Installing fieldbus — Part 2

Fieldbus is a wonderful technology with many benefits, but fieldbus installation requires some additional considerations over and above normal 4–20 mA projects, not the least of which is how to prepare for fault conditions.

In part 1 of this article we introduced fieldbus technology and discussed the cabling, termination and power supply requirements. But now that multiple devices are connected over a single cable trunk, we need to prepare for what may happen if a device fails and shorts the trunk, or if the trunk itself is damaged.

Short circuits

Short circuits are a common problem in any fieldbus installation. Maintenance technicians can jostle cables, corrosion can weaken connections and vibration from pumps and motors can loosen cables and connectors. Segment designers must be concerned about what might happen to an entire fieldbus segment if any single instrument shorts out.

It is highly recommended that the segment designer incorporate some form of spur short-circuit protection, which may be active or passive in design. Passive protection is very simple and usually provided by fuses on each spur which blow to disconnect any individual fault. This is inexpensive and very reliable, but it does require manual intervention — someone has to replace the blown fuse (hopefully after repairing the fault!).

Device couplers often provide active spur protection in two basic forms: ‘current limiting’ and ‘fold-back’. Current limiting and fold-back types both auto-reset after fault removal and both normally incorporate LEDs to indicate spur status.

Current limiting protection

The current-limiting technique limits the amount of power the short circuit can draw to between 40 and 60 mA (vendor dependent) but it also holds that fault on the segment continuously. Although this design protects the segment from the initial short, the additional current draw from the short can deprive other instruments on the segment of power, overload the segment power supply and possibly cause catastrophic failures on the segment. If current-limiting designs are to be used, ensure that your segment power supply can cope with these additional loads.

For example, a segment may have 10 measuring devices plus two valves connected via 1000 m of 50 Ω cable (say, 250 mA total). In this case, the trunk voltage drop equals 12.5 V, which provides 12.5 V at the farthest device. However, if a short occurs at a spur and an additional 60 mA load is ‘locked in’ to the segment, this takes away enough power so that devices receive less than 9 V (8.5 V for the farthest device), and some will drop off the segment. If two shorts occur, all the devices could drop off and an entire process unit might go down. Therefore, if current limiting protection is used in a device coupler, you must provide a 60 mA safety margin. That is, do not install as many instruments as the segment can theoretically power; instead, leave at least three spurs empty.

Fold-back protection An alternative design is the fold-back variety, where any faulty spur is switched off and that load is completely removed from the segment. The fold-back technique disconnects the shorted spur from the segment, thus preventing loss of an entire segment. The fold-back technique has a logic circuit on each spur (Figure 1) that detects a short in an instrument or spur, disconnects that spur from the segment and illuminates a red LED that can be seen by maintenance personnel. With fold-back device couplers, you don’t have to worry about spur failures and can have confidence about placing more devices on fieldbus segments.

Figure 1: A ‘fold-back’ circuit, available with some device couplers, removes a short circuit from the system. This differs from ‘current limiting’ short circuit protection, which limits the short to 60 mA but keeps it on the segment.

Since the cost of H1 cards (US$2500) and other segment hardware can be cost-prohibitive, being able to place more devices on a segment can save a considerable amount.

A typical Foundation fieldbus segment, consisting of an H1 card, power supply, device couplers and cables, can cost about US$5000. A large process plant may have hundreds if not thousands of devices. If the ‘safety margin’ approach is used, where the entire capability of fieldbus is not used, the cost of all the extra fieldbus segments can become substantial.

For example, assuming that a typical fieldbus segment with modern fold-back protection can accommodate sixteen 20 mA fieldbus devices, it requires 63 fieldbus segments to support 1000 devices, at an approximate cost of US$312,500. If a safety margin approach must be used because of current limiting protection, and each segment can now only accommodate 10 instruments, then 100 segments are needed, at an approximate cost of US$500,000. Simply by specifying fold-back short circuit protection, an end user can save US$188,000.

Redundant operations

Fieldbus systems offer many advantages to process companies, not the least of which is the elimination of ‘home run’ wiring and the rat’s nest of twisted-pair wiring in field-mounted marshalling cabinets. Fieldbus eliminates all this because it allows up to 32 devices to be wired together over a single twisted-pair digital ‘network’ or segment.

However, fieldbus systems present a problem: what happens if the segment cable or the power conditioner driving the segment cable fails? Depending on where the failure occurs, the entire segment — with all 32 devices — could go down. An entire process unit could then go offline.

One answer is to provide redundancy wherever possible, to ensure that any single failure cannot take down an entire process unit. Redundancy can be employed in two basic ways:

• Redundant power conditioners
• Redundant trunks.

A redundant power conditioner (Figure 2) has two power conditioners, both powered by a load-sharing pair of 24 VDC power supplies. Such a system can survive the failure of either 24 VDC power supply or either power conditioner. If a failure occurs, the unit automatically and ‘bumplessly’ switches all load to the backup unit. It also has an alarm output to indicate that a failure has occurred. If any of the individual modules fail, replacements can be hot-swapped into place without shutting down the segment.

Figure 2: A redundant power conditioner, such as this Trunkguard unit from MooreHawke, provides redundant power conditioners, and has two sources of supply. If any single part fails, it will continue to power the segment.

The power conditioner modules plug into a DIN carrier, which can accommodate four or eight modules, to provide redundant power for two or four fieldbus segments. For a redundant configuration, each pair of power conditioner modules requires two power supply inputs and one connection to the fieldbus segment. Installation is not difficult, because a redundant power conditioner requires no changes to be made to the fieldbus segment, device couplers or interface card.

However, in most cases (depending on the vendor), the DIN carrier can accommodate simplex (non-redundant) or duplex (redundant) power conditioners, but not both. That is, you cannot mix redundant and non-redundant power conditioners in the same DIN carrier. Therefore, when determining which critical fieldbus segments will have redundant power conditioners, take care to plan fieldbus wiring so that the critical segments are routed to the proper DIN carrier.

Redundant trunks

In a critical process segment, it may be necessary to provide redundancy on the main segment cable or ‘trunk’. This protects a process unit from going down if something happens to the main cable, such as a forklift running over the cable, water getting into the conduit, or any of a host of problems that can occur in the field. If the system can be switched to a backup or redundant segment, then the process can continue operating.

It is important to note that fieldbus instruments can continue to operate by themselves if communication to the host DCS is lost. In Foundation fieldbus installations, the field devices can talk to each other, and continue monitoring and control operations according to the last setpoints provided by the DCS. However, they cannot continue to operate if the trunk cable is broken, because the cable provides power to the instruments.

Entire segment redundancy One way to provide redundancy is to duplicate the entire segment (Figure 3). This requires a duplicate interface card (such as an H1 card for Foundation fieldbus), a duplicate power conditioner, duplicate cable, duplicate device coupler and duplicate field instruments. When one segment fails, the DCS switches over to the backup segment. While this is an extremely expensive hardware solution, it does provide redundancy for every device in the segment. No matter what fails, a backup exists. To install such a system, you must determine the conditions that will cause the DCS to switch segments, and program the DCS accordingly. Check with your DCS vendor to make sure the DCS can identify a segment failure. Some can only determine that an interface card failed. If this is the case, you must devise some way of determining that a segment failed. It is possible to set up a software scheme that periodically polls the fieldbus devices, asking for device status. If none of the devices respond, the software could conclude that the segment has failed, and call for the DCS to switch to the backup segment. However, maintenance procedures then become very complex, with special overrides to cater for out-of-service devices, etc.

Figure 3: One type of redundant fieldbus segment requires duplication of every component, from H1 cards to field instruments. If one segment fails, the DCS switches to the second segment.

Trunk redundancy An alternative method is to use a fault-tolerant segment with parallel interface cards, parallel power conditioners, dual trunks and one field device coupler (Figure 4). This eliminates the need to duplicate field instruments and avoids difficult maintenance issues, while still improving the segment MTTF by 7–10 times. The power conditioners determine when a cable break occurs, cut power to the failed trunk and use the backup cable immediately. This fault-tolerant approach simplifies installation, because it does not require any special programming of the DCS. When the fault-tolerant system detects a cable break, it deprives the H1 card of power, so the DCS knows that a failure occurred and can switch to the backup H1 card. It also gets an alarm from the power supply, indicating that a failure occurred. And, because the power conditioners have auto-termination capability, the proper segment termination is set automatically. The fault-tolerant system does not require any other special hardware; in fact, the DIN-rail power conditioner modules can be installed in the same DIN rack as conventional modules.

Figure 4: A fault-tolerant fieldbus system has two segments. If a fault occurs in one leg of the system, it automatically uses the other leg. It is not necessary to duplicate the field instruments.

No special installation wiring is necessary in the field. It is probably advisable to route the two segment cables differently, so that the same physical incident — such as a wayward forklift — does not take out both cables at the same time.

If a certain type of field instrument is prone to failure, a redundant instrument can be installed and wired into any spare spur on the device coupler. The DCS, of course, has to be configured accordingly, so it recognises a device failure and knows to switch to the backup instrument.

Simplify your installation

Many of the installation headaches discussed in this article can be minimised through careful selection of fieldbus equipment at the beginning of the project.

Few end users realise that fieldbus components, such as power supplies and device couplers, are not manufactured by the DCS vendor. Instead, they are provided by associated suppliers, such as MooreHawke, and others. Therefore, even if a user is buying an Emerson DeltaV or a Yokogawa Centrum or a DCS from any other supplier, it is possible to specify fieldbus components separately. Note that the choice of physical layer product makes no difference to the DCS operation. All fieldbus power conditioners and device couplers simply enable the fieldbus power and communications to work; they do not communicate with the DCS.

To simplify installation of your fieldbus system, evaluate the components carefully from the various suppliers.

Look for:

• Automatic segment termination on device couplers to eliminate termination problems during installation, start-up and regular maintenance.
• Fold-back short circuit protection (which disconnects a shorted device from the spur) to eliminate the need to leave spurs empty.
• Power supplies with built-in power conditioning, redundancy and surge protection.

Fieldbus is an exciting technology and there are many benefits which will accrue to end users. Implementation of real fieldbus systems is still a new experience for many engineering companies, and many subcontractors are coming to wire up devices without any real understanding of the different requirements and problems presented by fieldbus systems. Keep some of the guidelines described here in mind when ordering your fieldbus system and when dealing with your installation subcontractor.

Moore Industries Pacific Inc
www.miinet.com