Examples of how to troubleshoot two-wire instrument circuits

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This article is a reprint; the original title is: (Analysis and Troubleshooting of Circuit Faults in Two-Wire Instrumentation). Circuit faults in two-wire instrumentation are also common problems that arise, such as short circuits in the signal lines, short circuits between different lines, and short circuits between the lines and ground. Among them, a short circuit between the wire and ground is further divided into a short circuit from the positive terminal to ground and a short circuit from the negative terminal to ground; the fault symptoms in these two cases are completely different. If the prediction is inaccurate, it will definitely cause a lot of trouble during the actual repair process. I’d like to share with you a case of a wiring fault in a two-wire instrument that I encountered recently. The actual process of fixing it was full of twists and turns, so I’ll share it here for your reference: It involved a two-wire pH meter transmitter, which was powered by a DCS card, while the transmitter sent a 4–20mA signal to that card. The schematic is roughly as follows: http://www.hfybdl.com/u/9e2e642f-ce63-4fbc-bfe2-9749560b319f/image/636305197292365778.jpg The fault encountered this time is as follows: the power supply to the pH meter transmitter at the site is working properly, the gauge display is normal, and the pH value is 6.8. However, the DCS shows an open circuit condition, and the channel does not receive a 4–20mA signal. After assessing the general situation, the first thing to check is whether the voltage in the channel is normal; this involves locating the wiring terminals and card channel for this pH meter circuit within the DCS cabinet. Then disconnect the fuse at the terminal, and use the DC voltage function of a multimeter to check whether there is any DC voltage output from the channel, as shown in the figure below. The measurement result is 22V, which is within the normal range. http://www.hfybdl.com/u/9e2e642f-ce63-4fbc-bfe2-9749560b319f/image/636305198170803321.jpg After confirming that the voltage measurement is normal, use the mA current setting on the multimeter and connect it in series between the two ends of the fuse holder to check whether the current signal in the circuit is normal, as shown in the figure below. The measurement value was 11.8 mA, which is the same as the value displayed on the in-situ pH meter gauge of 6.8 (the pH measurement range is 0–14). http://www.hfybdl.com/u/9e2e642f-ce63-4fbc-bfe2-9749560b319f/image/636305198616496103.jpg At this point, troubleshooting has reached a dead end – is the configuration incorrect? Is it still the card slot channel that’s broken? Every possible method was tried: reconfiguring the installation, replacing fuses, swapping wiring channels, and so on – but nothing changed, and I have no idea why. It’s obvious that the direction being considered from the start was already off track. . . . . 1000 words are omitted here. After various attempts, line failures were considered, but at that time it had not yet been determined what exactly the problem was. I started checking the wiring; all connections related to the cards and field transmitters were removed. I checked the connections, measured the resistance of the wires, as well as the insulation resistance, and finally things became clear. Please take a look at the image below: http://www.hfybdl.com/u/9e2e642f-ce63-4fbc-bfe2-9749560b319f/image/636305198935828664.jpg Note: Pay attention to the grounding symbol on the negative terminal. Yes, when the wire resistance was measured, the resistance of the negative terminal connection to ground was only a few ohms, indicating a short circuit to ground. This can explain the aforementioned malfunction: since the negative terminal is grounded, there is no issue with power supply to the positive terminal, which is why the gauge on site shows normal readings. However, the 4–20mA signal did not return to the DCS card channel; instead, it flowed back to ground, so the DCS system displayed that channel as open circuit. In this case, there is no other option but to rewire it.