Selection of cable glands in Ex areas
April 2018, IS & Ex
Cable glands are the most common means of leading cables into hazardous area equipment. There are several conditions to be considered when selecting the right cable glands for the specific application. In principle there are three main types of cable glands:
• Cable glands made of plastic or metal for use with non-armoured cables.
• Cable glands made of metal for use with shielded cables.
• Cable glands made of metal for the use with armoured cables.
Considering glands for armoured cables, the important selection features are type of armour, cable outer diameter, cable inner diameter, thread size and wrench size. The gaskets at the enclosure play an important role regarding IP-protection. Here the installer has to differentiate between screwing the glands into threaded holes where a normal O-ring guarantees the IP protection and installation in through-holes with locknuts. In the latter case a flat washer gasket is required in order to compensate for the tolerances of such through-holes.
Hazardous area protection method
The above mentioned types of cable glands can be designed for either flameproof, Ex d protection or increased safety, Ex e protection, or a combination of both. Usually all Ex d certified cable glands are triple certified including Ex e and Ex t protection by enclosure, which is used in hazardous dust areas. Cable glands designed for Ex e applications are also certified Ex t in order to suit for both gas and dust applications. All Ex d cable glands require a thread engagement of at least five full threads in order to ensure the functioning of the flameproof gap between the male and female threads. Although there is no official need for a hazardous area certification for cable glands to be used with intrinsically safe circuits, the state of the art is to use Ex e certified glands for such applications.
Barrier cable glands
Barrier cable glands are Ex d glands for armoured or non-armoured cables, filled with a compound sealing around the individual conductors of the cable in order to maintain the safety of the flameproof equipment in which it is fitted. When correctly installed, the compound will prevent a potential internal gas explosion from exiting through the cable gland. It is mandatory to use barrier glands when the inner design and outside installed length of the cable, in conjunction with standard cable glands, cannot guarantee the safety of the equipment. The specific installation norm IEC 60079-14 (Explosive atmospheres – Part 14: Electrical installations design, selection and erection) explains in paragraph 10.6.2. that the use of normal Ex d certified cable glands is only allowed when the following conditions are met:
• The connected cable is at least three metres in length.
• The cable is sheathed with thermoplastic, thermosetting or elastomeric material; it is circular and compact; any bedding or sheath is extruded; and fillers, if any, are non-hygroscopic.
Otherwise barrier glands have to be used. The most critical point during the verification, if the use of barrier glands can be avoided, is the construction and the quality of the cable.
Cable manufacturers hardly give official confirmation for compliance with one or even all of the above conditions. Also they usually cannot give statements on cables which are proven for use with normal non-barrier cable glands, although some combinations might have been tested in real-life laboratory explosion experiments. It is well known in the community of experienced installers of hazardous area equipment that the installation of barrier glands, as well as of normal Ex d glands for armoured cables, is a source of numerous potential installation errors. Even if executed in a perfect manner, the explosion protection can depend on an elastomeric seal of merely 10 mm in length.
Cable glands need to protect the equipment against environmental influences which might intrude via the cable entries. Typical minimum ingress protection nowadays is IP66/IP68. In the past rubber shrouds were used as an option to protect metal glands against moisture and corrosion. Such shrouds cannot be recommended any more because modern metal cable glands are highly corrosion-resistant. Even worse, water and moisture can gather between the shroud and gland, leading to the development of an unwanted biotope. Hazardous area cable glands usually have a wide ambient temperature range. Plastic glands typically are suitable for -40 to 60°C, whereas metal glands are suitable for -60 to 80°C. Special cable glands are available for temperatures up to 200°C and above, as well as for very low temperatures.
Correct installation is key to a safe application. The simpler the gland construction, the fewer errors will occur on site. Good examples can be found with the installation of Ex d cable glands for armoured cables which contain up to ten different parts. These have to be combined in the right sequence and manner. Crucial requirements are the correct cutting of the armour, positioning the armour precisely at the end of the armour cone and the proper fixing of the armour by means of the corresponding tightening ring. A simple but common fault is the loss of inner components when disassembling the armoured glands prior to installation. That happens especially to the armour cone and tightening ring. Quality glands secure both parts with inner O-rings. As an additional advantage, the ingress of water is avoided. Using the correct tightening torques for each part of the installation is paramount, although in practice many installers prefer to rely on their experience rather than a real torque spanner.
Cable glands are among the most crucial components for ensuring a safe installation of electrical equipment in hazardous areas. Although everybody knows how they work in principle, errors can occur during the selection and installation process which can harm or even destroy the integrity of the explosion protection.