Prefabricated wiring: Time to plug and play

As the techniques for carrying out wiring have evolved, many contractors have turned to using prefabricated or modular wiring (otherwise known as ‘plug and play’), as opposed to traditional methods.

In recognition of this change and because there wasn’t a standard for the total system, BS 8488-1 was published earlier this year. Previously the only guidance for wiring had been BS 7671. Individual components such as the connectors or the distribution board have their own individual product standards.

Before looking at the new British Standard 8488-1 and its effect on prefabricated wiring, we must first consider the benefits of using radial topology over the ring circuit.

Ring versus radial circuits

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 the rest of the world. As the ring-main system is firmly entrenched in the set-up of electrical circuits, few contractors spend time considering its merits and drawbacks. However, there is a strong case for arguing that it is now time to start doing so.

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 balanced.

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

The results showed that relatively small changes in the resistance 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.

This means the heating effect in the path carrying the larger current is much greater than expected. In fact, Schneider Electric’s tests show that a standard 2.5 mm2 PVC-insulated cable used in a ring-main circuit protected by a standard 32 A miniature circuit board (MCB) can generate as much as 25% more heat than a similar cable used in a radial circuit protected by a 20 A MCB with the loads being equal. This additional heat will be insufficient to create a short-term hazard but it is still significant.

In the situation where a ring-main circuit is protected by a standard 32 A MCB, it would be easy to think the maximum total current that could flow in this circuit would be equal to the breaker’s nominal rating, that is, 32 A. In fact, this is far from correct.

A reading of BS EN 60898 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 the ring-main protected by a 32 A breaker, a current of up to 46.4 A could flow for as long as an hour.

This is clearly unacceptable, especially when coupled with the potential for current imbalance highlighted earlier. The main result in relation to a ring-main is additional cable heating. Again, does this matter?

Definitely, as the operating temperature has a large effect on the working life of cables. NICEIC figures reveal standard PVC cable conductors operating at 70ºC have a dependable working life of 69 years, based on an eight-hour day. While this may be acceptable, if the temperature rises by just 5ºC, the life of the cable is halved to around 37 years. If the operating temperature of that cable conductor reaches 100ºC, it will last just two years.

It is tempting to think such high temperatures would never be encountered in practice, but before making this assumption, take heed of these important facts. Modern buildings make extensive use of materials with excellent thermal insulating properties, and cables embedded in such materials can show remarkably large temperature rises.

Weighing up the cost

It might be possible to conclude the technical compromise of using a ring-main topology is offset by its cost benefits. This is another proposition difficult to support. Ring-mains can of course support multiple sockets, which suggest 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 the chances of all or even most of the sockets on a ring-main being used at the same time was slim.

Today, the situation is very different. Not only are all the sockets likely to be used, but there is a very good chance that some will have extension leads plugged into them to provide even more socket outlets.

It is also necessary to bear in mind that, in line with the 17th 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 definite in modern installations. When these factors are combined, the apparent cost benefits of ring-mains disappears.

These considerations lead to the inevitable conclusion that ring-mains are technically inferior and offer no cost benefits. The argument in favour of their continued adoption in new electrical installations is weak.

Female connector

BS 8488-1 specifies the safety requirements of prefabricated wiring systems and addresses concerns customers, users and safety organisations had about the system, laying down the minimum conditions that a manufacturer must conform to.

One of the main clauses in the standard specifies that the supply point of the circuit must be with a female connector. Some manufacturers who offer ring-circuit solutions have used both male and female connectors on the supply side, but under the new standard this is illegal. It prohibits ring-main circuits being wired with outgoing power on the male connector.

Additionally there is a safety concern over adding socket outlets to ring circuits. As we are all now aware, the practice of live working is not permitted by the Electricity at Work Regulations.

With a ring circuit, if an additional socket is needed, the male and female connectors need to be disengaged. At some point in this circuit there is the potential for a male connector to act as a supply connector, which is illegal. In a radial circuit the female connector always acts as supply connector.

Accessibility

The accessibility of connections involving junction boxes in prefabricated wiring, such as connector boxes for switching lighting circuits, has been open to speculation. The 17th Edition (clause 526.3) states: “Every connection shall be accessible for inspection, testing and maintenance.”

But there are exceptions such as if a product has its own standard. If a product does not, it will fall under the scope of BS 5733: Specification for general requirements for electrical accessories.

Schneider Electric has been an active member of the BS 5733 working group. The standard does not come into force until 2010, but it is something that manufacturers, contractors, architects and consultants should be aware of now.

Where junction boxes or similar items are used, they will only be considered maintenance-free if they have screwless terminals. Anything else will need to be accessible, and this will need to be incorporated into the fabric of the building.

There is a clear argument for the need to implement prefabricated wiring systems throughout new-build and rewiring schemes. Modular solutions can speed up the process of wiring an installation, so the earlier they are incorporated into the design, the greater the benefit, particularly when used as part of an integrated installation system.

Prefabricated wiring has developed as manufacturers continually push the boundaries of innovation, but by becoming more widely used it has been recognised that it requires its own standard. With the new standard in place, this approach should be embraced by the electrical industry.

Originally published in EMC September 2009 as "Time to plug and play"