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The enclosure failed on some bolts that caused a part to fly through the door at the back. After the bolt threads were redone and more bolts were added the enclosure passed all the tests and even a 10% overpressure test.
Lessons to take home: every bolt on a flameproof enclosure counts and it is vital that every bolt is inserted, tied down and the threads undamaged. Lastly get your enclosures tested at an accredited facility that actually do the tests! Flameproofing (Ex d) for you who don't now is a form of explosion protection applied to electrical equipment to prevent them from causing ignitions in explosive atmospheres like in coal mines, petrochemical plants and many other factories that handle and produce flammable materials.
A flameproof enclosure like this one can be installed in a flammable atmosphere and can "contain" an explosion (caused by ignition sources like switchgear) and so prevent ignition of the outside atmosphere.