Control.com - Nerds in Control

I want to know, what is the limit of current and voltage levels below which it is called as intrinsically safe and above which it is not intrinsically safe?

That is a loaded question.

The hazardous areas are divided into classes or divisions with further tags of applicable gas groups. It is these gas groups that decide what is the energy that must not be permited to go to the hazardous areas. This is decided by the 'iginition energy' of the gas. (Methane, propane, butane). The curve that is obtained is the result of the applicable voltage & associated current that is necessary to trigger an ignition. The energy used by the zener barrier of equipment has to be below this value at all times. The manufacturers play with the values of voltage & current, constantly making sure that they are well below the ignition energy of the particular gas group.

Notice: If you are a novice at this kind of thing pls make sure that you understand all the details. there are many people who use loose thumbrules as there is not too much understanding of this in the Indian Industry (assuming you are from India).
The zener element has to also be placed in the non-hazardous zones for the protection to be effective.

Various situations develop, each require their own solution.

Visit sites of Crouse Hind, Stahl or MTL.

Regards,
V Chavan