The equipment is simply contained in a heavy protective enclosure, usually made of die cast steel, occasionally plastic. If heat or sparks from faulty equipment within the enclosure ignite flammable gas present with it the resulting explosion is contained within the enclosure. In North America metal conduit must be used for field wiring. In Europe and elsewhere suitably rated cable is connected directly to the equipment using certified flame proof cable glands.
Advantage – simple to design the system, suitable for high power equipment
Disadvantage – equipment becomes extremely heavy & expensive; opening the enclosure while powered is not permitted
This approach limits the energy available to the intrinsically safe (I.S.) equipment, usually less than 2 watts, by means of a galvanic or zener barrier in such a way that under no circumstance will the equipment be able to generate sufficient heat or sparks to ignite flammable gases. Both the I.S. equipment and the zener barrier must be certified ‘Intrinsically Safe’ by BASEEFA, SIRA or a similar authority..
Advantage – considerably cheaper than comparable flame proof / explosion proof equipment, no special cabling required. Live maintenance permitted, no need to shut down the plant
Disadvantage – only suitable for low power equipment e.g. sounders, beacons and smoke detectors (which must be certified Intrinsically Safe)