Thinking inside the box: Finding creative ways to use Fieldbus in field enclosures

The dilemma engineers face is the need to meet the area classification for the devices in the field while still providing not only sufficient power to have the maximum number of devices on a single segment but also to have the longest possible total cable length while still meeting the requirement of a minimum of 9-volts at each device.

The now-traditional way of managing a reliable bulk power supply is to mount the power supply and power conditioner in the control room environment where conditions are well controlled and the uninterruptible power supply (UPS) is close at hand.

For intrinsically safe installations, there are two options available to end users; the IS (intrinsically safe)/FISCO (Fieldbus intrinsically safe concept)/FNICO (Fieldbus non-incendive Concept) completely live-workable solution, or the high energy trunk solution that uses Fieldbus Barriers in the field junction box to reduce the trunk voltage to acceptable intrinsic safe levels on each of the spurs. However, because the Fieldbus Barrier has both IS equivalent and non-IS energy sources in the same enclosure, you must take special precautions during installation and maintenance. The most important of these is isolating/separating the two types of cables.

Until recently, each of these Fieldbus barriers supported four spurs and you could install up to four Fieldbus barriers on a single Fieldbus segment. The reason I say recently is because there are now eight spur versions of the Fieldbus barrier on the market.

Despite the fact the Fieldbus specifications allow a maximum of 32 devices per segment, part of the reason for the four barrier limitation on the high energy trunk solution is that several host systems support a maximum of 64 devices per H1 interface card. Since each card has four ports, this works out to an average of 16 devices per port.

Depending on the gas group, FISCO and FNICO are able to provide the sufficient current to power 16 devices on a single segment. FISCO and FNICO power supplies work with the much simpler field segment blocks that are similar to traditional terminal blocks in their installation on the field enclosure.

However, almost all FISCO and FNICO power conditioners also serve as repeaters, so one possible way to overcome any length restriction is to install another FISCO/FNICO power conditioner at the appropriate distance along the home run cable to boost the signal back to its original strength. (The Fieldbus specification permits the use of a maximum of four repeaters on a single segment and an associated maximum cable length of 9,600 metres.)

If, as is normally the case, the bulk power supply is installed in the control room, and bearing in mind that the maximum voltage level for a FISCO or FNICO power conditioner is restricted by the requirements imposed by remaining below the safety factor and ignition curve limitations, the result of the lower voltage may be a restriction on the trunk or home run cable length to the range of 500-600 metres. This distance is normally not a problem in most installations, as the interface room is within this distance from the field devices.

The challenges, therefore, become obtaining a reliable bulk power supply in the field so that if required, the power conditioner/repeater will operate with the same level of reliability as the balance of the system and having a non-intrinsically safe energy source in a field enclosure. This second challenge is the same one the Fieldbus barrier solution faces.

One option is to have a field-based 24 VDC supply. Another is to run a two-pair cable to the field junction box; one pair with the H1 signal and the other with a 24 VDC supply. Of course, if you’re not using the 24 VDC supply (which you can now connect to the interface room UPS) for anything other than power, you can, if needed, leave the control room at a much higher voltage to supply 24 VDC at the field end of the cable.

Fieldbus is called so for a reason. It is a bus-based network designed for use in the field. What is required to make it work is that we as engineers, designers and maintenance workers must continually look for creative ways to use field enclosures for an “inside the box” solution.