may be recommended. Any alteration will require that the circuit affected meets current regulations. With old consumer units it is often simpler and more economic to change the old consumer unit for a new one.

When considering the changing the consumer unit, the age of the wiring must also be considered. The new consumer unit will give you more protection and be more sensitive to faults. If the wiring is old changing the new consumer unit is very likely to highlight any deterioration.