At Atexor we like to emphasize this: the certification is your friend. It provides (or should provide) proof that the Ex equipment you are buying is what it claims to be. An Ex certification (or ‘Ex-cert’) covers both the product and its manufacturing process. This is actually two separate certificates – one for the product, and one for manufacturing process (production quality assessment).
An Ex product cannot be certified without the process also being certified. This is critical and needs to be clear to anyone involved: product certification is based on sample units only. The manufacturing process needs to ensure that all units have the same, or very nearly the same, quality as those tested samples.
Where things can get a little less clear is the certification process. There are two options: self-certification or notified body (3rd party certification). The self-certification still needs to be signed by a notified body, but it is otherwise allowed for Ex Zone 2 where the explosion risk from gas is very low. Zone 0 and 1 Ex products must have 3rd party certifications.