selection of Fieldbus cable I.e. Type A, D,C

[amol baviskar]

Is there any selection criteria to use Type A , Type B, Type C & Type D FF cable in Hazardous area ( i.e Gr. IIA,IIB & IIC)

[rezabejd]

My guess is, it may depend on the criteria to which you're aiming to conform. FISCO as I recall limits spur length, so I suspect an assumption is made (or a requirement) of what capacitance / inductance properties of the spur are. In that respect there may be a "criteria".

The B, C, and D specs are there mainly for people with exisiting infrastructure (conventional 4-20 mA twisted pair shielded cable, for example) who want to re-use their wire. Again, whether such can be made to conform to FISCO or other IS requirements is beyond my expetise, and may involve some careful calculations. Most of us putting in "new" just spec the type "A" and are done with it.

[Mike ONeill]

With reference to A / B / C / D cable types, these definitions relate to the physical disposition of the cable and not to the hazardous area installation. Type A is shielded twisted pair, normally preferred by most people and likely to give the best results. Type B is a multi-pair twisted pair with overall shield, which is slightly poorer in performance, with Types C and D being much less preferred since they have non-twisted cables or no shielding. Many people would use multi-pair twisted pairs cable with individual shields to get back to Type A performance. Cable size is a matter of personal preference; 0.8sqmm is that used in FF standards performance testing, but if you want a large number of devices and have a long trunk, you might go to 1.5sqmm - bigger cable is lower resistance but at higher cost.

In hazardous areas, your other issues relate to the type of protection concept preferred. I.S. approvals demand that the cable parameters are taken into account, which isn't too hard if you do it in advance. If you have the FISCO form of I.S., there are limits to the cable length (for example, in IIC, only 1000m total and 60m spurs) and restrictions on the cable design with regard to inductance & capacitance, so most people would use a cable marked FISCO. If it's not an I.S. application, then your task is easier since the only significant parameters are the cable intercore filling (important in Exd and cross-zonal cables - cables with gaps can act like conduit & pass gas/vapour into boxes) and the external mechanical protection (steel-wire armor is very common).

Of course, there are other considerations as well; resistance to your local environment (refinery, marine, etc), local colour coding standards, etc.

Some cables suffer from 'cold-flow' where the internal plastic polymeric filling reacts to the pressure exerted by the gland seal and moves away from that seal point over time, thereby letting in water. For those cables you need a gland that has a diaphragm seal rather than a compression seal.

From personal experience, I would also suggest that you qualify the roundness of the overall cable sheath - cables with a very pronounced 'lay' are not the easiest to seal via cable glands of any type, nor is that seal as reliable in extended service.

[gion_ro40]

Dear Mike,

Please, may I have a copy of your paper <Installing Fieldbus> published in June 2008 issue of Chemical Engineering.

Regards,

Ion Grozeanu
email: Ion.Grozeanu@litwin.ro