Fieldbus Cables Specs and Selection

[spragujl]

I've a question to users and cable suppliers on the selection of cable for FF spurs.

We previously required that all our spur cables be labelled as 'Type A'. We buy it with an orange outer jacket and black/white conductor insulation. However, our projects had problems with cost, delivery times, and the limited number of vendors that supply this cable.

So we have recently begun using our standard twisted-shielded instrument cable for spurs, but we make sure it meets the ISA S50.02-1992 paragraph 11.7.2 specs (see below). We have had no problems whatsoever to date.

My Question: Are we missing anything important by using cable that meets the specs but is not labeled 'Type A'?

11.7.2 Cable: The cable used for testing fieldbus devices with a 31,25 kbit/s voltage-mode MAU for conformance to the requirements of this part of ISA-S50.02 shall be a single twisted pair cable with overall shield meeting the following minimum requirements at 25 °C:

a) Z0 at fr (31,25 kHz) = 100 W ± 20%;
b) maximum attenuation at 1,25 fr (39 kHz) = 3.0 dB/km;
c) maximum capacitive unbalance to shield = 2 nF/km;
d) maximum DC resistance (per conductor) = 24 W/km;
e) maximum propagation delay change 0,25 fr to 1,25 fr = 1.7 ms/km;
f) conductor cross-sectional area (wire size) = nominal 0,8 mm2 (#18 AWG);
g) minimum shield coverage shall be 90%.

[jberge]

Simply using a cable that meets the specification is sufficient. What material is the internal insulation? You will likely have best long-term performance using PE. If the internal insulation is PVC the characteristics may change over time. Consider using a cable with internal PE insulation and outer PVC jacket.

[rezabejd]

For me, these questions bring to light the usefulness of a physical layer diagnostic. As we start getting mileage on our FF systems, the integrity of the physical layer may be the biggest source of issues. Wouldn't it be useful to have an "alert" that flagged low signal-to-noise ratio, or high attenuation, before devices started dropping off the segment?

I just had an issue a week or so back, where devices started dropping off a segment. Turns out one of them had a water-filled termination housing (vendor uses no sealant on conduit plugs). This device had been sitting out there for over four years. Even if we did quarterly surveys of segment "health", I wonder if this problem would have shown up in time to remedy it.

[jsullslainte]

We have a new installation using a NON IS trunk (the P& F system) and we are having difficulting finding a type A cable with a mechanical protection. (European standards). In the past we have used BS5308 cable for I.S. (Hawke)installations but this is the first time to use the P& F system, I had heard that SWA cable is not suitable for FF due to noise and interference can anyone offer any advice.

[spragujl]

We have used our 'normal' (not type A) steel wire armored instrument cable for Fieldbus spurs (and trunks) without problem. We check the specs to make sure it meets the ISA-S50.02 wiring requirements.

I think Belden makes armored type A cable, but it might not be steel wire armor. See this link:

http://bwcecom.belden.com/college/Prodbull/NP196.pdf

[marisg]

Regarding fieldbus Type-A cable specification: The original physical layer standerd required less than 2 nF/km capacitative unbalance. This specificatiohn proved to be difficult to hold for production cable. The present IEC 61158-2, Section 22.7.2 calls for 4 nF/km. The accomamying Note says that this specifictions is only required when conformance testing is performed.

Cable testing we have done shows that capacitative unbalance makes no difference in the signal shape. In noisy environments the unbalance may contribute to the conversion from common-mode noise (noise that gets on the shield) to differential mode noise (the noise that affects fieldbus signals). Unless fieldbus is used in an extermely noisy environments with hundreds of volts common-mode noise, capacitance unbalance is nothing to worry about.

maris.graube@relcominc.com

[tibor]

jsullslainte,

Pepperl+Fuchs has armored "type A" cable as well as e.g.Kerpen.
Ask them. (or visit http://www.fieldconnex.info/)
If you need any further please let me know.

Tibor FARKAS
P+F agent
Hungary

[Mike ONeill]

There is nothing about Steel Wire Armoured cable that makes it more susceptible to noise & interference than unarmoured cable. In most cases, vendors take their stock vanilla cable (twisted pair(s), overall shield, overall sheath) and re-wrap SWA and outer sheath onto that. The shielding per pair, when correctly handled, will prevent interference issues.
The only other problem comes from ppl installing SWA cable using glands and fittings that are either not designed for armour, or where there is no provision for SWA continuity through the gland and then to local ground through the enclosure. As a gland manufacturer, we offer earthing rings and continuity plates for our plastic fieldbus enclosures.

Mike ONeill
Hawke International