What makes a device intrinsically safe?
This is based on UK/European practices: -

Firstly you need to decide what category zone and group you are likely to be working in. From memory Zone 0 is the most onerous and again, from memory, only Ex i (intrinsically safe) equipment is allowed. More than likely you are in group 2 because group 1 is reserved for mining environments.

I think these days you need to base things on the ATEX directive (94/9/EC) and it refers to Categories instead of zones, category 1 being the most onerous and requiring the greatest methods of protection.

There is also a very good guide from MTL on intrinsic safety that can be found here. I'd also take a look at this wall chart too - it contains some really good info like this: -



There is also MTL's Flammable facts poster that I remember referring to when designing Ex i equipment and contains such gems as this: -



Good luck.