Separate Intrinsically Safe Circuits

You need to consider the consequences of one fault applied and of two faults applied, Murphy. That is what the testing house will do in an attempt to cause an explosion in test conditions.

This may be a good place to start:

http://www.amazon.com/Intrinsic-Safety-Independent-Learning-Module/dp/0876646356

There are two ways of creating IS circuits.

1 Using an IS power supply where power is limited at source. The power supply can feed multiple circuits. The L/R cable ratios and capacitance criteria still apply. Be careful when connecting the common points. Each circuit should be directly connected to a busbar, not daisy chained, because you have limited energy and even small resistances of 0.15Ω at each joint can build up to give problems when wired in series.

2 Using IS barriers. These are only used on single circuits. Not because you can't wire more than one circuit, but there are so many possible combinations that barriers are only certified for use on an individual circuit.