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Problem with Instrument Control Loops

Discussion in 'Electrical Engineering' started by BIGEYE, Nov 22, 2005.

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    BIGEYE Guest

    Appreciate any help on solving a long standing intermittent problem.
    We have an instrument control panel that serves a treatment plant. We
    monitor gas pressures on seven analogue control loops.
    All the loops and PLC digital I/O's are supplied from a 24 VDC supply, and
    the 0 Volts is floating (not connected to earth).
    To explain one of the loops:
    24 VDC Positive to an Intrinsically Safe Barrier (MTL 787SP), out to field
    instrument, back to Intrinsically Safe Barrier, then to analogue input
    module on PLC, out of PLC, back to 0 Volts.
    There are seven of these loops. Intermittently, the PLC sees +20mA coming
    from the field instrument, and they all do this at exactly the same time. On
    checking at the field instruments, they are giving out nothing like 20 mA.
    As a quick check, I bypassed the IS barriers, and everything works fine. At
    first I thought that the zenner diodes may be leaking. So, changed all
    barriers and the DC power supply. Rechecked, PSU supplying correct voltage
    and not overloaded. On connecting the inputs to the IS barriers, with the
    outgoing loops disconnected, the plc is picking up milliamps and it is
    My next thought is that something in the plant is leaking to earth, and
    returning back through earth to the IS barrier earth. This is what the PLC
    is picking up.
    The IS barriers are commoned together with an earthing strip. This earthing
    strip is connected to the cubicle earth. If I remove this earthing
    connection from the barrier strip, everything returns to normal.
    Or if I earth the 0 Volts so that it is not floating, everything returns to
    normal. The PLC sees 0 mA with the outgoing field loops disconnected.
    I will probably never find the earth problem causing this, as it is a very
    large plant, and the PLC controls everything.
    Would installing a permanent earth to the 0 Volts on the PSU be the answer
    to my problem, or is there anything else I haven't considered.

    BIGEYE Guest

    1. I haven't looked at the analog card, but I think they share a common.
    2. This is how the circuits are connected.
    3. The only way I could get this problem to go away is to connect a
    temporary earth to 0 Volts on PSU.
  3. SQLit

    SQLit Guest

    Equal potential is what you need. You need to ground one side of your system
    and "float" the other end.
    I like to ground the PLC and all the shields to the cables at the power
    source. Shields are taped back on the termination on the remote device. Are
    you using non shielded control wire?

    Triac I/O in the PLC also can have some bleed over. ( in my experience) I
    do not use triac I/O

    Bleed over/ induction can be a factor in some high current situations.

    Your terminology is a bit strange to my ears, so I hesitate to point you to
    a direction. Do you have a drawing of the system?

    What changed recently?
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