Primary explosion protection (this takes preference):
• Measures that prevent an explosive mix arising, for instance by ensuring that there is an overpressure of clean air or an inert gas in a cabinet.
Secondary explosion protection:
• An explosive mix can still arise, but measures are taken to prevent the mix being ignited, for instance, by encapsulating the potential source of ignition or by limiting the electric potential to the source of ignition using Zener barriers.
Tertiary explosion protection (if there is absolutely no other way):
• These are measures that are aimed at limiting the result of an explosion to an acceptable level. This can be accomplished by, for instance, employing rupture discs, flame arrestors and/or explosion hatches.
In any Ex hazardous area training normally one goes for primary and secondary, tertiary is always less in focus (better say not at all).
How one inspector does his work, when tertiary is not part of his (her) studies?!
Keep up good work!