Ex d Cable Gland Selection

Ex d Cable Gland Selection: Has Ignition Risk Been Overlooked?

A technical discussion of BS EN IEC 60079-14:2024, historical research and the case for examining both ignition likelihood and flame transmission

Milaco Engineering | Technical insight | October 2026

Why this matters

Selecting cable glands for flameproof (Ex d) equipment is a critical part of hazardous-area installation. The latest edition of BS EN IEC 60079-14:2024 includes a selection procedure for Ex d cable entries and a low-pressure cable test in Annex C. These requirements prompt an important engineering question: should the selection process consider only the consequences of an internal explosion, or also whether the enclosure contains an ignition source during normal operation?

This article examines the historical evidence behind gland selection, the changes introduced in later editions, and why the distinction matters for equipment such as Ex db motor terminal boxes.

Two enclosures, two different ignition scenarios

Consider two Ex db enclosures in a Zone 1 location. Assume, for discussion, that a flammable gas mixture has entered both enclosures.

The first contains an electrical switch that operates regularly and can produce an incendive spark. Each operation while a flammable mixture is present presents an opportunity for internal ignition. Repeated explosions are possible if an ignitable mixture is replenished between operations.

The second contains only correctly installed fixed motor terminals. It has no normally operating switching contacts or intended arcing source. Although faults such as loose connections or overheating must still be considered, its normal-operation ignition likelihood is materially different.

Both enclosures must retain the required Ex db flameproof integrity. But their likelihood of experiencing repeated internal ignitions is not the same. This distinction is central to the discussion.

What the original research examined

British Electrical Research Association (ERA) work in the 1970s examined flame transmission and the effect of internal explosions on cables and cable entries. The IEC's later background review describes tests in which cable deterioration varied with enclosure volume and repeated explosions. A frequently cited example involved a 37-core cable: the cable was severely damaged following seven ethylene explosions in a 30-litre enclosure, whereas ten explosions in a 2-litre enclosure did not produce comparable visible deterioration.

Those results do not establish a universal safe enclosure volume. They do, however, demonstrate why repeated explosions, enclosure volume and cable deterioration were relevant considerations in the earlier technical debate.

The historic Ex d gland-selection chart considered whether the enclosure contained a source of ignition during normal operation. This was not a claim that fixed-terminal enclosures could never experience an explosion; it was a way of differentiating normal ignition scenarios when selecting cable-entry arrangements.

What changed, and why?

Subsequent German PTB and DEKRA investigations examined flame propagation through cable constructions following internal explosions. The IEC's background review describes the importance of gaps within cable construction and the influence of cable length. These findings informed the revised approach adopted in IEC 60079-14:2013, which moved away from the previous chart and placed greater emphasis on cable construction and length.

There is an important distinction between these questions:

• Ignition likelihood: How credible is an internal ignition during normal operation, and could repeated explosions occur?
• Flame transmission: If an explosion occurs, can flame travel through the cable-entry arrangement and ignite an external atmosphere?

The German results provide evidence relevant to the second question. The published IEC summary does not, by itself, establish that the first question became technically irrelevant.

The UK's recorded objection in 2014

The UK national Annex NA to BS EN 60079-14:2014 is particularly revealing. The UK committee stated that it had not seen documented test evidence justifying removal of the former chart or its replacement. It also raised the possibility of thermal damage to cable insulation and bedding from multiple internal explosions and expressly questioned why the new procedure applied to all flameproof enclosures rather than only those with a source of ignition in normal operation.

This is a documented technical objection, not proof that the replacement procedure is unsafe. It shows that the issue of ignition sources and repeated explosions was recognised and disputed during the standard's development.

What does the 2024 cable test actually demonstrate?

BS EN IEC 60079-14:2024, Annex C, specifies a low-pressure procedure to assist selection of Ex d cable-entry devices in conjunction with Figure 2. The test uses a representative cable sample and a nominal 5-litre test enclosure. Its initial pressure is 0.30 kPa (3 millibars), and the pressure must take at least five seconds to fall to 0.15 kPa.

Crucially, Annex C expressly says the procedure does not prove that a cable is gas-tight and must not be used for purposes beyond the specified cable-entry selection. The standard describes a correlation between breathing-test performance and limiting flame transmission; it does not present the test as a simulation of explosion pressure.

This does not make the test invalid. Indirect tests can be useful where their correlation with the relevant hazard is sufficiently established. The technical question is how broadly that correlation can be relied upon across cable constructions, gas groups and repeated-explosion exposure.

The Ex db motor terminal box example

An Ex db motor with a genuinely flameproof Ex db terminal compartment must comply with the applicable Ex d cable-entry requirements. It must not be confused with a motor using an Ex eb terminal compartment, for which different entry rules apply.

A fixed-terminal Ex db box may have no normally occurring ignition source, yet the current selection method does not provide a general exemption on that basis alone. Depending on the precise Figure 2 route, cable construction, cable length, supporting test evidence and equipment documentation, a barrier-type entry may be required.

Annex C evidence need not necessarily be produced by the installing contractor: the standard permits representative testing by the cable manufacturer or another competent person. Any selection must follow the actual standard, the equipment certificate and the manufacturer's instructions.

The technical question that remains

The debate is not whether Ex d cable entries must contain an internal explosion. They must maintain the protection required by the equipment's certification.

The question is whether the available research adequately explains why the presence of a normally occurring ignition source ceased to be a selection criterion, given that the earlier work addressed repeated explosions and cable deterioration, while later work emphasised flame propagation through cable construction.

A useful request to the standards committee would be:

What documented technical evidence supports removing the presence of a normally occurring internal ignition source as a factor in Ex d cable-entry selection, and how was the earlier evidence concerning repeated explosions and cable deterioration evaluated alongside the later cable flame-transmission research?

A further question is whether the Annex C acceptance criterion has been validated against flame-transmission performance across relevant gas groups and cable constructions, including after repeated internal explosions.

Conclusion

The historic British and later German research appear to address complementary aspects of the same hazard: how likely an internal ignition is, what repeated explosions can do to cables, and whether a cable entry prevents flame transmission if an explosion occurs.

It would be inappropriate to conclude from the available published summaries that the 2024 requirements are unsafe, or that the standards committee ignored the earlier research. It is nevertheless reasonable to ask for the documented technical basis for changes to the selection criteria and for the strength and limits of the Annex C test correlation.

At Milaco Engineering, our position is that Ex equipment must be installed in accordance with the applicable standards and certification, while the engineering rationale behind those requirements should remain open to informed technical scrutiny.