[FUN] FF Cable Types and Calculations - ARCHIVE

[Stephen Mitschke]

Hi All, (and Ian!!),I'm interested in hearing other people's practical experience withusing cable other than the VERY EXPENSIVE "Type A" and "B" subscribed to inISA S50.02 Part 2 Phys. Layer AND also by several FF field devicemanufacturers.I am wondering if anyone has experienced differential capacitanceproblems in using standard instrumentation cable on LONG segment/spur runs.I am proposing this in my EPC company to 2 of our client projects involvingvery large grass-roots oilsands projects in N. Alberta. I would like to use4 pr.homerun cables (4 segs. ea. with 16 or 18AWG indiv. twisted pr. w.indiv. shield & overall shield) and from the JB to device, use 16 or 18 AWGsingle tw. pr. w. shield ( this is the "Chicken Foot" topology we want touse). We have created a spec. which limits to MAX. 8 bus powereddevs/segment including 2 cont. v/vs max. Segment loading calcs. show thatvoltage loading of the individual devices would be OK with these standardcables. Some attenuation calculations appear to look OK for variousscenarios of distance and device counts. With std. I&C cable at the FF base freq. of 39kHz, I can't see capac.issues and attenuation being significant. Also, I can get cable which passesa Cold Bend Test of -40C, something the Type "A" cables such as BeldenTR381 do not do. Please pass me your comments and experience on this andthanks in advance!Best Regards,JimJ.E. (Jim) Jamison, P.Eng.- - - - -Embedded and attached is a PowerPoint slide I presented at an ISA Conferencewith some information on this subject. The data was extrapolated by anengineer at Belden cable off some graphs he had. I hope it helps.<<...>> IanIan Verhappen<<cable_specs.ppt>> - - - - Ian, All,We installed a fieldbus system with one segment that include three fieldbusdevices (2 valve positioners, 1 pressure transmitter) utilizing existinginstrument cable. This is a non-incendive system. The "homerun" instrumentcablewas multiconductor 20 ga. individual and overall shield approximately 2800ft.from the last field junction box, before the field devices, to the DeltaVcontraoller. There were a total of seven Jct. Boxes in the loop. We incurredaproblem with the total of the resistance of the wire being twice thecalculatedvalue. We checked and cleaned all terminations with no effect on theresisditance of the wire. We finally came to the conclusion that it was justpoor quality wire. Since the resitance of the wire created to much load onthenon-incendive rated loop power supply we decided to try using two pair ofinstrument wires to cut the resistance in half. We were concerned about thecapacitance effect, but it has not been a problem. This loop has been inservicefor ~ 1-1/2 years with no problems due to capactance. We did have onemysterycommunications alert that went away when we reset the H1 card. we have sincereplaced the H1 card with a later revision. In the future should weencounterproblems using existing Instrument wire over long runs we intend to userepeaters.RickRichard D. Shriner- - - - Hello Jim,On our Hazardous Area job we used standard 1.5sqmm Shielded Twisted PairDekoron cable with no problems. However our longest would have only been150m, powering 4 devices (max)Originally our DCS supplier suggested using Belden 3076F, but upon testingfound that the standard cable was OK over these relatively short runs. TheDCS supplier was happy to give us a performance guarantee for the FFinstall.RegardsMarc

[gsaravanababu]

Dear stephan,
Can please send the Fieldbus cable Specification which your planing to use for your project (North......).
Also please send your segment configuration . We are using Zone 1 FF configuration in the current Design .
Thanks & Regards,
G.Saravana Babu

[Heather Santos]

Dear Saravana,

The Fieldbus Foundation recently released a new test specification for H1 Cable.

The Foundation Technical Specifications can be purchased via the Foundation's website (http://www.fieldbus.org/index.php?option=com_content&task=view&id=144&Itemid=319).

[Rockbestos]

Hello Fieldbus users,

I would like to explain cable design and the question “should I use a regular instrumentation cable or a special Foundation Fieldbus cable for my fieldbus network?”
A fieldbus cable is designed to meet the electrical characteristics in the FF-844 H1 cable specification. The FF devices are designed to work with 100 Ohm impedance cables that match 31.25kHZ. If the cable impedance is different then 100 ohms at that frequency, impedance mismatch and signal reflections will occur which will increase signal attenuation and signal loss.
In order to meet the 100 Ohm impedance at 31.25 kHz, the cable manufacturer needs to adjust the insulation thickness and add to the calculation the specific gravity of the insulation material. Different insulation materials such as PVC, XLPE or EPR will require different wall thickness in order to meet the FF-844 specifications.

Why don’t all cables perform the same if they are all 100 Ohms at 31.25 kHz?
The answer is the value of the mutual capacitance between the insulated conductors. The mutual capacitance is a measured value and is different between materials. The lower the mutual capacitance number is, the faster the signal will travel through the cable with less attenuation. The FF-844 spec addresses the mutual capacitance through the attenuation measurement which is correlated to the mutual capacitance value.
Regular instrumentation cables will have lower impedance then 100 OHMs because the cable manufacturer will use the needed material that is allowed by the cable spec such as TC, PLTC, ITC etc. For short runs (less then 300 ft) regular instrumentation cables will probably work, but the attenuation will be high. For longer runs, 100 Ohms FF cable with low mutual capacitance will be a perfect match.


Please contact Rockbestos-Surprenant with any FieldBus cable questions.