Inductance parameter of fieldbus cables

[spark_msk]

Greetings!,

I have been trying to verify and update the fieldbus cable parameters in our database.

I do not know the original source of the cable data in our database. I notice that the parameter Lo is missing from the database. As I understand, this parameter is required while calculating the total segment inductance for Entity IS segments.

I searched up the IEC specs and the internet, but was unable to find values for Lo. Also the IEC lists just the 4 basic types of cables. I am not able to map our cables with any of the cable manufacturer's cables.

Can anybody help?

TIA!

Here is the list of cables that we have in our database.

FF Type A 0.8mm^2 (18 AWG) Polyethylene Shielded Twisted Pair
FF Type A 1.0mm^2 Polyethylene Shielded Twisted Pair
FF Type A 1.5mm^2 Polyethylene Shielded Twisted Pair
FF Type B 0.32mm^2 (22 AWG) Polyethylene Multi-twisted Pair, With Shield
FF Type B 0.50mm^2 Polyethylene Multi-twisted Pair, With Shield
FF Type B 1.00mm^2 Polyethylene Multi-twisted Pair, With Shield
FF Type B 1.50mm^2 Polyethylene Multi-twisted Pair, With Shield
FF Type C (26 AWG) Polyethylene Multi-twisted Pair, Without Shield
FF Type D (16 AWG) Polyethylene Multi-core, Without Shield" PATH
FF Type A 0.8mm^2 (18 AWG) PVCPVC Shielded Twisted Pair
FF Type A 1.0mm^2 PVC PVC Shielded Twisted Pair
FF Type A 1.5mm^2 PVC PVC Shielded Twisted Pair
FF Type B 0.32mm^2 (22 AWG) PVC PVC Multi-twisted Pair, With Shield
FF Type B 0.50mm^2 PVC PVC Multi-twisted Pair, With Shield
FF Type B 1.00mm^2 PVC PVC Multi-twisted Pair, With Shield
FF Type B 1.50mm^2 PVCPVC Multi-twisted Pair, With Shield
FF Type C (26 AWG) PVC PVC Multi-twisted Pair, Without Shield
FF Type D (16 AWG) PVC PVC Multi-core, Without Shield

[Mike ONeill]

I am a little surprised that you couldn't find the data you need from the cable manufacturers datasheets; all the ones I've read quote the cable resistance and the unit capacitance and inductance.
There is, of course, no common standard; anyone who makes twisted-pair copper cable with a shield of the right overlap can claim Type A, but the other paramaters flow from the grade of copper he uses (affects resistance), the type of plastic insulation he puts in the cable (affects capacitance) and the exact orientation of the cable within the overall shroud (affects inductance).
Note that for Entity I.S. systems, the paramaters need to be evaluated are capacitance and EITHER inductance OR L/R ratio. In I.S. fieldbus systems, staying under the inductance value is sometimes quite hard, but meeting the L/R value (usually in microHenry/Ohm) is usually quite easy. In our particular case with our RM100 Entity system, we always use the L/R ratio (which is also independent of length, and so once defined as a cable for the installation, you don't need to look at any spurs which might be longer).

[jberge]

My cable database is also missing inductance or L/R ratio because they are usually not stated in the cable brochures. I just checked Kerpen, Turck/InterlinkBT, and Belden again and there is noting on L or L/R. It could perhaps be because the standard does not put limits on it. I will ask around and see what I can find.

[Mike ONeill]

I re-checked my cable datasheets and inductance is definitely listed by Kerpen, Draka and Pirelli. Belden list it on some but not all, and you have to search for it.
e.g 3076F 0.19 microH/ft, 3077F 0.20 microH/ft, 3078F 0.30 microH/ft.
3076F has 7.3 Ohm/1000 ft so L/R = 0.19x1000/7.3 = 26 microH/Ohm.
(Belden datasheet Rev1 12/31/2002).

You will usually need the specific datasheet on that particular cable rather than the overall sales brochure. Happily, those data sheets are usually accessible through the wonders of the Internet!

L/R ratio is hardly ever listed and needs to be calculated - easily done from the specific resistance and the specifice inductance values; just remember that the result is required in microHenry/Ohm, so some conversion of quoted values is usually needed.

[jberge]

I asked around and managed to get the data from Kerpen, Turck, Belden, and Hitachi and I have now updated my cable database. Typical inductance (loop) is 0.7 mH/km.

[Mike ONeill]

Please be aware that in the field of Intrinsic Safety, 'typical' values are of no use. If you are using values as part of an Entity safety analysis, they must either be actual values related to the cable & equipment in uses, or maximum values that cannot be exceeded.

For a specific site, it would be normal practice to use one manufacturer for cable and so building the database of those datsheets shouldn't be too hard.

[Dan Daugherty]

I re-checked my cable datasheets and inductance is definitely listed by Kerpen, Draka and Pirelli. Belden list it on some but not all, and you have to search for it.
e.g 3076F 0.19 microH/ft, 3077F 0.20 microH/ft, 3078F 0.30 microH/ft.
3076F has 7.3 Ohm/1000 ft so L/R = 0.19x1000/7.3 = 26 microH/Ohm.
(Belden datasheet Rev1 12/31/2002).

You will usually need the specific datasheet on that particular cable rather than the overall sales brochure. Happily, those data sheets are usually accessible through the wonders of the Internet!

L/R ratio is hardly ever listed and needs to be calculated - easily done from the specific resistance and the specifice inductance values; just remember that the result is required in microHenry/Ohm, so some conversion of quoted values is usually needed.

Mike, you are using the shield resistance instead of signal conductors' loop resistance in your reference above. Is this intentional?

Regards,

Dan Daugherty

[Mike ONeill]

On my Belden datasheet (3076F Rev1 12-31-2002) it clearly states:

Nom. Conductor DC Resistance @ 20 Deg. C 7.3 Ohms/1000 ft
Ind. Pair Nominal Shield DC Resistance @ 20 Deg. C 7.5 Ohms/1000 ft

I used the conductor resistance value of 7.3 Ohm/1000ft (though it looks as if the shield is a great conductor as well!). 7.3 Ohm/1000ft equates to 23.725 Ohms/km so that seems about right. (Tell me quietly if I have made some elementary slip).

[Joe_WaZoo]

Here is the data page on the TURCK Foundation Fieldbus cables:

http://www.turck-usa.com/Products/Networks/FOUNDATION_Fieldbus/Cable_Specifications.htm

Thanks,
Joe_WaZoo

[Dan Daugherty]

Hi Joe,

I think that Turck webpage illustrates the problems we have with many manufacturers in finding parameter data for I.S. calculations. The DC resistance per conductor is explicitly listed, but one has to infer that the values to use for capacitances are the maximums permitted by Fieldbus Type A (probably worse than the actual cable capacitance). Then, an assumption of 0.7 mH/Km is probably about right (not to exceed), but there is no direct cable data published to corroborate that.

Then, it appears that Belden has reduced their conductor DC resistance value from 7.5 ohms/1000 ft in 2002 to 5.86 ohms/1000 ft in 2006. That's good if you are hoping to have less voltage drop in your H1 cable, but bad if your I.S. Entity calculations passed L/R when R was 7.5 but don't pass when R is 5.86. Getting accurate cable data is a challenge! And it looks like we might have to supplement the part number with a date when we document the calculations!

Regards,
Dan Daugherty
Fieldbus Consultant
Emerson Process Management

[Mike ONeill]

Thanks for that link, Joe.

I suspect that you have to work for Turck to be able to get any actually useful data. That link doesn't give much information on the cable other than the resistance, color and the OD size. Where do you find the capacitance/inductance or L/R values, minimum bend radius, mechanical abrasion resistance, etc, etc? There doesn't seem to be any further depth to the data available, or if it is there, the access method is not obvious.

[Joe_WaZoo]

Mike,

Below is what I found from TURCK. I haven't gotten an info on the inductance level yet. They will be updating their website to reflect this info. There is a new Product Manager of the Fieldbus media products there so he is going to get this corrected. I used to work at TURCK but no longer am part of their team. I hope to not offend anyone by trying to clarify this issue with a supplier of Fieldbus products.

Thanks,
Joe_WaZoo

CCC= Capacitance, Conductor to Conductor
C to Shld=Capacitance to Shield

490 CABLE:
CCC 17.2 pF/Ft +/- 2 pF/Ft
C to Shld: 45 pf/ft

492A CABLE:
CCC 17.2 pF/Ft +/- 2 pF/Ft
C to Shld: 45 pf/ft

492BA CABLE:
CCC 17.2 pF/Ft +/- 2 pF/Ft
C to Shld: 45 pf/ft

493 CABLE
CCC 17.2 pF/Ft nom
C to Shld: 50 pf/ft

496 CABLE
CCC 17.2 pF/Ft nom
C to Shld: 50 pf/ft


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[dweberl]

Hi I'm Deanne - Just wanted to let everyone know that many of the questions that have been posted relating the physical layer characteristics of a FOUNDATION Fieldbus segment are answered using the Segment Design Tool. This tool checks the segment layout utilizing the FOUNDATION Fieldbus rules governing cable lengths, power consumption and proper segment termination. This tool is free and is located at http://www.emersonprocess.com/systems/support/segment/. Give it a try and let me know what you think of it.