In the process instrumentation field, the most used methods of protection to reduce fire and/or explosion dangers are: Intrinsic Safety, Explosion Proof, Pressurization enclosures.
Below is a summary comparison of the considered protection methods in process control.
The method used for comparison is explosion proof enclosure "d" that perhaps is the most known and longest applied.
The analysis of the probability of ignition of a dangerous mixture could make one believe that a protection method has a degree of protection greater or lower than the others.
The Containment method, for example, has a much larger risk probability than Intrinsic Safety (10-7 vs 10-17) but from a statistical point of view, after over 50 years of use; there has been no report of any accident happening due to an explosion proof
enclosure. Therefore, considering a protection method safer than another is not correct because, if it is properly designed and installed, there is no practical difference.
The safety factor, shown above, considers only the human factor as the principle cause of a dangerous event. From this point of view, Intrinsic Safety presents a minor dependence on human error.
Pressurization and explosion proof enclosures require more maintenance and therefore are more subject to eventual incorrect maintenance that could endanger the safety of the system.
Pressurization is more flexible than explosion proof because it is not related to the type of dangerous atmosphere present and, despite the complexity, can be used where no other application is applicable.
Intrinsic Safety, even if related to the type of atmosphere present, is the only protection method that does not require a particular wiring, therefore allows a configuration of the system without any big problem, also for extremely dangerous areas such
as Zone 0.
The standard relative to Intrinsic Safety allows the installation in a similar way to the practice used for standard apparatus.
The explosion proof and pressurized enclosures require special devices (metal clad cables, conduits, cable clamps, etc.) and pressurization also needs a pipe line for the protective gas. These are the principle reasons for the higher installation cost
of these methodologies when compared to Intrinsic Safety.
Intrinsic Safety is the most advantageous because it allows live maintenance with no need of plant shut-down and also more reliable due to the use of over-rated components as prescribed by the standards.
Explosion proof enclosures require particular attention to warrant, in time, the safety especially for what concerns the integrity of the coupling joints and cable entrance.
For pressurized enclosures, there is an added cost for the maintenance of the protective gas supply system and its relative piping.
From the comparison of the 3 most used protection methods, it is evident that Intrinsic Safety, where applicable, is preferred for safety and reliability reasons and is the most economical for installation and maintenance.