Barrier Gland Volume Rule — IEC 60079-1 Explained

The 2 litre volume rule from IEC 60079-1 is one of the most tested topics in CompEx Ex01/Ex02/Ex03/Ex04. It determines whether a barrier or non-barrier gland is required for SWA cable entering an Ex d enclosure via a threaded entry.

The Rule

• Ex d enclosure, threaded entry, volume ≤2 litres — non-barrier gland acceptable for SWA cable
• Ex d enclosure, threaded entry, volume >2 litres — barrier gland required for SWA cable
• Ex d enclosure, clearance entry, any volume — barrier gland always required

Why Does Volume Matter?

The internal volume of an Ex d enclosure determines the maximum explosion pressure that could develop from an internal ignition. A larger volume produces higher pressure, which is more capable of forcing gas through the small spaces between armour wires in a non-barrier gland. Above 2 litres, the explosion pressure is sufficient to create this risk — hence barrier glands are required.

Finding the Enclosure Volume

The internal volume should be stated on the Ex certificate or equipment nameplate. If not immediately obvious, check the ATEX/IECEx certificate. For equipment in service, the volume may be marked on the label or available from the manufacturer's documentation.

Common Mistakes

• Assuming all Ex d threaded entries require barrier glands — not true for ≤2L enclosures
• Applying the volume rule to clearance entries — clearance always requires barrier regardless of volume
• Applying Ex d rules to Ex e equipment — Ex e does not use barrier glands
• Not checking the actual enclosure volume — never assume, always verify from the certificate