The technical experts at NAPIT look at different installation methods and the mechanical protection requirements for each.
Installation or wiring methods?
Whenever we speak to contractors or they ask our advice on an installation or wiring type, there is often some confusion over the different installation methods and how they can be used.
Let’s be clear, wiring methods are different ways of saying installation methods, so they are generally taken as being the same principle.
What’s a wiring system?
These differ slightly from installation methods and refer to a cable or system containing both basic and mechanical protection when installed and commissioned correctly.
PVC/PVC flat twin and earth cables are classed as a wiring system as they possess all of these properties and also include a cpc, as do SWA, MICC, soft skin fire cables (FP200 and similar) and many other similar cable types.
The systems we use to support these cables are referred to as containment because they contain the cable wiring system. A containment system or method is not a wiring system, and in most cases, it can be used for, or with, many different wiring systems.
One such example of a containment system is the cable basket, as shown in Fig 1, where the open metal frame design offers less surface area so increases air circulation and prevents both heat and dust build-up.
Another cable containment system is the cable tray, as displayed in Fig 2, which are commonly used to support a multitude of light-weight cables. In the same way, we may choose to use cable clips or cleats. These can be used for many differing wiring systems; the clips or cleats themselves vary by design, but the principle behind their use is basically the same.
For heavy duty and high volume cables, cable ladders are typically used as a containment system, as shown in Fig 3.
When we use singles, 6491X and similar, these are only half of the system.
As 6491X cable only possesses basic insulation and no mechanical protection, we have to install them inside suitable mechanical protection. We can’t call 6491X cable a system, unless we also refer to its mechanical protection part at the same time.
There are two basic principles for mechanical protection of 6491X single cables, which are conduit and trunking. These again are split into two types of material construction, leading to four distinct types in total.
Fig 4 shows the two material types for conduit protection, metal and plastic.
For 6491X to be called a system, it needs to be referenced with one of these four mechanical protection parts. These two mechanical protection parts of a system, conduit or trunking, although they serve the same purpose, have very different properties and skill sets needed to install them.
Fig 5 and Fig 6 indicate the two material types of trunking, metal and plastic. Indeed, the different materials used to construct each of them also have very different installation techniques and skill set requirements.
We generally wouldn’t use a separate containment for 6491X/conduit systems, as the system itself is the containment. A conduit system, whether metallic or plastic, will use saddles to secure them, but these are considered to be integral to the wiring system and not optional. We wouldn’t lay conduit or trunking onto a cable tray or basket, for example, as this is not a proper use of either piece of equipment.
Flexible metallic conduit is often utilised for connections to equipment, although there has been an increase in its use for connecting solid metallic conduit between expansion joints within buildings and similar.
One thing to remember is that flexible metallic conduits are unsuitable for use as a cpc; therefore, any links between fixed metallic conduits would require earthing links to ensure the earth continuity of the containment system.
What about the cpc?
With metallic conduit and trunking, the structure itself takes on the role of the cpc, so only the line and neutral of the circuit are carried internally, with a connection made at the accessory to either the conduit or the trunking.
Plastic conduit and trunking are obviously non-conductive, so the cpc for each circuit must be taken inside with the line and neutral conductors to each accessory served.
Which one do I use?
A designer can use whichever they choose to, but will generally be driven by the following:
● Client needs or requirements
● Usage
● External influences
● Durability
● Accessibility for maintenance, repairs or alterations.
Whichever method is used, the installer must possess the correct skills needed. Complex wiring systems need high levels of competence and experience to ensure compliance with BS 7671.
Both plastic conduit and trunking arguably require less complex equipment and training to construct; however, they are not easy to install correctly. Metal conduits generally require specialist equipment to install, and metallic conduits will likely involve powered cutters or tooling.
When dealing with containment, the same applies, although generally speaking, power tools will need to be used to install most cable tray and ladder products.
Can you mix and match?
Mixed systems are often seen and indeed necessary, for example, when going from a trunking system to a conduit system to disseminate final circuits for point of use, etc.
We also see cable systems inside trunking to reduce the need for unnecessary containment, where the circuits wired in a cable system are limited. It’s usual to see the odd SWA, MICC or other such cables in trunking alongside 6491X singles, especially where modifications and alterations to an installation have been made.
Things can get trickier with conduits, as the friction from pulling a sheathed cable system through them can damage them and any other 6491X singles in the same conduit.
In both cases, the designer needs to take into account any thermal constraints, as a wiring system may have its current capacity impaired by the enclosed effects of being encased in a conduit or trunking. Likewise, the thermal constraints of the 6491X conductors may be affected where a sheathed wiring system is introduced to the same conduit or trunking.
We can’t mix and match 6491X or similar singles by placing them on either a cable tray or ladder, as they would not have mechanical protection. So, from a mix-and-match perspective, wiring systems can use trunking and conduit as a containment method if thermal constraints allow, but singles must be in either conduit or trunking.
Is twin and earth only for domestic?
Often seen as a domestic-only wiring system, PVC/PVC flat twin and earth, is actually far from it. Twin and earth and similar variations are wiring systems in their own right, and can be used in commercial and industrial installations, where the designer sees fit.
The only problem you will find is that they are less flexible, given their solid conductors, below 4 mm². Other than that, they can be used anywhere a similarly specified cable wiring system can be used, providing they meet the requirements for low smoke and fumes.
Conclusion
We have several options for the type of installation method we use; each has its merits, drawbacks and varying levels of difficulty of installation. This is beneficial as it gives the designer options to choose the best possible solution.
For more information on NAPIT scheme registration, click here
