Prefabricated wiring systems offer huge advantages to an installation, but make sure that the system you specify is both safe and compliant with the Wiring Regulations, argues Steve York.

Using plug and play technology has always been a common solution for lighting circuits, but increasingly, prefabricated wiring systems are extending this principle right back to the distribution board.

Most notably, some suppliers claim installation time savings of up to 80% when compared with conventional hard-wired solutions. Such installations can also use semi-skilled labour for cabling, freeing up skilled labour for more value added tasks.

Meeting the regulations

With these advantages, why wouldn’t you use such systems? Unfortunately, there have been some concerns in the past and ultimately it is the designer and/or specifier who must ensure that the system is safe and fully compliant with BS 7671. Reversed polarity, under rated couplers and incorrectly sized wiring sections have all been areas of concern.

With this in mind, the technical committee responsible for BS 7671 agreed that there was need for a product standard. In response, a BSI Committee developed a new British Standard BS 8488-1. Importantly, this was then amended in November 2010 to become BS 8488:2009+1:2010.

The 17th Edition now identifies the 2010 version of BS 8488 as the version to specify.

Some of the key safety requirements and tests in BS 8488 include protection against electric shock, insulation resistance and electric strength, clearance and creepage distances and resistance to heat, fire and tracking.

You need to ensure that the system meets all of these requirements. To do this, you should check the organisation’s competency to test and issue certificates of conformity. One suggestion is to insist that the laboratory used is independently recognised to BS EN ISO/IEC 17025 for BS 8488.

You also need to check that the prefabricated wiring system provides the following details:

• Instructions for safe use
• System design information to validate conformity with BS 7671
• Information to facilitate inspection and testing.

Each cabling section should be clearly labelled and if the installer cross-checks this against an installation drawing, then this should help prevent installation errors. Labelling each wiring section is a requirement of BS 8488 anyway, which states that it must be marked with the rated current and corresponding reference method from BS7671:2008 Table 4A2.

Designing the system

The cabling is central to such systems and you need to check that all of the calculations have been done for each wiring section.

When it comes to designing the system, the rated current and cross sectional area of the wiring section conductors are determined by the following factors:

• The number of loaded cores defined by the manufacturer
• Not being grouped with other wiring systems or cables
• Not being in contact with thermal insulation
• The ambient temperature not exceeding 30^oC
• The frequency of operating being not greater than 61 Hz.

The rated current (A) must be marked on each individual section.

The system designer must then determine the required current carrying capacity of a system section by applying all relevant rating factors – such as grouping, ambient temperature and thermal insulation. This current carrying capacity may be different from the rated current.

To ensure compliance with the Wiring Regulations the designer must consider:

• The cross sectional area of live conductors
• The voltage drop
• The earth fault loop impedance for protection against electric shock (fault protection)
• The protective conductor cross-sectional area for protection against earth fault current
• The cross-sectional area of live conductors for protection against short circuit current.

Two areas that require particular attention in designing such systems are for the use of multi-core cables and the use of flexible cables.

To complete all of the calculations needed, dedicated calculation software would make the job a lot easier. If it is used, then you need to check that an independent specialist has validated the calculation methods used by the software.

System products

In addition to the cabling, you will also need to consider all of the other elements in the installation, such as the LV switchgear, marshalling boxes, other connection systems and, of course, the connectors themselves.

Generally circuits in a prefabricated wiring system are distributed in similar ways. What the specifier needs to ensure is that all of the individual elements in the system conform with the relevant product standards (see Table 1).

Table 1

To comply with BS 8488 and therefore BS 7671, you must ensure that all of the elements of a prefabricated wiring system meet relevant products standards. These are summarised in the table below:

In conclusion

Prefabricated wiring systems are here to stay – they simply offer far too many advantages not to. You need to be aware, however, that as the specifier, you are liable for the system complying with BS 7671.

All of this may seem like a lot of detail, but you need to do it if you want to reap the rewards of a prefabricated wiring system and ensure its safety and compliance.

Hager has produced a free technical guide to prefabricated systems.