With the sheer number of brands of consumer units available on the market, each offering different features and benefits, the process of picking the right consumer unit can at times be likened to the feeling that Jose Mourhino gets when trying to select his Chelsea side every week. Alan Roadway, ABB’s circuit protection specialist throws some light on what electricians should look out for when trying to pick out a match-winning consumer unit.
Establish the environment
One of the first thing electricians need to consider before making that trip to the wholesaler, is where the consumer unit will actually reside. The physical location plays a vital part in deciding what consumer unit is right for any particular application. For example, when installing in a school environment, a more ruggedly constructed consumer unit capable of withstanding harsh conditions, perhaps even mounted in a separate enclosure should be chosen.
On the other hand for domestic applications, a more pleasing-on-the-eye plastic enclosed unit might be suitable. A flush mounted consumer unit may be more suited than a surface mounted unit in certain locations where depth is perhaps an important factor, like behind a door. It’s also worth looking out for units that offer that little bit extra to make life easier.
For instance, ABB’s Housemaster range of moulded consumer units has been designed specifically with the electrician in mind. They are available with a sliding access door that stays in place either in the open or closed position, or any other position in between, thus avoiding the risk of breaking which is possible with the commonly found flap style doors. Another feature of the Housemaster that will be of interest to electricians is that they are delivered comprehensively equipped with blanking plates and all required labels to meet BS7671 regulations.
Putting a lot of thought into where the unit will be based at the start of a project will undoubtedly save time, money and one’s reputation in the long run. Just as location, location, location is a popular phrase amongst the estate agent fraternity, the same can be said when selecting a consumer unit.
Providing protection for people
Nowadays most circuit protection is provided by the use of miniature circuit breakers (MCB) and residual current devices (RCD). The two devices are used in order to protect both people and property in the event of a short-circuit, overload or earth-fault.
As you would expect, protecting human life is paramount in any electrical installation and as such RCDs are an essential part of any domestic application. They are designed to protect people by reducing the risk of electrocution and are very sensitive requiring maximum 30mA in order to operate. If choosing a consumer unit equipped with RCD protected circuits, there are various options available to an electrician. It is possible to have RCD protection to the whole consumer unit. However, with this option, in the event of an earth fault, every circuit will be shut down. If it is absolutely necessary to have a RCD main switch, it may be appropriate to use a time-delayed device together with a secondary RCD (a split load configuration) in order to achieve discrimination in the event of an earth fault.
The most common choice of consumer unit when it comes to providing RCD protection is a split load unit. This has a main isolating switch feeding some circuits – like the lighting – and the remainder RCD controlled. The main benefit of this is that it allows some circuits to carrying on functioning even when others have tripped.
Equipment protection
When providing outgoing circuit protection, the two main options open to electricians are fuses or MCBs. Traditionally overload and short circuit protection of outgoing circuits has been achieved using fuselinks
, however nowadays MCBs are the preferred choice. There are three types of MCB used to protect circuit wiring from the effects of excessive current. These are Type B – for domestic applications, Type C – normally used for commercial/industrial installations and Type D – restricted to industrial applications.
Control and command devices such as timers, relays, contactors, bell transformers and surge suppressors are also becoming more popular within consumer unit applications
Size is everything
Once protection levels have been determined, the consumer unit can then be physically sized. In determining the dimensions the main points to consider are:
- How many outgoing ways (modules) will be required
- If using a split load consumer unit, how many ways are on the RCD protected side and how many are on the non-RCD protected side.
- How many spare/future ways required?
- How many ways are needed for control /command functions
- What are the width, height and depth constraints of the installation
- Confirmation of supply and outgoing cables i.e. position/size/direction
Sizing is normally assessed as modules or ways with each module being nominally 18mm wide. The standard module width for a single pole circuit breaker is 1 module (18mm) and main switches 2 modules (36mm). It’s also worth noting that some control/command functional devices can be 3 or 4 modules wide.
Summary
In a market where there is a mass of products to choose from it’s easy to get overwhelmed. Electricians should however bear it in mind that knowing exactly what they’re after and shopping around to see which manufacturers have really taken the time out to think about them, can mean picking the right consumer unit every single time.
For more information on issues relating to circuit protection or consumer unit selection, contact Alan Roadway, ABB Ltd 02476 368500.
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