Issues with DCS output to field signals
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The DCS outputs an AO signal of 4–20 mA; by removing the remote cable at the terminal cabinet, the current measured at the terminal holes is consistent with the DCS command. However, the terminal hole current measured after rewiring did not match the DCS command. If the valve is instructed to be opened to 50%, the direct measurement at the DCS terminal is 12mA ； After connecting the terminals, the reading was 8.33 mA; at the site, the current at the valve connection point was 10.1 mA. The valve in question has been tested and found to be functional – could it be a problem with the cable?
Check following steps: 1 Determine whether the resistance value of the cable meets the requirements, and also check its insulation condition. 2 Does the DCS side have safety barriers? This could be the fault point – an issue with the actuator of valve 3; try replacing it.
Unplug it at the valve wiring end for testing; it cannot be tested while still connected, as that won’t give accurate results. Remove it and measure how many mA it is; if the value is much lower, it’s likely that the resistance of the wire is too high, and there might be a short circuit somewhere along the wire. Is there a spare cable that can be connected to give it a try?
Is there an intermediate junction box? If so, check whether there are any issues with the wiring in it. It is best to measure the current at the locator; the control room should send a continuously changing signal to check whether the current changes, and also check whether the voltage is too low. If everything is fine, then it is the locator that is damaged.
After removing the remote cables from the terminal cabinet, the current measured at the terminal holes was consistent with the DCS commands, and this measurement is reliable； After connecting the terminals, measure the current again. How is it measured? Should the ammeter be connected in series in the circuit? ? If so, an excessively high internal resistance of the ammeter will cause measurement errors (most likely the current value will be too low) ； If not, it is equivalent to connecting a measuring instrument in parallel to the circuit, which will cause measurement errors (most likely the current value will be too low).
If there is an intermediate junction box, check the wiring, and examine the resistance and insulation of the cables
Is the load-bearing capacity insufficient?
If the measurement method is fine, 1. It’s a potentiometer issue. 2. Cable issues lead to a decrease in current, and the main reason for this is changes in resistance values; these are the two factors that affect the resistance values