FM Approval 3611 - Instricially safe

[guilio2010]

If you look at the ignition curve for resistive load, Figure B-1 page 28, I'm surprised by what the FM shows vs. some of the post that are listed on this site for intrinsically safe. Most intrinsically safe that I have worked with keeps the power minimal, in the mW, however, from the graph, 12V @ 5A will not have enough power to spark/ignite the gas? If this was the case, then 60V should be able to go to 1A, however, this is not the case according to the graph. The only thing I can thing of is maybe 12V is not enough to cause a arc between the air gap, ie switch closing and the voltage gradient increase as the switch gets closer to the contact point? Has anyone actually proved or done a test of this nature with explosive gas and arcing 12V with 5A?

Thanks,
guilio

 

[DRWeig]

As far as I know, those curves came in print originally from the US Bureau of Mines. That was a long time ago, and I think that bureau is gone. FM and UL may have reproduced those tests...

Best to you,

Goober Dave

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[zappedagain]

I work off IEC 60079-11. Their Figure A.1 and Table A.1 also shows 12V 5A for Group IIC apparatus classified as Simple Circuits. Section A.2 has a note stating that simple circuits may not apply to practical circuits. As soon as you start dealing with inductance (that every wire has) the current limits go way down.

I believe you are correct that the 12V won't spark; I have an old reference showing about 34KV/in at room temperature and sea level, so 12V won't jump a gap larger than 0.3 mils.

Z