Anatomy of a Consumer Unit

What is a Consumer Unit?

A Consumer Unit or Fuseboard is the single most important electrical device in any building. It protects electrical appliances from damage and more importantly, human beings from electrical shock.

Whilst each installation requires individual design and set up there are highly standardised and regulated approaches to configuration. Let's first look at what makes a consumer unit and what goes inside it.

Components of the Fuseboard

1. Enclosure - The 'box' which houses all the components. This used to be either metal or plastic, (also known as 'insulated'). However, on 1st January 2015 a significant change to the 17th Edition of the IET wiring regulations came into practice. BS 7671: 2008(2015), known as 'Amendment 3' stated that all consumer units must be made form non-combustible material. As a result, all enclosures are now formed in steel.
2. Neutral Bar/s - The common connection point for the neutral cables from the earth leakage protection devices (RCDs & RCBOs).
3. Earth BarEarth Bar - The common connection point for all circuits' earth cables. All circuit protective conductors (CPCs) will connect on the earth bar along with earth bonding connections.
4. Din Rail - The metal bar onto which all the devices are mounted. This sits on the inside back of the consumer unit, facing outwards. It does not carry any electrical current.
5. Busbar - A copper 'comb toothed' bar which electrically connects the 'circuit devices' to the 'isolation devices'. The busbar is cut to size by the electrician, when installing the consumer unit and 'plugs in' to the bottom of all devices. In our image below you can see the plastic insulation of three strips of busbar, connecting the main switch to the RCBO and the two RCDs to their banks of MCBs.
6. Main Switch - As the name suggest this is the master switch for the consumer and all devices housed within it. Turn this switch off and all power is cut to the devices, meaning that all circuits protected by this consumer unit are now dead. Electricians will usually use this 'master off' when carrying out works in premises with which they are unfamiliar.
7. MCB - Mini Circuit Breaker - Protects appliances from overload. They come in a range of amperages from 6a to 50a in domestic installations, but are available in much higher amperages for commercial environments. Read our article: 'What is an MCB and how does it work?'
8. RCD - Residual Current Device - An RCD protects humans against electric shock by measuring the total outgoing current against the total incoming current. If the two are different, obviously some electricity has gone missing - known as 'earth leakage'. The RCD rapidly cuts power to those circuits. An RCD protects a bank of MCBs and these days it is common to see two RCDs in a consumer unit. (Larger duplex boards of over 24 circuits can have 3 RCDs). Read our article: 'What is an RCD and how does it work?'
9. RCBO - Residual Current Overload - An RCBO is a clever piece of kit which combines the functionality of both an MCB and RCD, protecting against both overload and earth leakage. They are used on any circuit deemed critical by the homeowner such as fridge freezers, alarm systems, tropical fish tank pumps or certain lighting circuits for example.
10. Blanks - Blanks are pieces of plastic which clip to the din rail in just the same way as the MCBs, RCDs and RCBOs and allow for future changes and additions to the circuit design. Imagine the home owner adds a circuit of outdoor lighting for example - the consumer unit and busbar are ready to accept the MCB which would protect this circuit without any reconfiguration of the consumer unit.