Intrinsic safety (Ex i) is the most elegant method of protection: it does not rely on an enclosure. Instead the circuit is designed so that the electrical and thermal energy in it is physically incapable of igniting an explosive mixture — in normal operation and under fault conditions alike.
Levels of protection
- ia
- Safe with up to two independent faults — permitted in Zone 0
- ib
- Safe with one fault — Zone 1
- ic
- Safe in normal operation — Zone 2
The practical advantage
The great advantage of Ex i is that equipment can be worked on live, without shutting down the area — because the circuit is inherently safe. That makes it the usual choice for instrumentation, sensors, transmitters and portable equipment.
One important note: an intrinsically safe circuit is a whole system — device, cable and barrier together. Replacing any one element with an uncertified part defeats the entire protection.