[FUN] Grounding rules

[Stephen Mitschke]

Could you coment something about grounding rules for FF.Can any vendor use they owm methodology?Is it possible to use floating ground?Thank you.Magda Pe=F1aloza

[Stephen Mitschke]

Ian,Could you add me to the FUN list - Maris forwarded this to me to comment on:There are three issues with Foundation Fieldbus (FF) H1 (31.25kHz)Grounding:1: ShieldThe Shield wire should be grounded at one point only. This is typicallydone in the control room, but can be done elsewhere if it makes sense foryour installation. Products that contain surge abatement, may require otherground connections, but these are only electrically connected when a surgeevent occurs. Multiple grounding of the shield will likely cause groundloop currents which will induce noise into the FF signals.2: Twisted PairThe FF twisted pair provides DC power to devices and also allows signalingbetween devices and the host. The Standard requires that these two wiresare Isolated from ground. No ground connections should be made to eitherwire in the twisted pair. 3: Power SupplyPower is supplied to the twisted pair using a combination of a 24VDC powersupply (PS) followed by a Power Conditioner (PC). Power flows from the PSto the PC and then to the twisted pair. A PS typically converts the Mainsvoltage to the 24VDC and is transformer isolated from the Mains voltage.However, many plants have a policy of grounding the negative lead of the PS.This is a requirement for some other equipment and if you are installing amix of FF and other equipment, you will be presented with a grounded PS. Ifthis is the case, then the PC MUST provide galvanic isolation from the 24VDCinput - this is called an Isolated Power Conditioner (IPC). If you canprovide isolated 24VDC power to the PC, then it need not provide galvanicisolation. However, there are some other issues that come up withNon-isolated PC's. Although they are cheaper than IPC's, our recommendationtoday is to use IPC's. If high reliability is a concern, then also considerRedundant Isolated Power Conditioners.Bonus Material:Just because you have designed a FF system correctly (with respect togrounding), do not assume that it is installed that way. A procedure shouldbe in place to verify that the wiring is correct. This is easily done witha DMM. Verify that the shield is grounded and only in one place, and alsothat the twisted pair is isolated from ground. Verifying the wire beforepowering up the segment can eliminate problems down the road. An especiallydifficult area is what happens to the shield at the device connection. Yourdrawings should specify that the shield is cut off and the cable is coveredwith heat shrink to prevent the shield from contacting the terminals insidethe device, or the device housing. Exposed shield wire or foil can causeintermittent problems which are the worst kind to troubleshoot.Mike Strauser Relcom Inc.P.S. HAPPY THANKSGIVING to our American friends.

[Stephen Mitschke]

I would like re-enforce Mike Strauser's point on correct termination of theshield. On Malampaya we initially used a braided insulating sheath over thefield end of the shield. We had numerous problems that were very difficultto identify. Eventually we discovered that the braided shield had slightlyhygroscopic properties, which in our high humidity environment producedproblems. Changing to the heat shrink type of sheath resolved theseproblems.Ken JonesSnr. Instrument EngineerShell Philippines Exploration BV