3.3V overvoltage limiter circuit topology
The circuit needs to be verified formally against failures of single
  components and component ratings against EN60079 regulations (see
  §7.6d of IEC 60079-11 Ed 6). Countable faults and infallible assembles
  are explicitly requiring that the circuit is well known and each
  source of fault is described in detal. ICs. are explicitly banned for
  the purpose.


It sounds like you are trying to build a zener barrier and zener barriers use two zeners for category i(b) and three zeners for i(a) intrinsic safetly category. Zeners are not regarded as infallible and barrier designers use more than one and each one is individually rated to perform without the others.

Zeners of course fail (they are not regarded as infallible) and the same is true for ICs so, I would avoid the problem you appear to be creating for yourself and work along the lines of using an IC and duplicating or triplicating the circuit in order to reach the level of intrinsic safety required for the device.

ICs are NOT explicitly banned BTW.