Steel Wire Armoured Cable: All you need to know

SWA Cable, abbreviation for Steel Wire Armoured Cable, is a hard-wearing power cable designed for the supply of mains power, or to quote most electricians a pain in the ar… neck. swa stripping tool

SPOILER ALERT: A solution for Electricians, Engineers, Contractors, Construction Managers, Project Managers and anyone dealing with SWA Cable at the end of this post (SWA Stripping Tool).

What is an SWA Cable?

SWA is a power and auxiliary control cable, designed for use in mains supply electricity. It is one of the various protected electrical links – including 11 kV Cable and 33 kV Cable. Found in underground systems and frameworks, cable networks, control systems, power networks, outdoor and indoor applications, and cable ducting.

Steel Wire Armoured Cable’s design features have mechanical protection, which explains its common use for external applications. The armour reduces any risk of pinching or damaging the cable, while the steel protects the armoured cable. SWA cables are heavy, which makes them extremely difficult to bend; therefore they are most suited to underground cabling or fixed to outdoor walls using cable cleats.

Other widely used terms for it are: Mains Cable, Armoured Cable, Booklet Armoured Cable and Power Cable. The name Power Cable though, applies to a large variety of links including 6381Y, 6491X, NYCY, NYY-J Cable.

(For our fellow Americans: replace Armoured with Armored)

Using Steel Wire Armoured Cable as Earthing

Using armour as the means of providing earthing/ grounding to the equipment supplied by the cable, is a subject of controversy within the electrical installation industry. This function is technically famous as circuit protective conductor or CPC. 

Usually an additional core within the cable is the CPC (for example, using a three core cable instead of a 2-core one for line and neutral, and the armouring as the CPC) or an outside earth wire runs alongside the cable, serving as the CPC. The main concerns are:

• The relative conductivity of the armouring compared to the cores (which decreases as the size of the cable increases),

• Reliability and Health & Safety issues. 

According to recent articles and research by authoritative sources, a detailed analysis of the practice concluded that, in most cases, the armouring is suitable to serve as the CPC under UK Wiring Regulations.

(For more information on Earthing click here)

Construction of SWA Cables

The typical construction of a Steel Wire Armoured Cable includes:

1. Conductor:

   Class 2 Plain Stranded Copper (Cu) Conductor, complying with BS EN 60228:2005.

2. Insulation:

   Cross-Linked Polyethylene (XLPE) is used as insulation in many power cables, due to its excellent electrical properties and water resistance. It also ensures that conductors and other metal substances do not come into contact with each other.

3. Bedding:

   Polyvinyl Chloride (PVC) is used as bedding to provide a protective boundary between inner and outer layers of the cable.

4. Armouring:

   Steel Wire Armour (SWA) used for mechanical protection. That way the cable can withstand higher stresses, be buried directly and used in external or underground projects. The armouring

   usually connects to earth and can be used as the CPC (as above).

5. Sheath:

   Polyvinyl Chloride (PVC) Sheath holds all the components together and provides additional protection from external damage.

Sheath Colour: Black (Carbon loaded for UV stability)

Voltage Rating: 600/100V

The construction of a Steel Wire Armoured Cable depends on the intended use. For example, when the power cable needs to be installed in a highly populated and /or enclosed public area, a Low Smoke Zero Halogen (LSZH) equivalent, called SWA BS 6724 Cable must be used (SWA Cable BS 6724 has LSZH bedding and a black LSZH sheath). All London Underground cables have to use LSZH sheathing after the fatalities due to toxic gas and smoke inhalation during the King’s Cross fire in London in 1987.

What do BS5467 and BS6724 stand for?

The use of terms BS5467 or BS6724 is very common when referring to Steel Wire Armoured Cable. These phrases mean that the SWA cable meets the corresponding British Standard requirements, for both construction and testing.

Cores

Multi Core

Steel wire armour is only used on multi core cables. Multi core swa cable, is a cable with two or more cores:

2-Core SWA Armoured Cable is live and neutral for Class II and Double Insulated appliances which do not need earth connection.

3-Core SWA Armoured Cables are live, neutral and earth, unlike 2-Core. Three core cable is for Class I or Single Insulated which must have earth connections.

4-Core SWA Armoured Cable are perfect for low voltage or low current signal applications. Made of 4 copper individual colour coordinated cables hence the name four core cable.

5-Core SWA Armoured Cable is most commonly used in low-voltage transmissions and uses a three-phase line with one zero line for the power supply.

7-Core SWA Armoured Cable’s main purpose is for low-voltage connections with a max of normal 50Vdc. Each core is from copper conductors within the steel armouring.

Single core (Aluminium Wire Armoured)

Steel wire armour is only suitable for use on multicore versions of the cable. When a cable has only one core, the use of aluminium wire armour (AWA) instead of steel wire is preferrable. This is because the aluminium is non-magnetic. A magnetic field is produced by the current in a single core cable. This would induce an electric current in the steel wire, which could cause overheating.

Cutting & Stripping SWA Cable Easily & Safely

The increasing number of electricians’ injuries while working with SWA cable along with the updated Health and Safety Regulations that forced most major contractors to ban blades and cable knives on site, made the need for a new SWA stripping tool an urgency. The SACS Tool was a welcome breath of fresh air to tackle this issue while countering all the drawbacks of previous SWA cutters:

Follow link for more Blog Posts on: Tooling, Standards & Regulations.

Armoured Cable vs. Unarmoured Cable: What’s The Difference?

With the rapid development of optical communication, more and more fiber optic cables are increasingly used in different environments. What if under harsh conditions? Then it’s crucial to ensure your cables smooth and reliable operation when transmitting data. This is where armoured cable comes into play. An armoured cable, as its name suggests, is protected against mechanical damage, whereas an unarmoured cable not being protected. What is the difference between them? And why should we choose armored cable over unarmoured cable? You my find answer in this post.

Amoured Cable Overview

Armoured cable has an extra layer of protection to keep it from being cut or abraded. The armor layer of coax cable is a foil wrap that is ribbed like corrugated metal to allow for flexibility, around the inside and outside of that wrap is a flooding compound to keep moisture from penetrating the cable and causing an impairment. The internal structure of 4 core armoured cable consists of many layers to prevent the cable from damage. The outer jacket provides protection against rodent, abrasion and twist, which is usually made of plastic. And the armoring materials are mainly come from kevlar, steel, and aluminum foils, aiming to protect the armored cable from being stretched during installation.

Difference Between Armoured Cable And Unarmoured Cable

Structure

Many people may think that armoured cable just has metal protection. To be precise, the armoring material doesn’t have to be metal, it can be fiber yarn, glass yarn, polyethylene etc. The only thing that makes armored cable different from unarmored cable is that the former has an additional outer protective layer for optical cable. The 4 core armoured cable tends to be more expensive than unarmored cable, while the armoured cable with steel strip and aluminum is much cheaper than armored fiber cable with Kevlar, which is usually used for special occasions.

Application

Armoured cable is installed in locations exposed to mechanical damage, such as on the outsides of walls, as an alternative to conduit. Armoured cable usually has a small metal ribbon to ensure electrical continuity of the safety ground. (You must run a separate ground wire in flexible conduit too; you can’t depend on the continuity of the conduit.) In HT & LT distribution, 4 core armoured cable is preferred. Inside walls and in other protected locations, less expensive unarmored electrical cable can be installed instead. Unarmoured cable is mainly used for control systems.

Why Should Use Armoured Cable Over Unarmoured Cable?

There are a couple of reasons that armoured cable should be used. The biggest reason is about strength, because armored cable was used more extensively in past decades when cable was simply directly buried under dirt and not used through a conduit. Nowadays most local municipalities require conduits to be trenched in prior to installing network components, thus eliminating the need for unarmored cable in most applications. Secondly, rodents or animals can and will chew through cables so the armor protects the cables from damage by animal or shoveling in direct bury applications. Thirdly, the most uncommon reason it would be used is in an RF environment that has an off air RF signal that is powerful enough to interfere with your network, the armor when grounded can provide another layer of RF protection.

Conclusion

Armoured cable can be regarded as a kind of strengthened cable, which is harder and stronger than standard optical cable. With an unparalleled protection against physical damage without sacrificing flexibility or functionality within fiber networks, 4 core armored cable is a perfect addition to any fiber network in hazardous environments.

What is the difference between unarmoured and armoured cables?

As the name implies, an armoured cable has added protective armour that helps protect the cable core. This is important in places where there could be accidental damage to the cable due to mechanical stress or impact. To put it in context, typically armoured cables would be used when you require the cable to be buried directly underground, for outdoor installation or in tunnels. There may be instances where the ground is opened up again, and in the process, a spade or mechanical excavator may hit the buried cable by accident. The armour in this case would help protect the cable conductor core from being exposed easily and prevent electric shocks and interruptions in power delivery.

For standard power cables, you may come across aluminium wire armour (AWA) or steel wire armour (SWA). Steel is naturally a stronger material, but you will notice that for single-core cables, it is ALWAYS paired with AWA and never SWA. This is because aluminium is non-magnetic.

What is AWA vs SWA for armoured cables?

If a single-core cable is paired with steel wire armour, there will be a magnetic field induced when current flows through - the whole cable becomes a large magnet. The higher the current, the larger the field. The magnetic field will induce an electric current (eddy current) in the steel, which would cause overheating and massive derating of the cable. Aluminium, due to its non-magnetic nature, would not cause this issue.

However, once we move past single-core into two or more cores, it is safe to revert to galvanized steel wire armour because magnetic fields produced in such multi-core cables will have a cancelling out effect by the other cores’ fields thereby preventing the magnetic flux.

Armoured cable construction

This would be a typical armoured cable construction at Keystone:

Conductor: Class 2 stranded plain annealed copper
Insulation: cross-linked polyethylene (XLPE) is recommended over polyvinyl chloride (PVC) to provide a higher maximum operating temperature, better water resistance and stronger dielectric properties
Bedding: a protective layer between the insulation and the armour.
Armour: steel or aluminium armour to provide the mechanical protection
Sheath: PVC or LSZH (Low Smoke Zero Halogen) outer sheath that holds the cable together. LSZH would be recommended for public areas or in tunnels. At Keystone we would also include UV stabilizer and anti-termite additives in our cable sheath material.