Building Services - Preassembled standards - reliable and cost effective electrical installations

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Modern Methods of Construction (MMC) is the term  used to describe a range of technologies and processes, all of which are designed to aid the efficient and cost-effective construction of buildings. The MMC concept has received many accolades and much encouragement, not least in Sir John Egan's ‘Rethinking Construction' report which was produced for the government in 2001. Keith Ball and Tom France from Schneider Electrical Building Systems and Solutions investigate

Until now, MMC has largely been confined to the building fabric, and has been variously defined as off-site construction (OSC); pre-engineered building; system building; volumetric; modular; and - its original name - prefabricated construction.

Now, in a development that represents a significant and important shift within the electrical industry, Schneider Electric is applying the principles of MMC to electrical installations for buildings. Of the above definitions, those which most closely match Schneider Electric's MMC initiatives are offsite construction and system building.

However, the company's development of Preassembled Standards, which lie at the core of the company's MMC offer, are far more than just a way of simplifying electrical installations. For this reason, this paper is divided into two parts. The first deals with fundamental thinking about the design of electrical installations, the second with the provision of tailored, factory built, easily connected electrical system solutions.

Circuit design
For more than fifty years, the UK has been one of a very few countries to adopt a ring-main topology for the majority of electrical installations, rather than the radial topology favoured in most of the rest of the world. So firmly entrenched is the ring main system that few specifiers or engineers ever spend time considering its merits and demerits. As we shall see, there is a strong case for arguing that it is now time for them to start doing so.
Let's look at some of the facts behind this statement. By its very nature, a ring main delivers power from a distribution board or consumer unit to the loads via two parallel paths. There is an implicit assumption in the design of ring main systems that the current in the two paths will be more or less balanced.

To test the validity of this assumption, Schneider Electric carried out extensive tests in its own ASTA-accredited laboratory. A conventional ring main was simulated with all of the socket outlets loaded, and the current in each of the two circuits linking the sockets to the power source was carefully monitored.

The results were surprising. Relatively small changes in the impedance of the wiring were all that was needed to produce a large imbalance in the currents. In fact, ratios of up to 2:1 could be produced under conditions that could easily be encountered in a real installation.
Does this matter? It certainly does, because the heating effect in the path which is carrying the larger current is much greater than might have been expected. In fact, tests show a standard 2.5mm2 PVC-insulated cable used in a ring main circuit protected by a standard 32A MCB can generate as much as 25% more heat than a similar cable used in a radial circuit protected by a 20A MCB, the loads being equal.

Almost certainly, this additional heating will be insufficient to create a short-term hazard but it is nevertheless significant, as we shall see shortly.

Now let's look at another aspect of ring main circuits protected by standard 32A MCBs which comply with BS EN 60898. It would be easy to think that the maximum total current which could flow in such a circuit for other than a very short period of time would be equal to the nominal rating of the breaker - that is, 32A. In fact, this is far from correct.

A reading of the standard will reveal that an MCB can carry a current of up to 1.45 times its nominal rating for up to an hour without tripping. So, in our ring main protected by a 32A breaker a current of up to 46.4A could flow for as long as an hour. This is clearly undesirable, especially when considered along with the potential for current imbalance discussed earlier.
The principal result of the shortcomings we have identified in relation to ring main systems is additional cable heating. Again, does this matter? Emphatically yes, as operating temperature has a large effect on the working life of cables.

This is an aspect of electrical installations that is rarely considered, but it certainly deserves closer attention, as figures provided by NICEIC reveal. These show that standard PVC cable conductors operating at 70ºC have a dependable working life of 69 years based on an eight hour usage day.

That may be acceptable. But if the temperature rises by just 5ºc, the life of the cable is halved to around 37 years, which is less likely to be acceptable. In fact, if the operating temperature of that cable conductor should reach 100ºC, its life will be just two years.
It's tempting to think that such high temperatures would never be encountered in practice but, before making this assumption, it's important to remember that, in their construction, modern buildings make extensive use of materials with excellent thermal insulating properties. Cables embedded in such materials can show remarkably large temperature rises.

Let's recap on the issue of radial versus ring main topologies. Ring mains, as we have seen, can have higher cable temperature rise characteristics. With radial circuits, the maximum potential temperature rise is lower, because the protection is provided by a 20A MCB rather than a 32A MCB, and it's also much more predictable, because there is no uncertainty about how the current will divide in the cabling.

If ring main topology is a rather poor technical compromise, surely this is offset by its cost benefits? In fact, this is another proposition that is difficult to support.
Ring mains can, of course, support multiple sockets, which suggests that they are economical in the amount of cable required. When they were first introduced, this might have been true, as far fewer electrical appliances were in use, which meant that the chances of all, or even most of the sockets on a ring main circuit being used simultaneously was slim.
Today, the situation is very different. Not only are all the sockets likely to be used, there's a very good chance that some will have extension leads plugged into to provide even more socket outlets.

It ‘s also necessary to bear in mind that, in line with the 16th Edition of the IEE Wiring Regulations, radial circuits can feed more than one socket. In fact, multiple sockets per radial can be taken as a realistic figure for modern installations. When these factors are taken together, the apparent cost benefits of ring main installations disappear.
These considerations lead to the inevitable conclusion that ring main systems are technically inferior and that they offer no cost benefits. There can, therefore, be very little argument in favour of their continued adoption in new electrical installations.

Equipment pre-assembly
The superiority of radial wiring topology is reflected in the design of the products developed in line with the principles of MMC. Essentially, these products centre around factory-assembled distribution boards and consumer units, which are delivered to site ready for immediate use, and which use plug-and-socket connectors for connections to external circuits.

The products are individually configured at the factory to suit the user's specific requirements. They can be supplied with incoming connectors rated at up to 63A and outgoing connectors rated at up 20A. They are, therefore, ideally suited for use in installations where a radial topology has been adopted for power distribution. To ensure maximum versatility, the boards can also incorporate other protective devices such as RCCBs and RCBOs along with simple control circuitry.

The boards and consumer units are, however, only part of Schneider's MMC solution. Junction boxes and sub-distribution boards, all with tool free connectors, are also available, as are pre-wired socket outlets.

The socket outlets are fitted with a cable of user-specified length, which is terminated in a tool free connector. This means that, on-site, the installation of power outlets involves no more than mounting the socket, running the cable to the nearest distribution board or junction box, and plugging in the connector. No conventional wiring is needed.
This approach offers substantial benefits over the traditional practices of assembling consumer units and distribution boards on site, and carrying out wiring on an ad-hoc basis.
Among the most significant of these benefits are the large reduction in the amount of costly on-site labour required, and the big reductions in overall time needed to complete the installation work. Indeed, on beta test sites for the new system, which include major school projects, time savings of up to eight times have been achieved.

Another less obvious but equally important benefit is the reduced reliance on skilled labour. All of the elements making up the system are not only supplied pre-assembled and wired, but also factory tested. This eliminates the risk of errors such as fitting MCBs with the wrong ratings and means that the new system is, for all practical purposes, plug-and-play.
As no conventional wiring is used, the level of electrical expertise needed to use the system successfully is comparatively small. With shortages of experienced electrical contractors currently being widely reported, this is a benefit that can only be expected to increase in significance.

All of the equipment included in Schneider Electric's pre-assembled standards range is, as its name suggests, custom assembled from standard products to meet the user's precise requirements. Since only standard products are used in the assembly, and efficient streamlined manufacturing techniques are employed, delivery times are short and installed costs are very competitive.

Pre-Assembled Standards extend the benefits of MMC, which have for so long been available only in the major constructional elements of building projects, to electrical installations. Suitable for virtually all power and lighting applications in healthcare facilities, schools, offices and other public or commercial buildings, they are already helping users to deliver solutions which combine technical excellence with convenience, flexibility and competitive pricing.
Further information is readily available. Schneider Electric welcomes the opportunity to consult and advise on MMC at an early stage in any project. Why not take advantage of this free service to discover advantages that Pre-Assembled Standards could provide in your next project?