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AI HART CHARM MA offset reading for 4 wire transmitter output.

I am reading about a similar problem in the EEx365 blog but have not read /found a solution to the following problem I had on Friday, Dec 31st.

A 4 wire, SIL rated, 4-20 MA output transmitter is read in DeltaV as 10 to 12 MA higher when connected to Term 2 & 4 of a 4 point Charm Terminal Block, ( Not an IS AI Charm) . Changing out the 4 pt TB & HART AI Charm has no change in the reading. The AI higher value reading is verified by breaking in series with a Fluke 87 VOM.  However, the transmitter AI output is very accurate from 4 to 20 MA when the transmitter output is in series with ONLY the Fluke meter.  Also, using ONLY the Fluke in MA simulate output mode is read very accurately by the DeltaV from 4 to 20 MA.  The shielded twisted pair loop from the DeltaV does not read any voltage across the pair of wires (at the transmitter)  or to earth or shield ground at the transmitter.  The resistance across the shielded twisted pair from the DeltaV Charm is 300 ohm at the transmitter.  The 300 ohm is the value that I would expect due to the wire resistance and 250 ohm Charm input for the A/D circuit.  The Transmitter is powered by a 24dc Phoenix power supply that also provides the active 4-20 MA signal to the DeltaV AI HART Charm.  Unfortunately, the Transmitter does not have a passive MA output.

I suspect a ground loop but I have never seen this problem with any other 4 wire instruments and a HART AI Charm input.  I am also wondering how well the Charm is isolated for stopping ground loops. The Charm wiring diagram shows, for a 4 wire input,  #2 TB pt (+) is going to ground through a 250 ohm resister for the A/D input and #4 TB pt (-) is coupled to ground.  If there is a ground loop, then I wonder is there another or better way to isolate the 4-20 MA loop using only a Charm input and not having to add a separate isolation device to the circuit.   I have not found another AI Charm that may be more isolated. (HART is not needed.) I have considered using a precision 250 ohm resistor ( 1-5 volt input) with an isolated Voltage AI Charm but that would be a unique solution to the standard plant wiring.  Plus, I understand that new Charm deliveries are now quoted at 4 to 6 months.

I will continue troubleshooting on Jan 4th when I return from the holiday, and have plan some other tests, but I am hoping for any suggestions others may have.  I will update this when I find a solution to this interesting problem.

George

8 Replies

  • Cedric Dawnhawk
    I think you have proved that the problem is with the transmitter, not DeltaV. The 4-20 mA output from the transmitter needs to be isolated from ground. Alternatively the output of the transmitter connected to terminal 4 needs to be connected to ground due to the arrangement of the transmitter circuit and power supply although this is not a very satisfactory arrangement.

    Try disconnecting the wire from the transmitter to terminal 4 leaving the wire to terminal 2 connected. The DeltaV indication should go zero. If it doesn't, there is some kind of connection to ground at the transmitter end.

    Is the transmitter/power supply combination used elsewhere without problems or is this a one-off?
  • Cedric Dawnhawk

    In reply to Cedric Dawnhawk:

    Forget to mention. Have you examined the transmitter itself? We had a similar problem and the transmitter was wet inside causing a partial connection to ground.
  • ha nguyen
    I am very interested in knowing the solution or the culprit of the situation that you described. We are using the same 4-20 mA AI CHARM module with the same terminal block type. The reading at one 4-20 mA AI CHARM module is about 3-4 mA higher than what the transmitter is outputting. We used the same technique that you described. With a Fluke process meter in series and without the 4-20 mA AI CHARM module in place, we have a lower, accurate value; and when we snap the 4-20 mA AI CHARM in place, a 3-4 mA higher value is seen.

    Worse, we are also troubleshooting an adjacent 4-20 mA AI CHARM module next to the one I am describing above. When both 4-20 mA AI CHARM modules are in place and receiving inputs from two separate transmitters, both would read an erroneous value, but they would be exactly the same. Even worse, when these two CHARM modules are in place, another adjacent 4-20 mA AI CHARM module’s reading would become erroneous. This reading is correct when the other two 4-20 mA CHARM modules are removed.

    We replaced the questionable 4-20 mA AI CHARM modules with new ones, but the results are the same.

    Any information you have would be very appreciated.

    H. Nguyen
  • Cedric Dawnhawk

    In reply to ha nguyen:

    I noticed that I had made some comments to the original post which still apply to your problem. In particular, try disconnecting the wire from the transmitter to terminal 4 leaving the wire to terminal 2 connected. The DeltaV indication should go zero. If it doesn't, there is some kind of connection to ground at the transmitter end.

    The sketch shows how this can occur. The additional current i2 flows back via the ground connection and so is not controlled by the transmitter. If a number of transmitters share a power supply, the connection to ground does not have to be in the "faulty" transmitter, it could be in one of the others. This agrees with your observations that adjacent CHARMS are affecting each other.

  • ha nguyen

    In reply to Cedric Dawnhawk:

    Thank you for your response.  

    I did look at your earlier response after my posting and did some studying up on the design and build of our CHARM I/O cabinet (by a local Emerson vendor) for possible ground loop issues.  Below is the current design of the cabinet's internal wiring and our field wiring.  The wiring between the CHARM baseplates and the intermediate terminal blocks were all pre-wired at the vendor's shop.  Due to a facility configuration, we had to request intermediate terminal blocks (we could not wire straight to the baseplates which would minimize any ground loop issues in my opinion).  

    From what I saw, it looks to me the shield (SH) terminals on the intermediate terminal blocks should not have been grounded to the DIN rail (and to cabinet).  I would very much appreciate your feedback.  Thank you.

  • Andre Dicaire

    In reply to ha nguyen:

    In my opinion, you have a wiring, grounding fault. The drawing above does not follow recommended grounding as you have grounded the internal shield section at both the CHARM terminal and the Terminal Strip DIN rail. I would not expect shield length in the cabinet to cause any induced signal, but you do not have proper shield wiring.

    As for the erroneous signal, having shown the CHARM is working correction with a test signal, the issue must lie in the field wiring/transmitter grounding. Measuring the transmitter with a Fluke also eliminates your wiring, and if the signal is grounded in the transmitter, the Fluke would not show any error.

    Try testing for voltag dif between DeltaV DC common and the return signal wire of the transmitter. Disconnect from terminal 4 and measure voltage to DC Common. If there is voltage, you have a ground loop. Transmitter is likely grounded internally to the voltage source or has been wired to the transmitter chassis. Or there is a ground fault in the wiring.

    If there is voltage checking at DeltaV, disconnect return wiring at the transmitter. If voltage disappears, issue is in transmitter. If voltage remains, fault is in wiring.

    For the shield, I suggest you disconnect at the CHARM base plate and leave all shields grounded at the DIN rail. This will be least disruptive and get things to a signal ground. The Din rail should not rely on mounting screws for ground connection but have an explicit ground terminal designed for wiring the DIN rail to AC ground. (Check DeltaV Quick reference guide for power, grounding and surge for more information on Shield grounding)

    Andre Dicaire

  • ha nguyen

    In reply to ha nguyen:

    I should clarify that, in my opinion, the proper way to do this is to have the SH terminals at the intermediate terminal blocks not grounded such the shield wire for each twisted pair is only grounded at the baseplates and no where else.

  • ha nguyen

    In reply to Andre Dicaire:

    Thank you, Andre for your input. I just saw your note.

    I agree with your suggestion that the least disruptive thing to do to eliminate ground loops is to disconnect the shield wire at the baseplate. Not an ideal situation but it will get the grounding wiring to a better condition. I will check and see if mounting screws were used as grounding connections or actual ground terminal blocks.

    Best.

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