types of protection according to EPL Ga do not require a second independent means of protection E.g Ex ia (IEC 60079-11), Ex ma (IEC 60079-18), Ex da (IEC 60079-1).
application of 2 independent types of protection EPL Gb: Ex d + e shall work too: an example of common mode failure is if an Ex d enclosure containing arcing components is installed inside an Ex e enclosure. should the Ex d enclosure be compromised, it would also compromise the Ex e enclosure.
one interesting question: have you ever seen an Ex ia rated JB (junction box) installed in a Zone 0?
keep up good work!