Monitoring Control Loops for Leakage Current and Increased Resistance

Control systems rely on the signals coming back from measurement devices to perform control operations and provide feedback to operators. Unfortunately the signal from these devices can be impacted by several conditions.

These conditions include damage and corrosion, water or condensation in the junction box or terminal compartment, power supply deterioration and grounds or electrical shorts.

This 4:08 YouTube video, Rosemount 3051S and 3051 Pressure Transmitters – Power Advisory Diagnostic, shows how these Power Advisory Diagnostics continuously monitor the electrical loop to detect changes that compromise the integrity of the transmitted 4-20 mA output signal.

The video shows two scenarios. The first is where corrosion causes a leakage current in the loop. An additional electrical path is created adding 3mA of current to the 15mA coming from the transmitter. This parallel path means that the control system receives 18mA from the loop resulting in a reading much higher than it should be.

The second example is where the loop incurs increased resistance due to age and wear. This increased resistance can limit the current flow through the loop below the level it should be at.

The Power Advisory Diagnostics continuously monitor the transmitter electrical loop to identify these types of problems. The diagnostics use a characterization process where the transmitter quantifies the electrical characteristics of the 4-20 mA loop at installation and then monitors the loop for future changes in order to help protect the integrity of each measurement.

Power-Advisory-Diagnostics

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5 comments

  1. Jonas Berge says:

    Looks like the video link was broken. I think this link now applies:
    https://www.youtube.com/watch?v=MEnIETGmFwg

    I agree. The leakage current and increased resistance result in a wrong measurement as the result of loose or corroded screw terminals or ground faults, so for a 4-20 mA circuit a transmitter might be trying to send a certain value but as a result of excessive impedance you might get less at the receiver resulting in a wrong PV indication giving operators the wrong picture. Control and alarms will also not work appropriately. Therefore, if 4-20 mA is used, this kind of signal circuit diagnostics is a good idea. In my personal opinion, more 4-20 mA devices should have this capability.
    These 4-20 mA loop failures are undetected (covert) which is generally dangerous. Such analog signal problems are hard to detect because they are ‘on-scale’ meaning they are wrong, but still within the range of 4 mA and 20 mA so they appear to be correct to the system even though they are invalid. That is, a problem with a 4-20 mA signal may go undetected. Increased impedance may be due to loose or corroded screw terminals, or it could be ground fault. Water ingress creating a partial short is another common cause:
    http://www.emersonprocessxperts.com/2014/10/wired-versus-wireless-risk-analysis-for-process-instrumentation-measurements/
    Use of a digital bus or wireless protocol can prevent the problem. Digital communication statistics such as provided in DeltaV detects installation issues affecting the communication. Failures in communication systems are generally overt which is generally safe. If the data received by a final element or any receiving device is not timely, and is not valid, it is very easy to detect. The ability to diagnose the digital communication to detect errors is an important advantage of digital protocols (fieldbus or wireless) over hardwired systems (4-20 mA or on-off).

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