While preparing a training for a customer, I made a thorough research of the current status of Fieldbus/Profibus device coupler’s technology and availability and spent some time working out a clear way to present the Ex ic fieldbus concept which is not as conceptually clear as the older Ex i a/b concepts.
Nowadays everybody involved with fieldbus infraestructure components is familiar with the Entity, FISCO and High Power Trunk concepts, at least for Zone 1 applications.
Entity fieldbus was not practical, it was way too limited in voltage and current to be really an option for large projects.
FISCO was developed with the purpose to make IS fieldbus economically feasible, by increasing the allowed voltage and current in the segment and thus enabling the use of more field devices in it.
But FISCO power supplies are expensive and complicated.
Enter the High Power Trunk concept.
The idea, although radical when first presented, now seems almost obvious: In order to increase the number of field devices in a Zone 1 segment and to avoid the current and voltage limitations of both FISCO and Entity concepts, the solution was to eliminate the need of an IS trunk and put the IS galvanic isolation into the device coupler, obtaining IS spurs and using Ex e protection concepts for both the trunk and the device coupler’s junction box.
Work in a fieldbus segment usually is done at the spur level, the trunk remains untouched, so HPT its a safe and sound approach.
So far so good, fieldbus became easier to implement in hazardous areas.
Then a version of the HPT concept appeared as the Ex ic protection method for Zone 2 applications.
Until then, Zone 2 fieldbus was sort of a melange of different solutions that mixed Ex na, Ex nl protection methods, the FNICO approach from one of the main suppliers and some influence from the NEC non incendive protection concept and cable installation techniques.
It seemed very logical to standardize and establish a global approach for fieldbus Zone 2 applications, because in real life the majority of fieldbus segments are mounted in Zone 2 (Class I Div 2 or Class 1 Zone 2 for NEC 500 and 505 fans), at least in the O&G industry.
Zone 2 fieldbus was tricky, and some users avoided dealing with Zone 2 specific technique in order not to deal with the fine letter of the standards and went for Zone 1 methods that although costlier, seemed to be more straightforward and foolproof.
Then the IEC 60079-11 and 60079-FISCO presented the Ex ic approach. That is: intrinsic safety for Zone 2.
It sounded very good: to apply the well known IS methods for Zones 1 and 0 for Zone 2, but with added benefits due to the less stringent requirements of the environment.
But the approach followed by most suppliers was to modify as little as possible their currently available solutions in order to fulfill the Ex ic requirements and in doing so the resulting solutions turned out to be a sort of compromise.
One of the most publicized advantages of Fieldbus technology is the notion of interoperability, that is, to be able to use whatever brand of device in combination with other brands and remain confident that they will work together.
When the HPT concept appeared and the subsequent competence between this approach and the FISCO approach took place, it was very clear how both methods worked and so users could check which one fulfilled their needs in the best way. Even a mix of power supplies and isolated device couplers (a.k.a. fieldbarriers) from different suppliers could work without significant issues.
But with Ex ic, solutions are not so obvious. One has to use an Ex ic certified device coupler, which will limit the spur’s available current to Zone 2 approved levels, but if the connected field devices are FISCO or Entity, different power supplies should be used, because the usual approach is to split current limitation from voltage limitation.
So the trunk and the device couplers must be installed according to Ex e protection methods.
Voltage limitation can be implemented at the power supply, an approach that requires two versions of it, one limited at 24 V for Entity devices and another limited at 17,5 V for FISCO devices.
Or it can be implemented by the use of a trunk mounted voltage limiter at 17,5 V that enables the device coupler to comply with Ex ic with FISCO devices, but should not be mounted if we are using Entity devices. In both cases the power supply must be limited at 24 V. It gets trickier if an Entity device has to be used in this scheme: an Entity to FISCO adapter must be connected at the spur.
One supplier directly avoids offering an Ex ic solution for FISCO devices. Evidently he doesn’t want to deal with the need to support 2 power supplies with different voltage levels at the trunk.
So, for Ex ic applications, interoperability between the infraestructure components of different suppliers its not easy or clear, or even possible in some cases.
Ex ic is a logical approach but, in my point of view, its originally intended simple nature became affected by the mixture of inherited approaches, preexisting hardware and the need of different solutions for FISCO and Entity devices
Although significant savings can be achieved by its use, Ex ic needs a special care when selecting infraestructure components and requires great care in order not to mix FISCO and Entity devices in the same segment.
The great benefit in the end is that whatever solution may be adopted, users will get the benefits in intrinsic safety, that is, the possibility of live working at the spurs and a simpler cheaper installation when compared with Ex d or similar methods, but they’ll have to do their homework.
Mirko Torrez Contreras is a freelance Process Automation consultant with special focus on Fieldbus and Intrinsic Safety. He has a somewhat ambivalent point of view about the industry’s ability to create cute acronyms for any new concept that is being introduced to the market.