Calculating maximum permissible track amperage for intrinsic safe PCB
I cannot figure it out how the current calculated

They're probably not calculated because they include strong safety margins for maintaining intrinsic safety thus, the current won't produce an excessive over-temperature (for a given track width) that might ignite a contacting gas. It's highly likely (that along with spark ignition testing on various gases), these currents were deemed &quot;infallible&quot; in that a particular ignitable gas couldn't ignite it AND, this testing was quite likely to have been done in a small ignition chamber hence no calculations. I've seen the chambers used for spark testing and I suspect a similar method was used after all, why not.