Considerations in Foundation Fieldbus Cable Design

The Fieldbus Foundation has an active LinkedIn group where users of this digital bus communications technology share expertise. I saw a recent question [hyperlink added]:

i came across a document which mentioned that in reality IEC 61158 specified cable is actually not required. They may bring installation benefits, but is not required.

Emerson’s Dan Daugherty, who leads the Advanced Services development efforts, responded [hyperlinks and picture added]:

Foundation Fieldbus Capacitance Inductance ResistanceThere are only three things that make a difference to fieldbus: inductance, capacitance and resistance. Inductance is mostly influenced by construction – twisted pair or not. Twisted pair is better. Resistance is less of an issue than it was 10 years ago, because that mostly only affects how many devices you can put on a segment for a given length (due to voltage drop) and you can buy higher voltage fieldbus power supplies now so you can tolerate more voltage drop (please refer to DC model).

The real killer is capacitance. It eats and distorts the signal (please refer to the simplified AC model which ignores the lesser effects of relatively small inductance and resistance). If you want a robust design margin, you want lower capacitance than what the IEC spec says. As a rule of thumb, calculate the total capacitance you would get from 1000 meters of Belden 3076F, and make sure your segment length of whatever cable you choose has no more than that. Or conversely, for the segment length you choose, make sure you don’t exceed that total capacitance.

You will find that if you exceed that capacitance, your segment will probably still work, but in my opinion, it is better to have more design margin. Fieldbus tends to work in somewhat adverse conditions and you can’t tell by the error rate alone how close you are to having problems.

I have seen some engineers install standard 16 AWG shielded twisted pair instrument cable for fieldbus and it works fine. But they also make sure their segment length is around 500 meters or less. The reason for using it is because it is the same cable you use for your other instruments — not because it has lower resistance. If you use multi-core cable with an overall shield, the actual capacitance seen by the fieldbus transceivers will be higher than what the cable spec says for one pair.

Dan followed up his comment with a second post [hyperlinks added]:

One more thing, homerun cable in tray is better than cable in metal conduit. The metal conduit will increase distortion. Again, it is a matter of segment length, so for relatively short segments, you have a high degree of freedom in choice of materials and construction.

I didn’t mention Intrinsic safety. If you’re using the field barriers, there’s no change in what I said above. If you’re using FISCO, you are required to use approved fieldbus cable. But still take the trouble to calculate total capacitance (especially for multi-core cable with overall shield) for your segment length and make sure it doesn’t exceed the capacitance the FISCO power supply is permitted to be connected to. Don’t just assume that if a manufacturer says they meet the IEC spec that it will work well for you.

Dan shared with me that the Advanced Services has several fieldbus consultants on staff, who provide a range of services from consulting prior to installation to troubleshooting of existing installations. Connect with Dan to discuss your project.

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