04 June 2008
Fieldbus Wiring
Fieldbus cable may be virtually indistinguishable from 4-20mA cable, but field wiring techniques and accessories are definitely different. Fieldbus systems are simple to design because all the device wire-pairs are connected in parallel but, in practice, any attempt to fill a box full of terminals and just “jump” between all positives and all negatives will result in a “rats nest” of cables within the enclosure. This may be acceptable in some plants, but will lead to all sorts of maintenance problems once the installers have left the site.
A better idea is to use Device Couplers - junction boxes specifically designed for fieldbus implementation. These units automatically provide the necessary system interconnections without confusion and greatly speed up the process of device installation. They should incorporate the required terminator with either manual or automatic activation.
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 “foldback.” Current-limiting and foldback types both auto-reset after fault removal and both normally incorporate LEDs to indicate spur status.
The current-limiting technique limits the amount of power the short circuit can draw to between 40 and 60mA (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 1000m of 50 Ohm nominal cable (say, 250mA total). In this case, the trunk voltage drop equals 12.5V, which allows 12.5V at the farthest device. However, if a short occurs at a spur and an additional 60mA load is “locked in” to the segment, this takes away enough power so that devices receive less than 9V (8.5V 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 60mA safety margin. That is, do not install as many instruments as the segment can theoretically power; instead, leave at least three spurs empty.
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 5) 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. Since the cost of H1 cards ($2,500) 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 $5,000. 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 16 x 20mA fieldbus devices, it requires 63 fieldbus segments to support 1,000 devices, at an approximate cost of $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 $500,000. Simply by specifying fold-back short circuit protection, an end user can save $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 snake’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 off line.
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 6) has two power conditioners, both powered by a load-sharing pair of 24Vdc power supplies. Such a system can survive the failure of either 24Vdc 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. The power conditioner modules plug into a DIN carrier (See Figure 2), 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 require 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.
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