Monitoring Control Loops for Leakage Current and Increased Resistance

by | Nov 24, 2014 | Measurement Instrumentation, Pressure | 0 comments

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.


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