Hi there. A big welcome back to all the Haz Area masochists out there. I know you’re excited to hear our rant about compound barrier glands, so I’ll get right to it.
In our experience there is a common misconception about the reasons for using barrier glands in an installation. Lots of people associate them primarily with sealing to prevent migration of flammable gas or liquid from a hazardous to a non-hazardous area, and they are sometimes used for that purpose. BUT by far the most common use for them is in flameproof installations, where they are used to prevent flame from the explosion that may occur inside the equipment from travelling down the cable. We commonly use a barrier gland to seal the cable entry in an Ex d box to prevent that flame propagation.
Sealing cables to prevent migration of flammable gas or liquid is not usually needed for general equipment like lights, junction boxes, motors etc. It is sometimes needed for process-connected equipment like instrumentation, where there is only a single process seal, like an o-ring seal in a pressure switch for example in contact with fluid under pressure. If the process seal fails, flammable liquid or gas can be forced up into the instrument by the process pressure, and from there pushed down the cable. In that scenario we need to provide a second seal in the wiring to prevent flammables from migrating to the non-hazardous area.
Compound glands are sometimes used for that purpose, but we recommend a conduit seal or y-seal as a safer option based on recent experiences where compound glands have failed to prevent migration of fluids under pressure. Lots of compound glands are designed with a metal compound tube, which fits inside the entry element and forms a flamepath. This means that although the centre of the cable is sealed, there is a flamepath gap which can allow fluid under pressure through, into the back section of the gland where the armour clamp is. The very back of the gland is the IP seal, which, if it’s tight, can prevent the fluid from getting out, instead forcing it under the cable sheath into the armouring layer, which allows a migration path. The centre of the cable is sealed, but a path may still exist through the outer layers of the cable.
We should point out that we’re not singling out any particular manufacturer here – most glands on the market use similar designs, so we think they’re all potentially unreliable in preventing migration. They’re fine for Ex d cable entries, but for migration issues we use y-seals which will stop anything.