Erik Malund wrote
QUOTE

I doubt very much that you can make a current loop that meet the high impedance requirements for intrinsically safe.

UNQUOTE

Getting a little off-topic now, but allas. Do not doubt, not even a little!

For zone 0, at 5V open clamp voltage on the Ex-supply output, it's still safe to draw several hundreds (!) of milli-amps. You may verify this in the explosion characteristics. The impedance in that case is quite limited even if one would use just a simple metal wire resistor for current limiting. But as you may know it is also possible - and allowed - to use electronic current limiting, as long as it's fail-safe in design, provided with a suitable galvanically separation and sufficiently fault tolerant, meaning that the limiter has to be doubled (for zone 1 applications) or tripled (for zone 0). Away with high impedances!