What is an Electrical Consumer Unit?

Fuse Boxes Explained

A consumer unit is an electrical box with breakers between the electricity meter and all the other electrical circuits in the house. Bigger installations may use a distribution board instead of a consumer unit. It is often called a fuse box, but modern boxes do not contain fuses anymore, but plug-in Miniature Circuit Breakers (MCB's) or RCD's (Residual Current Device). MCB's are better than fuses, because they can be reused after multiple trippings, whilst a fuse must be replaced.

A series of MCB's and usually one RCD are fitted onto a DIN rail, but in the case of a split consumer unit you may have more RCD's.

The MCB is an electrical switch, designed to protect an electrical circuit from overload or short-circuit. When it detects a fault, it interrupts the electrical flow by tripping. Contacts inside a MCB must carry the load without getting too hot and must also withstand the heat produced by the arc when it trips.

What happens when a short circuit occurs?

An arc is generated when the current is interrupted. This is to happen in a controlled way by containing, extinguishing and cooling it well enough to enable the gap between the contacts to again withstand the voltage in the circuit. This arc can be controlled in various mediums, including air, vacuum, oil or insulating gas. Once the cause of the arc has been dealt with the contacts must be closed again to restore power by simply switching on the MCB.

When short-circuits occur, It is usually accompanied by a current many times greater than normal. Although the contacts open and interrupt the electrical flow, an arc can still form between the contacts, causing the current to continue. This can further lead to conductive ionized gasses being created and metal being vaporised and this can cause further continuation of the arc, or short-circuit. Ultimately the circuit breaker may explode, with all the equipment surrounding it.

To prevent this the arc must be effectively extinguished, Air-insulated circuit breakers often use metal plates or ceramic ridges as an arc chute structure to extinguish it, with magnetic arc blowout coils to deflect the arc into the arc chute. Other larger circuit breakers may use other means, such as oil to extinguish the arc. Some of the oil is vaporised, which forces a jet of oil through the arc.

Other types of circuit breakers

Vacuum circuit breakers are very effective, with minimal arcing, because there is nothing to ionize apart from the contacts and they are used extensively in bigger installations. Gas circuit breakers use an inert gas like sulfur hexafluoride to sometimes stretch the arc, using a magnetic field, while the dielectric strength of the gas quenches the stretched arc. Circuit breakers still in good condition should be able to interrupt the current in less the 150 ms.

To open these contacts when a fault is detected, mechanically stored energy within the circuit breaker is utilized to separate the contacts, such as compressed air or springs. Sometimes some of the energy required may come from the fault current itself. Circuit breakers have different values depending on what it is used for. For instance, MCB's of 6A may be used for lighting circuits, 16A for single sockets, 32A for 7kW electric cookers and 40A for larger electric cookers.

The Residual Current Device

The Residual Current Device in the consumer unit is an extra protective device that replaces the isolator switch. In some cases where it does not protect all the circuits, an isolator switch will be included to disconnect everything and this is then called a "split load" consumer unit.

RCD's are designed to prevent electrocution. It can detect current flowing to earth, which is not supposed to be the norm, but that is what happens when someone touches the wrong parts. They can be made far more sensitive than MCB's - usually 30 mA as opposed to, for instance, the 6 amps for lighting MCB's. With this sensitivity other problems can be detected, such as a breakdown of the live/earth, which can generate heat and cause fires. This makes them unsuitable for protection against overloads and therefor must be used in conjunction with MCB's.

Only one RCD is required with each section containing MCB's, but in the UK they are also required to conform to regulations. With a separate RCD that protects only the lights and a few other things, you may have lights when everything else has tripped. They are mainly used to protect sockets that may supply power to equipment outside the house, like lawn mowers, sheds and lights.

RCD's are not guaranteed to protect against electrocution and should be trip-tested once a month using the test button. This may be inconvenient when you have to reset certain devices in the house after a test. Death from electrocution usually results from the duration of contact and the current and RCD's react within 0.04s with a leakage of 30mA or more.

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