Figure 1: The selection criterion of EN 60079-14 standard concerns the choice between the cable gland with sealing ring and the barrier cable gland for entry into explosion-proof enclosures or equipment

Using cable gland with sealing ring, the criticality in the explosion phase is due to the possibility of the flame passing through the space between the sealing ring of the cable gland and the outer sheath of the cable. Once this risk has been averted, the possibility remains that the flame passes through the interstices inside the cable. Tests conducted in explosion-proof enclosures where explosions have occurred repeatedly have shown that the cable tends to be thermally damaged and no longer guarantee sealing in the event of subsequent explosions.

The barrier cable gland is installed by eliminating the sheath in the cable for a certain distance and the solid filler (figure 2). In their place, for a certain length inside the cable gland, the sealing is carried out with a special compound (figure 3, the section of sealing inside a barrier cable gland is highlighted in red).

The selection criterion between barrier cable gland and cable gland with sealing ring is described in the EN 60079-14 standard. A schematic representation of it is in Figure 4.For the reasons mentioned above, the type and length of the cable also falls within the selection criteria. 

The longer the cable, in fact, the less the possibility that the flame seeps into the interstices inside the cable.

Figure 4: flow chart based on the selection criterion of the cable gland [2], the cable glands must be certified as ‘Ex db’ equipment

It is right to remember that, until 2016, (year of cessation of conformity of the EN 60079-14: 2010 standard) the old criterion for selecting the cable gland had always been applied in Europe which, as indicated in figure 5, took into consideration the volume of the enclosure, the gas group and the presence or absence of scintillating material.

Currently in the countries that adhere to the Atex scheme it is allowed, except for national derogations, to apply only the new selection criterion.It is fair to report that the new selection criterion of the cable gland, summarized in the flow chart of figure 2, has aroused considerable perplexity in the circle of technicians.

In fact, the old flow chart in figure 3 went into more detail on the merits of the application and the risk associated with the formation of an explosive atmosphere.

The presence of ignition sources inside the enclosure, the gas group (group IIC has a higher probability of flame passage in the cable as per the tests conducted at the ERA laboratories [3] in 1976), the installation area and finally, the internal volume of the enclosure (volumes greater than two liters brought significant thermal damage to the cables during the explosion). [4]

The familiarity that many technicians had matured with this selection criterion is certainly in favor of the old flow chart, as well as the fact that, since its appearance in 1996 within the second edition of IEC 60079-14, it had guaranteed a high level of safety.

In this perspective we can understand the decision of the British national committee to insert the old flow chart as NA (NA = National Amendment national special conditions) of edition 6 of EN 60079-14 implemented in the UK.

Figure 5: flow chart for the choice of the cable gland up to the 2008 Edition of EN 60079-14 standard

We have reviewed the technical reasons behind the choice of the ‘Ex db’ cable gland to use, but indeed economic considerations also come into play, as the barrier cable gland is more expensive and more difficult to install.It is therefore necessary that designers and installers are updated on recent regulatory developments so that they can always make their choice in the sign of maximum safety and consistency with current legislation.

Notes, reference standards and bibliography

[1] THE EX-D PROTECTION METHOD THE SEALING FITTINGS - Cortem 24/3/2022 - Figure 4

[2] EN 60079-14: 2014 10.6.2 (b)

[3] ERA Laboratory in UK

[4] Significant and controversial changes to explosive atmospheres installation and inspection standards - Peter Roberts / HazardEx 10/2014