The simplest way to determine between Explosionproof (called flameproof in Europe) and Intrinsically Safe is as follows;
Flameproof. This protection concept does not prevent the ignition of gas within the flameproof enclosure but the enclosure design is such that if an explosion takes place, the cooling effect of flamepaths - specifically designed, high tolerance gaps in the enclosure - mean that by the time the expanding gas/flame propegate from inside the flameproof enclosure to the plant outside, there is insufficient energy to re-ignite the surrounding gas and cause damage to the plant.
It is a common misconception that if a flameproof enclosure is used, it is acceptable to install any component within that enclosure, the thinking being that since the enclosure can contain and cool and explosion, anything it allowed within. This is unfortunately not that case, take batteries for example specifically secondary cells (rechargables), it is not permitted to charge these cells within an flameproof enclosure as the off-gassing produced by some cells during charging can change the gas mix within the enclosure causing the potential for a higher energy explosion and as such the calculated flamepaths may not be sufficient to cool the gas as it expands.
Another example is the Temperature Class or Autoignition Temperature. To obtain a T-Class for your flameproof device it is nessecary to quote the overall power within it so that the test body can establish how that power will translate into an external surface temperature.
Intrinsically Safe This protection concept relates to the limitation of power to sub-incendive levels which means that the device itself cannot create a spark or surface temperature to cause an explosion. For fixed devices located permanently in the field, this is normally achieved by using either a Zener Barrier or a Galvanic Isolator, the former requires an IS Earth/Ground the latter does not. These devices effectively limit the power available to the field device by way of a series of diodes/resistor networks which prevent an explosion.
Again, a common misconception is that if one incorporates a barrier then the downstream device itself does not need to be certified. This is not true, each part of the system must be matched. Similarly, the term "simple device" is not an excuse for the removal of a barrier altogeter, a "simple device" - such as a basic switch - is only simple when protected by a suitable barrier.
For handheld devices, this becomes more complicated still as we must protect the device within itself rather than relying on a high level protection device located within a safe area (barrier). Everything from the battery itself through to inductive and capacitive loads, creep and clearance and even oversized track widths must be considered when designing a handheld device intended for cerrtification as intrinsically safe.
To cap it all off, in Europe according to the ATEX Directive there are two types of Intrinsic Safety, Ex ia and Ex ib which defines the level of safety and hence the areas within which a device may operate. Ex ia devices can deal with a double failure of the protection system whereas an Ex ib device can deal with a single failure, as such Ex ia devices can be used in Zone 0, Zone 1 or Zone 2 areas whereas Ex ib devices can only be used in Zone 1 or Zone 2 areas.
This is not the case in the US where there is only a single level of protection covering all Classes and Divisions.
For more information on IS vs XP, you can visit our website and look through the knowledgebase or alternatively I can send our IS vs XP explanatory whitepaper directly if you email me on marcus.halliday@cord-ex.com
I hope this was helpful.
Marcus Halliday
CorDEX Instruments Ltd.
www.cordexinstruments.com
marcus.halliday@cord-ex.com