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

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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 protection 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 Y, X, 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 :.

2. Insulation:

   Cross-Linked Polyethylene&#;s (XLPE) use as insulation in many power cables offers 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&#;s (PVC) choice as bedding provides 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

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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 Cable must be used (SWA Cable BS 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 .

What do BS and BS stand for?

The use of terms BS or BS 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. Single-core cables armoured with steel wire or steel tape shall not be used for an AC circuit. This is because aluminium is non-magnetic, while the steel wire or steel tape armour of a single-core cable is regarded as a ferromagnetic enclosure. For single-core armoured cables, the use of aluminium armour may be considered.

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.

Hello Dave,

&#;I wired out the supply to my workshop with 6mm armoured cable. The total cable distance from the consumer unit and destination is comparable to yours and voltage drop calculated at under 5 volts. Remember to buy 3 core cable so that you have a copper cored earth, the armour is steel sheathing and not really good enough for a reliable earth. and will get failed if you put it up for test certification.

&#;You will also need the appropriate size of glands at each junction point.

&#;Regards
Brian

I'm not sure your advice about not using the armour as a protective earth is quite right there.

If you use SWA, it would certainly be bad practice not to use the armour as an earth conductor. It is very much more than sheathing. Indeed it is the very definition of an electrical protective earth. It is virtually impossible to damage a phase conductor without first cutting the protective sheath. The very act of encircling the phase conductors with the earth offers a method of electrical protection and essentially guarantees fast circuit breaker action if the cable is accidentally cut.

There is much uncertainty out there on this matter, so lack of awareness can be excused. One thing is certain, poor implementation will not be excused by mother nature under fault conditions.

People have generally noticed that corrosion can set in at glands and where the outer plastic sleeve becomes damaged. This can compromise the earth bond, and provide less than adequate protection.

Some recommend using an additional copper earth conductor inside the armour, others an additional copper earth conductor outside the armour.

Both schemes have problems;

1) If the armour is electrically damaged then neither scheme can guarantee to expose the earth conductor to the phase if the cable is cut. If the digger bucket scrapes along the length of the conductor then phase can be exposed without shorting to the protective earth. One is dependent on soil resistance to operate the circuit breaker in this situation, and it is not good.

2) If the parallel wired copper earth conductor leads an electrical test operative to believe that the armour is intact during test, then no-one may know that the armour has actually become damaged.

In conclusion; if the armour is not good enough to do the job because of corrosion, then it has not been monitored and maintained properly.

In most cases there is no reason why a steel wire armour cannot perform the function of a protective earth. If the calculations show that the armour can trip the fault circuit in the prescribed time, then my opinion is that it is safer only to use the steel wire armour.

Like all engineered installations it is important to periodically check and maintain equipment to ensure safety.

Edited By Andy Ash on 20/03/ 20:13:54