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Can any body tell me the minimum level of wattage of signal (power or control) to ignite the hydrocarbon atm (like refinery complex), beacuse it is required to decide the IS / NIS device suitablity?

You are not interested in the wattage so much as in the energy that can be released into a spark. This depends on the voltage and capacitance, and, separately, on the current and inductance. It also depends on the
particular gases involved. The relationships are complex and are given in many standards covering Intrinsic Safety. A link to the ignition curves is given in:

http://www.gminternationalsrl.com/?p=intrinsic_safety_basic_princip les.

Hope this helps,
Bruce.

It is the amount of energy available to the fuel mix thats important and that can be delivered not just thru sparks but hot surfaces also. The hazards present in the atmosphere need identifying first and the extent of them. In addition for intrinsic safety there are other considerations to take into account (other than energy/wattage) in order to design a safe system. It sounds like you need some education.

Go to the MTL instruments web site, google potentially hazardous atmospheres or flammble atmospheres & equipment for a basic grounding. Also you need to check your local regulations. The "global" standard is the IEC60079 series of standards but these may not be applicable in your part of the world - check.

Hope this helps

DaveMH