This particular loop has a problem, for the valve remains in the full-closed (0%) position regardless of what the DCS tries to tell it to do.
A technician begins diagnosing the problem by taking a DC voltage measurement at terminal block TB-10 in this loop circuit:
Control Valve Loop
The technician knows a reading of 0 volts could indicate either an “open” fault or a “shorted” fault in the wiring.
Based on the location of the measured voltage (0.00 VDC), determine where in the wiring a single “open” fault would be located (if that is the culprit), and also where in the wiring a “short” fault would be located (if that is the culprit).
For the next diagnostic test, the technician disconnects the red wire of cable 41 where it attaches to the screw terminal on TB-10, and re-measures voltage at TB-10.
After disconnecting the wire, the new voltage measurement at TB-10 reads 24.9 volts.
Determine what this result tells us about the nature and location of the fault.
Based on the first measurement (only), we could conclude the wiring fault may be an “open” in cable 26 or cable 30, or a “short” in any cable.
After taking the second measurement, we must conclude the fault is a “short” (not an “open”), and that it lies somewhere between TB-10 and the control valve (most likely in cable 41).
Questions for you
1. Explain why it is critically important to determine the identities of the valve and DCS card as being either electrical sources or electrical loads when interpreting the diagnostic voltage measurements.
2. Identify some of the pros and cons of this style of testing (measuring voltage at a set of points before and after a purposeful wiring break) compared to other forms of multimeter testing when looking for either an “open” or a “shorted” wiring fault.
3. Identify a fault other than open or shorted cables which could account for all the symptoms and measurements we see in this troubleshooting scenario.
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Credits: Tony R. Kuphaldt