Hi,
Many years ago I attended an Intrinsically Safe Equipment course regarding the design of equipment to be used in explosive atmospheres - it was a bit of an eye-opener.
The course started with a photo of the chard remains of a victim of the Flixborough explosion (just to grab your attention!).
It was pointed out that hydrogen only ignites above a surface temperature of 400C and it was demonstrated that a steel hammer hitting a piece of aluminium can ignite an explosive atmosphere.
Apparently to stop a spark causing an explosion you have to limit its energy measured in joules and for each type of gas mixture there is a different limit. Well, 1 joule/sec is 1 Watt and Watts equals volts x amps as we all know.
Any Intrinsically Safe Equipment has to be capable of (if I remember correctly) having three faults within it, but still limit the available energy by means of resistors. This also means considering any devices that can store energy i.e. capacitors and inductors (motors).
If you can limit the available voltage to 1.2V it is then deemed "Simple Equipment" and is exempt from the regs. Also, if the equipment is enclosed in an air-purged enclosure it is also allowed in the explosive area, though if the air-purging fails the equipment has to be automatically isolated. Zener barriers, which limit the available supply voltage, can also be used.
You can have a metal mesh as a barrier, so if a flame developes it energy is sapped away by the mesh (remember the Davy Lamp).
To work out the available spark energy you would have to know the inductance value of the motor across the commutators, but thats getting a bit silly!
If you ensure that the motor ran in a pure butane/propane atmsophere that would be ok! Our generators ran at 23 thousand volts in a hydrogen atmosphere quite happily (except once!) and we used to make our own hydrogen and pump it up to over 400 psi.
In the early development of my computer controlled steam boat the "Vital Byte", I had trouble with the computer behaving erractically and finally traced it to a steam jet causing an electrical discharge much in the same way as those tank washing jets.
The final idea is to use a canary - if it stops singing you know you're in trouble.
Ian G.