Blog

Mar 14, 2024

Steel wire armour cables: Maintaining earth continuity

Regardless of whether the armouring of a steel wire armoured cable (swa) is employed as a protective conductor or not, it should always be effectively earthed to ensure the requirements of ADS are

Regardless of whether the armouring of a steel wire armoured cable (swa) is employed as a protective conductor or not, it should always be effectively earthed to ensure the requirements of ADS are met.

Where swa cables are connected to metallic enclosures, such as items of switchgear and control gear that also serve as the protective conductor of the circuit concerned, all the requirements of Regulation 543.2.2 (relating to the suitability of a metallic enclosure to provide effective earthing), should be met.

Where cables are terminated into enclosures such as consumer units and distribution boards, it should be confirmed that the connection at the interface between the steel enclosure and brass gland, as shown in Fig 1, is mechanically and electrically sound.

Many steel enclosures are coated with finishing products, such as epoxy resin or powder coated paint, to protect them from corrosion, but as such coatings are generally non-conductive they are likely to act as a barrier at the interface connection, between the brass gland and the steel enclosure. As a result, the resistance value of the armouring may be increased beyond that acceptable to meet the requirements of ADS.

To ensure a low resistance connection is achieved between the steel wire armour and the steel enclosure an earth tag washer (‘banjo’or earthing nut) should be included in the gland assembly, as shown in Fig 1.

Where swa cables are terminated into non-metallic enclosures, such as for example where an isolating device for a piece of fixed equipment is comprised of a non-metallic enclosure, the continuity of the armouring should be maintained across the enclosure by the use of earth tag washers and an attached adequately sized conductor, as shown in Fig 2.

The connection should be made by fitting the appropriate size of cable gland in accordance with the manufacturer’s instructions, and ensuring the cable is adequately supported to prevent undue strain on the gland connection (Regulation 522.8.5 refers).

Bends in swa cable should comply with the minimum values recommended by the cable manufacturer based on the cables overall diameter. Otherwise, damage may be caused to the armouring, conductors, or the cable insulation during erection of the installation or at some other time, such as under fault conditions.

It should be noted that non-metallic enclosures may be used for the purposes of divorcing one earthing system from another, such as where it is desired to arrange a TT earthing system from a TN-C-S supply.

The minimum radius dimension calculated from the table shown relates to the surface of the cable on the inside of the bend (see Fig 3) and can be calculated in terms of the overall diameter of the cable.

As an example, consider an armoured thermosetting insulated cable having copper circular conductors and an overall diameter of 32 mm.

From the table, it can be seen that the recommended minimum internal bending radius for the cable is six times its overall diameter (i.e. 6 x 32 mm = 192 mm). Had the conductors of the cable been of the copper shaped type, the minimum recommended internal bending radius would have been eight times the overall diameter.

British Standards for cables recommend that for a cable having a standard thermoplastic (pvc) sheath, installation (including bending) should take place only when both cable and ambient temperature are above 0° C and have been so for the previous 24 hours, or when special precautions have been taken to maintain the cable above 0° C.

While identification of the armouring is not required (Regulation514.6 refers), the cores of swa cables should be identified using:

• colour as required by Regulation 514.4 and/or

• lettering and/or numbering as required by Regulation 514.5.

The bi-colour combination green-and-yellow shall be used exclusively for identification of a protective conductor and this combination shall not be used for any other purpose (Regulation 514.4.2 refers).

Other conductors should be identified by colour in accordance with Table 51 of BS 7671.

This article will discuss the identification of extraneous-conductive-parts to avoid the unnecessary installation of protective bonding conductors and the potential introduction of hazards.

NICEIC has produced this guide covering the risks that need to be considered when mounting electrical equipment in loft spaces in domestic premises.

The technical experts at NICEIC have created this guide to the appropriate use Minor Works Certificates.