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Domestic Consumer Units and Circuit Protection Devices – A Glossary


Consumer Units, Circuit Protection Devices and Accessories – A Glossary

Consumer Unit

A consumer unit is the electrical device used to distribute power throughout a domestic dwelling. Acting as a safety device, protecting against injury through electrical shock, a consumer unit houses protection devices which also protect household appliances from overload.

A consumer unit’s first function is the organisation of electrical ‘circuits’, such that the different ways in which we use electricity throughout the home may be safely and more easily managed. A typical domestic installation, for example will have the following circuits: Ring main for power sockets and items like washing machines and TVs, Lighting, smoke alarms, cooker, shower etc.

Main Switch

This is the ‘Double Module’ device which sits on the far right hand side of a consumer. It usually has a big red bar switch and acts to isolate the entire board. This means that by manual operation it can cut power to every circuit in the installation. Main Switches are almost always rated at 100 amp, allowing it to isolate an installation which pulls no more than 100 amps in total across all circuits.


Mini Circuit Breaker. An MCB protects its circuit against overload. When for example a bulb blows on a lighting circuit the MCB cuts the power to the that circuit, thus protecting the other lights. No other circuits on the consumer unit have been affected. An MCB does not protect people against electric shock.


Residual Current Devices protect multiple circuits against earth leakage. This is a vital function of a consumer unit as it is earth leakage which causes electrical shock. RCDs measure the amount of electricity flowing into the consumer unit against the amount flowing out. When it detects an imbalance between the two it cuts the electricity supply to the bank of circuits it’s protecting.

It should be noted that this means that a fault on one circuit will result in a cutting of the power to any other circuit protected by that RCD. For example an RCD may protect the ring main and indoor and outdoor lighting circuits. In this instance a fault on the outdoor lighting circuit will knock out the indoor lighting and all power sockets.

It is for this reason that the 17th Edition Regulations demand ‘Division of Circuits’ to avoid ‘Nuisance Tripping’


Residual Current Breaker with Overload (RCBO). This clever device combines the functionality of an MCB and RCD and therefore allows for independent protection on individual circuits against both overload and earth leakage.

Din Rail

The mounting rail upon which are fitted the Main Switch, RCDs, MCBs and RCBOs.


The device which connects electrically the Main Switch, RCDs, MCBs and RCBOs.

Neutral Bar

The terminal bar which accepts all the neutral wires from the RCBOs and MCBs.


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