1. The selection of the cable gland
In the plant concept it has always been of primary importance the physical separation between safe area and classified area. This concept was applied in the electric energy transport systems by means of cables buried in sand bed and then derived to the user (an electric motor or a junction box or an end user) with the use of a conduit tube suitable for the insertion of electrical cables (Freez-Moon) free from internal ridges and terminated on the end user through a physical separation made with explosion-proof sealing device (for example a sealing fitting).
With the advent of the IEC/EN 61241-0 and IEC/EN 60079-0 standards, as well as the IEC/EN 60079-1 standard, changed the concept of cable entry in electrical and electronic equipment suitable for areas with danger of explosion, allowing to have the cable entry with appropriate sealing cable gland. Lately, the IEC 60079-14:2013-1/EN 60079-14: 2014-03 standards, which describes the design, selection, and installation of electrical systems in explosive atmospheres, introduced, in the paragraph 10.6.2, the selection criterion of cable glands to be used for cable entry into electrical and electronic equipment suitable for installation in such atmospheres. This means that substantial changes have occurred over time. The effect of regulatory unification, both in Europe and internationally, changed the concepts of classification from a "Rigid" system, a system that provided unique dimensional parameters, to a "Calculation" system according to the type of gas, the concentration, the aeration capacity of the classified area and other process parameters that have become a prerogative of the process engineer.
For a greater clarity and to give a precise description of this conceptual variation, in the graphs below we show the differences between the previous version of the standard IEC 60079-14: 2007-12/EN 60079-14: 2008-10 and the version currently in force of the IEC 60079-14:2013-11/EN 60079-14:2014-03, fully reporting what is described in these standards.
In the current version, in paragraph 10.6.2, it is described as follows*:
"10.6.2 Selection of cable glands"
The cable entry system shall comply with one of the following:
a) Sealed cable glands provided with a special compound (barrier cable glands) according to IEC 60079-1 and certified such equipment.
b) Cables and cable glands that meet all the following:
- cable glands according to IEC 60079-1 and certified as equipment
- cables used according to the specifications of Article 9.3.2 a) of IEC 60079-14 (Note 1)
- the length of the cable is at least equal to 3 m (Note 2).
c) Indirect entries using a combination of an ‘Ex d’ explosion-proof junction box provided with sealing bushing and a ‘Ex e’ junction box.
d) Mineral insulated cables and metal sheath, with or without plastic coating, fitted with appropriate flameproof cable gland compliant to EN 60079-1.
e) Explosion-proof equipment specified in the documentation or according to IEC 60079-1. The sealing device must incorporate a compound or other suitable sealing means, around the individual cores of the cable, which do not allow the passage of anything. The seal must be installed at the point of cable entry into the equipment.
Note 1 The minimum cable length is specified to limit the potential hazard due to flame transmission through the cable (see also Annex E of the same standard).
Note 2 If the cable gland and the cables in use are certified as a part of the equipment (enclosures), the compliance to 10.6.2 paragraph is not necessary.
*Note: the versions of the mentioned standards, within this description, must obviously be those aligned with the date of issue of these rules.
Note 1 With a sheath in thermoplastic material, thermosetting or elastomeric. They must be circular and compact. Any padding or sheathing must be extruded. Any fillers must be non-hygroscopic.
It is therefore evident that, following the entry into force of the new version of the IEC 60079-14 standard, the scenario has changed, and the use of barrier-type cable glands results of primary importance.
2. The methods of sealing of the barrier-type cable glands to achieve a perfect flameproof protection
There are two different methods to be used for the realization of the "Barrier": the use of the semi-solid sealant with two components and the use of a two-component resin in the liquid state.
Using the semi-solid sealant must be performed the following operations:
- mixing the two semi-solid components, thus causing a chemical reaction of unification;
- inserting a minimum part of this compound inside the cable cores and, subsequently, covering all the surrounding parts;
- inserting the whole into the housing provided in the cable gland.
This operation, after hardening, does not allow a complete filling. In fact, air pockets could form. Moreover, during the polymerization phase of the compound, the cables must not be touched at all, to avoid the formation of air interstices between the cable sheath and the compound.
Using the bi-component resin must be performed the following operations:
- preparing the "cable head", inserting the special pre-drilled cap for the effective diameter of each individual core.
- filling with the bi-component resin prepared previously or from the single package supplied with the barrier cable gland. This operation must be carried out directly inside the housing designed for this function.
The resin of the liquid type guarantees a perfect penetration between the individual cores and outside them, guaranteeing a correct finishing in full compliance with the reference standard.
Elfit, after a careful analysis on installation problems and requirements, has preferred the second system that allows the installer to carry out a correct system and, at the same time, verify that it corresponds to the installation instructions given along with the cable glands.
We conclude by recalling that all activities of sizing and selection are the prerogative of the designer who must always refer to the specific mandatory regulations. He is entirely responsible for the analyses, calculations and the consequent executive project, certifying the compliance with the regulations.
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