Why does water collect inside a sealed enclosure (condensation)?
A sealed enclosure keeps moisture in as much as it keeps water out. As the temperature changes, the resulting pressure difference draws humid air in through gaps at the seal and gland; when the enclosure cools, this moisture condenses on the inner surfaces. The solution is a breather plug or breather gland with an ePTFE membrane; it equalises the pressure and does not pass liquid water.
This is the situation that surprises people most during installation: the enclosure is IP65 or IP67, yet water collects inside it. The cause is not leakage from outside but thermal pumping. The air inside the enclosure expands as it warms during the day and escapes; when it cools at night, the resulting negative pressure draws humid air in from outside. Each cycle carries a little more moisture inside, and when the temperature falls below the dew point it condenses.
Countermeasures:
- Breather plug: fit a breather plug or breather gland with an ePTFE membrane. The membrane passes air and water vapour but not liquid water; since the pressure is equalised, the pumping stops.
- Position: place the plug not on top of the enclosure but on a surface where water will not collect and which will not take a direct jet.
- Entries downward: position the cable entries facing downward and give the cable a drip loop before the enclosure. Water collects at the lowest point of the loop and drips off, never reaching the entry hole.
- Sealing discipline: use the gland with a cable matching its sealing diameter, and close unused holes with blanking plugs.
- At the moment of assembly: do not leave the enclosure open in humid weather; the moisture you shut inside will condense later.
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