arpadveress 
Do you follow all points listed by manufacturers? All in line with their conditions set in Ex certifications and related manuals?
As long as it is not, you may not have an Ex device with you even if you purchased the one next to you as an Ex device…
One Example: one light fitting is certified with a Philips tube, you do maintenance and replace it to Osram. Both are lighting. So visual inspection shall be fine. BUT it works not anymore according to valid Ex certificate. So not Ex.
That simple.
Keep up good work
Arpad
