Armoured Cable Regulations
SWA, or Steel Wire Armoured cable, stands out as a go-to choice for electricians in various settings due to its versatility. However, amidst its flexibility, there are specific regulations often overlooked yet crucial to ensuring its safe and effective application.
Embark with us on an exploration into the Armoured Cable Regulations outlined in the 18th edition. Discover the remarkable versatility and safety that this cable type offers when adhering to the correct usage guidelines.
What Is SWA Armoured Cable?
Armoured Cable, often referred to as SWA (Steel Wire Armoured), stands out as a cable specifically engineered for environments prone to mechanical hazards such as impact or abrasion.
Internally, its structure comprises inner cores cushioned within a PVC surround, akin to the material used in typical multicore flexible cables, albeit slightly softer. While this bedding offers less rigidity than standard PVC multicore flex, its strength is supplemented by an additional two layers within the cable.
Wrapped around the PVC bedding is a winding of galvanised steel armour wires. Though individually thin, the multitude of wires wound around the inner bedding imparts considerable stiffness to the cable, given its size.
Encasing this assembly is a robust, thick PVC outer sheath, delivering the ultimate shield that safeguards the steel armour wires against external elements.
Contrary to initial appearances, the armour wires don’t serve as a conventional mechanical ‘shield,’ akin to what one might expect from armor plating. Instead, they serve a precise purpose.
The correct termination of this cable is paramount to its performance under fault conditions. Hence, various armoured cable regulations outline specific guidelines to ensure its optimal functionality.
One commonly overlooked aspect is the need to earth the armour wires. Understanding why this is essential is pivotal, as explained below
Why Do We Connect The Armour To Earth?
An essential aspect to grasp is the necessity of connecting SWA armour to earth. The armour wires serve a critical role by providing an encompassing path to earth. In the event of any impact damage that breaches the cable, these armour wires swiftly establish a low impedance pathway to earth for the inner cores. This rapid connection to earth is pivotal as it ensures the protective device can promptly trip, mitigating potential hazards.
Consider a scenario: an armoured cable buried underground. In the event of accidental contact, such as striking it with a shovel while digging, relying on the shovel or even your body to act as a path to earth obviously isn’t safe! Instead, the tightly bound armour wires come into play, guaranteeing that any fault current finds a reliable pathway directly to earth through the armour wires themselves.
Surprisingly, one of the most commonly disregarded aspects within armoured cable regulations pertains to the treatment of armour wires during installation. Often, installers pass the armoured cable through a stuffing gland into an enclosure. Once inside, the armour wires tend to be stripped back and covered with tape to prevent them from catching on the inner conductors within the enclosure. This precaution is essential as the cut ends of galvanised steel wire can be sharp and potentially cause snagging issues.
However, overlooking this crucial step in the armoured cable regulations essentially bypasses the cable’s inherent design. Neglecting to address the armour wires not only compromises their intended functionality but also inhibits the cable’s ability to establish a reliable low impedance path to earth. Without the proper treatment of the armour wires, the cable’s fundamental safety mechanism is undermined.
How To Properly Terminate SWA Cable
In order to correctly provide the path to earth, and securely terminate the cable into an enclosure, correct SWA glands must be used.
These come in 2 distinct types:
- BW (Internal glands) – these are suitable for terminations into enclosures where there is no exposure to moisture. For example, connecting to a distribution board inside a commercial unit. BW glands must not be used outdoors or in areas where there is a likelihood that the gland will be exposed to rain/moisture
- CW (External glands) – these have another rubber ring within the gland which tightens onto the outer sheath of the cable. These glands are designed for use outdoors, or in other areas where exposure to water is a possibility. Whilst BW glands cannot be used in these type of environments, there is nothing to stop you using CW glands wherever you see fit, indeed they can often be easier to terminate into as the body of the gland is longer.
Whichever gland you use, you need to strip back a small section of the outer sheath to expose the armour wires. Between 20-25mm should be sufficient, any more and you run the risk that the armour wires will sit outside of the gland on CW types.
The body of the gland is fed over the outer sheath of the cable. Within this, there is a small olive which catches the armour wires between the gland body and the inner piece which sits into the body of the enslosure.
I’ll be honest, this is quite difficult to explain in words so I’m just going to share a link to a YouTube video which shows the process really well:
If you couldn’t see the video for any reason, check out https://www.youtube.com/watch?v=WQuoQvppvRQ
Hopefully once you’ve watched this you will understand much better than just trying to read my awkward explanation! This video is a great example of how to properly terminate SWA into a gland in accordance with the armoured cable regulations
There is an alternative to using the ‘banjo’ earth tag included with most SWA gland packs, which is not only time saving, but also offers a few other benefits which the older method cannot always guarantee.
Piranha earthing nuts replace the banjo tag and the traditional securing nut with a wider nut that has a small grub screw which secures onto the threads of the gland. The nut also has a screw (or thread and bolt on larger nuts) which allows connection of a crimped ring terminal/lug.
The benefits of using an earthing nut as opposed to the original method is that the IP rating of the enclosure can be kept intact without having to drill for a bolt through a banjo tag. The earthing nut also stays clamped to the gland with the grub screw so even if the enclosure is damaged or compromised in any way, the armours path to earth remains.
Using SWA Armour As A CPC
In much the same way that you can use galvanised steel conduit as the CPC for a circuit, you can actually use the armour wires
Even if you end up using one of the cores as the CPC (as you would with a normal cable), or even running an external CPC alongside the SWA, as discussed above, the armour still needs earthing to ensure that it provides a path for the fault current should the cable get damaged.
If you are only using the armour for ‘protection’ of the cable, then it is satisfactory to connect it to earth at one end only, this usually being the ‘supply’ end.
However, this raises the question, why not connect to earth at both ends and use the SWA armour wires as a CPC in their own right?
Although a common practise, a number of variables are often overlooked when using SWA armour as CPC. This can lead to situations which are non compliant with the armoured cable regulations, and in the worst instances can actually be dangerous.
Depending on the nature of the installation and the circuit in question, it might not be desirable to connect the earth at both ends. One example of this would be where the SWA cable was feeding an outbuilding which may need to be a different type of earthing arrangement (for example, TT).
Connections to different types of earthing arrangement should not be accessible within the same installation. In this instance, there are particular glands you can purchase which enable you to ‘isolate’ the incoming SWA earth from that of the enclosure to which it is to be attached.
If you do choose to use the armour wires as a CPC, it should be noted that the steel wires do not have the same effective CSA as copper due to differences in conductivity. The table below outlines the minimum cross sectional area of armour wires to comply with table 54.7, and also the actual CSA of a selection of popular sizes and number of cores. (for what it’s worth, you can see that in every instance the size of the armour wire CSA is above the minimum required to comply)
Cross Sectional Area of SWA Armour as a CPC
|CSA Of SWA Conductors
|Min CSA of Steel CPC
To Comply With Table
|2 Core SWA
|3 Core SWA
|4 Core SWA
There are a number of other notes that should be taken into account when using the armour wires as a CPC to ensure compliance with the armoured cable regulations and these are as follows:
- The operating temperature of the cable does not exceed 70’C.
- The CSA of conductor cores does not exceed 95mm2.
- The cable is not installed in the floating section of any marina.
- The cable is correctly selected for the correct value of R1 + R2 for the circuit protection device protecting the cable.
- The cable does not connect different installations together which have separate means of earthing unless sized for the potential earth fault current.
- The cable is not used as a combined CPC and bonding conductor on a PME installation unless suitably sized.
- The cables are properly terminated in correctly sized manufactures brass glands selected for the external influences prevailing.
- Where a removable gland plate is present on the enclosure to which the cable is attached the earth ring (banjo or earthing nut) supplied with the gland is used and is connected to the earth terminal in the enclosure with a separate CPC. This CPC is to be terminated in an eyelet to a bolted connection to the earth ring.
- Any protective coating on the enclosure surface is removed under the contact surfaces of the gland and associated earth ring to expose bare metal.
- All the wires of the armouring enter the gland and the gland nut is adequately tightened. The cable is adequately supported up to the gland with cable cleats to prevent mechanical strain on the gland.
Burying Armoured Cable
One of the things that makes armoured cable so popular is the wide variety of ways in which it can be installed. In particular, whilst most other cables require housing within ducting when being buried in the ground, SWA is one of the only cables suitable for direct burial without any ducting.
It’s worth noting that despite this, I would still recommend that it is installed within a duct, purely as this enables another cable to be easily pulled in should there be an issue with the original, whereas direct burial means that you will have to dig the trench up again in order to replace the cable.
So what do the armoured cable regulations say about burying the cable? Well, to be fair, they are quite vague in this aspect, and do not give any hard and fast minimums for a burial depth for the cable.
What should be ensured is that the cable is buried below a depth where it may be struck accidentally during digging, usually only considered manually with a spade as opposed to using a mechanical digger! From this, best practise is to bury the cable at least 450mm below the surface, and to be fair 600mm may be more appropriate.
The cable must be covered with ‘cable below warning tape’ sat a couple of inches above the cable itself, which helps to identify that there is a cable buried underneath should any digging take place in that area.
Armoured Cable Regulations
As you can see, there are quite a few armoured cable regulations to be aware of.
However, if you follow the rules surrounding it’s use, SWA cable can be one of the most versatile and safe methods of providing power, either in a domestic garden or else in a commercial warehouse. In fact anywhere that presents a higher risk of mechanical impact is a great place to use this cable.
If you are looking to learn more about SWA armoured cable then check the following links: