NAPIT’s Principal Technical Author, Richard Townsend, looks at the importance of enclosing single insulated cables.
What have we been missing?
Often overlooked is the importance we should place on enclosures to ensure any conductors and their respective terminations are adequately protected from mechanical damage and accidental contact, especially from ordinary persons.
The most common problem we see when we look at installations, either new or existing, are single insulated cables not taken into an adequate enclosure.
Whether the exposed single insulation is the result of 6491X cable improperly used, from missing trunking lids (which is very common), or from a sheathed cable with the sheath removed, exposing single insulated cables, such as 6242Y flat twin and earth, 6945X armoured or 6181Y meter tails, the problem is the same.
Why is this a problem?
Single insulation does not provide any mechanical protection, which means it’s susceptible to the rigours and abuses of everyday life. I say abuses because the seriousness of injury from electricity is very much maligned, and there is an assumption that the cable will protect you from the electricity flowing through it against all things – from walking on it to hanging things off it.
As single insulation does not possess any mechanical protection properties, by its design, we need to ensure it’s protected in some way, and that’s where enclosures come in and why they’re so important.
BS 7671 is very specific when it mentions and defines an enclosure. According to Part 2 Definitions, an enclosure is:
‘A part providing protection of equipment against certain external influences and in any direction providing basic protection.’
That’s not enough on its own, though, so greater clarity is given in Regulation 526.8, which in turn refers to 526.5, requiring single insulation to be treated as any termination or connection in a conductor. In requesting this, the single insulation must be taken into either:
-An accessory complying with a suitable product standard
-An equipment enclosure complying with an appropriate product standard
-An enclosure partially formed or completed with non-combustible building materials, or
-A combination of two or more of the above.
What’s the solution?
A reason for leaving single insulation outside of an enclosure that we often hear is that there isn’t room in the given enclosure to fit the cables and terminate them adequately. As we look at enclosures, we can agree that this is an issue in some cases, but there certainly is no excuse to do it, as there are devices to help overcome this.
Where there isn’t room in a manufacturer’s enclosure, distribution board (DB) or consumer unit (CU) to adequately encase the single insulated cables, extension and spreader boxes can be used. These items provide an extra section of enclosure that can house single insulated cables to allow a tidy and adequate connection inside the main enclosure.
Fig 1 and Fig 2 illustrate the two main types of spreader boxes.
Individual entry spreader boxes are designed to allow the sheath of larger cables to be stripped back so that the single insulated conductor can enter the enclosure. This facilitates good termination of the conductors without tight radiuses or undue stress on the terminations.
Multi-cable spreader boxes, or extension boxes as they are sometimes called, effectively extend the enclosure. This allows the sheaths of multiple cables to be stripped back so that the enclosure, generally a CU, is free from masses of cable entries that can cause stress on the eventual terminations and from the issues associated with the close confines of switchgear.
Although most of these pieces of equipment are designed to be used with a specific manufacturer’s enclosure, there are alternatives.
Where a consumer unit, especially in a domestic setting, requires multiple armoured cables, simple trunking adequately installed can do the job justas effectively.
Providing the correct lids and endplates are used, different trunking sizes can be re-purposed to provide a tidy interface between multiple cables and aCU/DB, especially for CU/DB changes, where space and existing cable lengths are challenging.
If we look at Fig 3, we can see that a CU/DB has been installed using a section of 100 x 100 mm metallic trunking. Fitted neatly along the top edge of the CU/DB, it provides a useful interface to terminate several armoured cables.
This, in turn, allows the single insulated conductors of these types of cables to enter the CU neatly without causing any obstructions. In the same way, flat twin and earth cables have also been installed this way, which stops the sheathes from cluttering the inside of the CU/DB.
The following are some useful outcomes of this type of design for an installation practice:
-Removes unnecessary clutter from the CU/DB or enclosure
-Reduces stress on terminations and conductors
-Promotes air circulation and cooling for terminations, conductors and devices within the main enclosure
-Allows clear identification and access for future periodic inspections.
Although the example in Fig 3 uses metallic trunking as a spreader, because of the extra stress incurred by the armoured cable glands, this does not preclude the use of plastic trunking.
Generally, plastic trunking is more than acceptable for utilisation in this way, especially where flat-sheathed cables, such as flat twin and earth used in domestic and less complex installations, are the only cables used in the installation.
Using plastic trunking in this way isn’t a problem in domestic installations with metallic CUs as the trunking does not contain switchgear, and as such, it is not required to be of metallic construction, as is the CU.
Something to remember, however, is that whenever a site-manufactured spreader/extension box is used in this way, it must be fit for purpose and include the correctly fitted end plates and trunking lid. The key to a safe installation lies in its design. Without a solid understanding of design principles, we run the risk of falling short of the most basic requirements.
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