CABLES & CABLE ACCESSORIES
7
Electrical earth connections through cable glands F irstly, there is no need to earth a plastic cable gland (as it is not conductive), and it is generally not necessary to earth
If the cable has a braid armour/ screen, the associated cable gland cannot have a higher rating than Category A, as the cable’s braid will not be capable of carrying the currents required to meet Category B. In other words, the limitation is typically dictated by the cable construction, not the cable gland. Cable glands for SWA or AWA cables can have a Category B rating as long as the connection between the cable gland and earth is sufficiently good. This can be achieved by either securing the cable gland to a threaded entry on the (earthed) equipment or using an earth tag. Since cable glands can be fitted to clearance holes and secured with a locknut, their electrical rating must default to the lowest value. Therefore, they are often specified as Category A if no earth tag is used and as Category B if an earth tag is fitted. A Category C rating is only achievable with a heavy-duty earth connection cable and a very low resistance path to earth. This can be attained using screw- on earth tags or cable glands with an integrated earth stud. In all these installations, it is vital that the cone and cone ring armour clamp are of good design and quality. Independent tests have shown that glands with loose cones and plastic-to-metal threads cannot carry an earth fault current. Earthing for EMC protection. EMC, or electromagnetic compatibility, is the ability of equipment, cables, or
cable glands used to terminate multi-core unarmoured cables, although installers may choose to do so as part of good practice. The requirements for earth connections in cable glands are defined in the general standards for cable glands, e.g., SANS 1213 and IEC 62444. Why earth a cable gland? In general, cable glands used with armoured cables are earthed for one of the following three reasons: - 1. To prevent induced voltage build-up, possibly leading to sparks. 2. To allow them to carry a fault current to earth in the event of an electrical fault. 3. For EMC protection. We shall look at each of these in turn, but first, a quick reminder about cable construction. The majority of armoured cable construction types fall into two types: those that use steel (SWA) or aluminium (AWA) wires laid in a slow helical form around the cable to form an armour, and those that use much thinner but a larger number of wires that are arranged as a braid around the cable bedding. Less common are armoured cables with a metal tape wound around the bedding, but they are important in certain applications, as we shall see in the section on EMC. It is also worth reviewing how a cable gland is earthed. This can be done by any of four methods. 1. Electrical contact through the armour clamping cone and cone ring, and through the threads of a threaded entry hole in an earthed enclosure when the cable gland is tightened into it. 2. By face-to-face contact with an unpainted (earthed) metal enclosure when the entry holes are clearance holes rather than threaded. Usually, the cable gland is held in place with a locknut and, preferably, a serrated washer to improve electrical contact and prevent slackening due to vibration. Note that this electrical contact is often improved by fitting an earth tag. 3. Using a slip-on earth tag (sometimes known as a ‘banjo’). This is fitted between the cable gland and the enclosure and provides a superior electrical contact with the cable gland. Earth tags are typically made from brass or nickel-plated brass and can be bent, if needed, to allow easier access for connecting a dedicated earth cable. This arrangement provides a visual indication of both an earth connection and an electrical one. An earth tag is essential if the enclosure is not made of metal or painted. 4. The final earthing method is by using a high-current earth connection. This would typically be used when the installation involves power cables with armour used as the earth connection and forming what is known as the CPC, or Circuit Protective Conductor. In this type of installation, the armour may have to be capable of passing high currents to operate a trip or breaker in a fault condition. This means that the cable gland and its connection to earth must also carry the same high current, and a standard earth tag is not capable of doing so. Instead, a screw-on earth tag can be used, or, alternatively, the cable gland can have a dedicated earth lug fitted, usually known as an Integrated Earth connection.
Non-Armoured Cable
Screw-On Earth Tag
Semi-Flexible Braid Armoured Cable
Armoured Cable
Earth Tag
1. Earthing for the prevention of an induced voltage. The armour of a cable is designed to protect or shield the cable and normally does not carry any current. Power cables (excluding single-core cables, which will be discussed in the section on when to isolate a cable gland) are usually constructed so that any magnetic fields produced by the alternating currents in the cores are minimised. Of course, in practice, achieving perfectly balanced electrical fields is Impossible. This means that in a power cable, a voltage may be induced in the cable armour even if it is not electrically connected to the main current-carrying cores. The induced voltage might be sufficient to cause an electric shock if touched, and if a spark occurs, it can harm sensitive electronic equipment. In a hazardous environment, it could act as the ignition source for an explosion. Cable glands for use with armoured cables provide an earth clamp connection by means of a cone and cone ring inside them, so that the armour is electrically connected to the cable gland. If the cable gland is also connected to earth, then, as a consequence, the cable armour will also be earthed, and a potential difference (a voltage) cannot be generated in the armour. CCG Cable Gland Armour Clamping Cone and Cone Ring and metal-to-metal thread engagement. 2. Earthing allows the armour to carry a fault current to earth in the event of an electrical fault. This is applicable when the electrical installation uses the cable armour as a protective earth, and the current that is required to be carried is defined in Table 5 of IEC 62444. In general, the connection between the cable gland and the cable armour is not the limiting factor in selecting the rating category for a cable gland.
systems to operate satisfactorily within their electromagnetic environment without causing unacceptable electromagnetic disturbances to other equipment in the same environment. With the increased use of variable speed drives (VSD), instrumentation and control, wireless data acquisition, and communication in industries, reliance on EMC cables and enclosures to maintain signal and power integrity has become increasingly important. Although cable glands do not emit
CONTINUED ON PAGE 10
ai177330124516_CCG Ind.180 x130 Advert.pdf 1 2026/03/12 09:40
Cable diameter mm > 4 to 8 > 8 to 11 > 11 to 16
Category A minimum kA rms
Category B minimum kA rms
Category C minimum kA rms
-
-
-
0,5 0,5
3,06 3,06 3,06
10,0 13,1 13,1 13,1 13,1 43,0 43,0 43,0
> 16 to 23 0,5
> 23 to 31
0,5
4,0 5,4 7,2
> 31 to 43 0,5 > 43 to 55 1,8 > 55 to 65 2,3
10,4 10,4
> 65
2,8
SPARKS ELECTRICAL NEWS
APRIL 2026
Made with FlippingBook flipbook maker