Practical Industrial Data Networks:Design, Installation and ...

Foundation Fieldbus overview 209Once these checks have been performed; switch on the power supply and check thewiring system. The Fieldbus tester (or an alternative simpler device) can be used toindicate:• Polarities are correct• Power carrying capability of the wire system is ok.• The attenuation and distortion parameters are within specification.A few additional wiring tips and suggestions with reference to the diagram below.+24v FIELD BUS SEGMENTPOWERSUPPLYFieldPOWERCONDITIONERControlRoomHomerun1900 metersMaximumSPUREXPANSIONTERMINATOR{grounded}TERMINATOR(Isolated)Figure 14.7Overall diagram of FieldBus wiring configuration (courtesy Relcom Inc.)1. It is not possible to run two homerun cables in parallel for redundancyunder the H1 standard. H1 Fieldbus is a balanced transmission line thatmust be terminated at each end. In some cases, it is a good idea to run aparallel cable for future use. In case of physical damage, you need todisconnect the damaged cable and put in the undamaged one. Ensure that,if this is the philosophy, you do not route both cables in the same cabletray.2. Do not ground the shield of the cable at each Fieldbus device. The shield ofthe cable at the transmitter (for example) should be trimmed and coveredwith insulating tape or heat shrinkable tubing. The only ‘ground’ thatoccurs on the segment is usually at the control room Fieldbus powerconditioner.3. Note that the ground that is connected to the isolated terminator at the farend of the segment does not connect the shields of the Fieldbus. It onlyallows for a high frequency path for AC currents.4. There has been no provision made for lightning strikes. However, youshould specify a terminator that has some type of spark gap arrestor, whichwill clamp the shields to about 75 V in such a high voltage surge.5. A quick way to check that the grounding is correct before powering up is,doing a resistance measurement from the ground bolt on the powerconditioner to the earth ground connection point. This measurement shouldbe of the order of megohms. You can then connect the earth protectiveground to the power conditioner bolt. Once this has been done, measurethe resistance from the cable shields on the isolated terminator at the farend of the segment to a nearby earth ground point. A very low value ofresistance should be seen.6. A standard power supply cannot be used to power a Fieldbus segment. Astandard power supply absorbs most of the Fieldbus signals due its lowinternal impedance. It is possible for a standard power supply to provide