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The Egan report identified off-site prefabrication as one of the key methods that could be employed to improve the construction process. One area where this is becoming prevalent is cabling.

Established for over ten years in the UK now, pre-fabricated wiring systems for lighting installations have gained widespread acceptance. Faster, cheaper, easier to install, flexible and adaptable – these are the stated benefits of pre-fabricated wiring systems.

Installing a system
The initial step in the installation of a wiring system, and usually the first fix item, is the distribution of the power.

One method of achieving this is to connect a primary distribution hub (PDH) to a distribution board via a home run cable. The home run cable is supplied prewired to the primary distribution hub to a predetermined length and is connected to the distribution board in the traditional manner.

The length is determined from the lighting layout drawings, and typically an additional five metres is supplied to cater for unforeseen on-site circumstances. It is usually laid within basket trays or fixed directly to the building slab.

The PDH provides power from the distribution board locally. It has a number of outputs that can supply up to eight independent circuits. These outputs may be mixed to provide both lighting and power for variable air volume or fan coil units, although connectors with differing mechanical codings are used to prevent misconnection.

Having distributed the power as needed for the installation, it is then simply a matter of plugging in the relevant components to achieve the desired functions and lighting configuration.

There are two typical configurations used: the spider and daisy chain approaches.

The spider approach
The typical spider approach is illustrated in figure one. The pre-wired flex (power from the PDH) is simply plugged into the local PDH supplying power to the zone distribution hub (ZDH).

The spider lead is a male to free end cable assembly. The free end is typically hard-wired into the luminaire (normally by the luminaire supplier) so that the fittings can be simply plugged into the ZDH on site.

The possibilities for the control input are numerous. Of course they are also inter-related to the luminaire's functions, such as dimming. However, local one-way switching, passive infrared control and infrared manual switching are all possible even with the most basic of systems, and in most instances are simply plugged in.

While this configuration is common, if on-site simplicity and standardisation is to be achieved, it is important to note that there are some restrictions to the capabilities and ultimate flexibility possible.

For example, normally only one type of control input is possible. This means that an entire group of luminaires is controlled by this single form of input. Should there be a future need to partition the area the luminaires were originally intended for, and some of the luminaires are to be used in this newly partitioned area, another ZDH may be required, together with more hardware to enable this function.

The configuration also assumes that the PDH has sufficient spare capacity to cater for this unknown future possibility. Each ZDH is only able to support a predetermined number of luminaires.

That said, for a repetitive application this approach may prove most viable.

Daisy-chain approach
With the daisy-chain approach, again the pre-wired flex is simply plugged in. However in this instance there is no need for the ZDH (see figure 2).

The extender leads are a male to female cable assembly. These simply plug into the luminaires, which can be pre-fitted with 'T' modules.

The 'T' module is simply fitted to the luminaire via a 20 mm bush and lock-nut. This operation is commonly undertaken by the luminaire manufacturer so the fittings arrive on site ready to be simply plugged in. These modules have a locking facility and are colour-coded to prevent accidental misconnection. There is also a version available to adapt downlights.

The possibilities for control input encompass all those achievable with the spider approach. However, they are extended and two-way, intermediate switching and dimming can easily be introduced. In figure 2, one-way local switching is shown.

There is really no downside to this approach. Should we wish to partition our hypothetical office, a plug-in module is available which provides an uninterrupted power output. This can be used to tap-off to the lower four luminaires and easily plug in any type of new switching requirement for these.

The amount of hardware is reduced as no ZDHs are required, which in turn means less installation is needed. Additionally, there is no restriction on the number of luminaires that can be connected using this method other than the electrical properties of the connectors and cable.

A foolproof system?
Fortunately there are many differing colour and mechanical codings available in the market, ensuring that it is impossible to misconnect any device or product.

Within the Metway Net-Works' product range a number of conventions have been adopted to guarantee a safe and simple installation. For example, brown connectors are used for two-way and intermediate switching; green connectors for presence detection and control; black and white for input/outputs; and so on.

Of course, with any system it is most important to follow the manufacturer's instructions and installation guidelines. Additionally, it is commonsense to actively involve the wiring system supplier at an early stage of a project and throughout its duration. These companies are there to support you and it is in everyone's interest to ensure that the project runs smoothly and to time.

Proper co-ordination and communication is also important, particularly between the luminaire supplier and the wiring systems manufacturer. The systems supplier does need to know which company's fittings are being used, the fittings specifications, and so on.

None of this is rocket science, but without proper communication no project can progress as smoothly as it otherwise might have.

Standards and BS 7671
At present there are no standards which specifically cater for prefabricated modular wiring systems. This area is being addressed though, with the active involvement of the BRE and the ECA.

So what can you do in the meantime?

• Ensure that your wiring systems supplier manufactures within the framework of the ISO 9000 procedural system.
  
• Ensure the components used carry some recognised international approval, for example, Nemko, Demko or CS IMQ.
  
• Ensure that all cabling is low smoke and fume (LSF) or low smoke zero halogen (LSOH).
  
• Carry out voltage drop calculations at the design stage.
  
• And most importantly, treat the installation as if it were a fixed installation.

That means the same good practice should be applied to this as any other type of wiring installation, complete with proper testing.

Applications
So where can I use a modular wiring system?

Generally speaking, the most common (and most cost- effective) projects are commercial offices with suspended ceilings. This is the bread-and-butter application and usually where most benefits are realised.

In the case of speculative developments, the almost infinite flexibility and easy reconfiguration of a wiring system can be fully exploited. If the tenant wants presence detection, cellular offices with user-controlled, infrared switching on their desks, or just a big off switch, all of these facilities can be easily introduced in minimal time and with little disruption, so there's no need to worry about ruining the paint job.

That said, such wiring systems have been used in hospitals, warehouses, department stores and hotels. The viability will depend upon many factors so talk to your supplier.

So what does the future hold?

Already we can satisfy the majority of requirements for lighting and small power. The next generation of wiring systems will likely feature fully integrated plug-in lighting control from one manufacturer to cater for that increasingly common specification sentence: 'install a wiring system that will allow the introduction of lighting control by the tenant at a later date'.

Prefabrication and wiring systems are well established and here to stay. It is one of the few products where there really is no downside for anyone.

Datacomms, cctv, security...no doubt these systems will be encompassed within wiring systems at some point in the future. The only question is: when?