Is there a power threshold below which a circuit is considered to be intrinsically safe? For example, if one were to power a tiny Arduino circuit at 5vdc / 30mA = .15W could such a device be considered Intrinsically Safe and used in ATEX Zone 2 or Zone 0 WITHOUT going through certification?
Alternatively are there IS enclosures that are also RF (2.4Ghz) 'transparent'?
As has already been mentioned, you need to have a recognized certifying authority test your device to ensure that it meets the requirements for Intrinsic Safety.
Your power level is fine but you have to ensure that the total amount of stored energy is below the limit for your voltage rating. That means that capacitors can't be larger than some value; same for inductors.
Another option that you can explore is to use a certified Explosion-Proof enclosure. Again, there are rules that specify how the connections are made to such an enclosure (epoxy-sealed conduit, etc). However, if you are building only a few devices, this can be a less expensive method than going for full Intrinsic Safety certification.
FWIW - it costs us anywhere between Can $5,000 to $10,000 for Intrinsic Safety certification for each of the hazardous-location devices that we manufacture. There are also quarterly inspections that each cost several hundred dollars for devices that are type-approved.
What would this circuit be powered by and the energy stored in the source is it substantial ? Low voltage isn't going to be an issue if you have no boosters or chargpumps that can fail and push voltage to 20V or so even temporarily. Other issue though is short-circuit currents through high enough inductances that may unload the energy stored in forms of small enough sparks, but small enough to ignite say hydrogen etc. the 60079 standards provide empiricaly sources graphs for ignition of various gases with respect to inductances and/or capacitances. there will be no way you could bypass a certification body, but a simple circuit will always help with spedding up things and keeping costs down. Simple enough would be an alkaline cell with maximum possible voltage of 1,65Volts and an LED and resistor in series, and this would require probabaly the least risk analysys of any useful circuit, but then certification would still cost in the orders of a few thousand (just for getting the stamp on a paper)
Intrinsically safe device does not create sparks of sufficient energy to ignite certain explosive atmosphere (what gases and what energy depends on classification).
With no certification approval, you could not go to zone 0, however the zone 2 of Ex i in concept can be self-declared by manufacturer in Europe (ATEX).
