DCS system DI, DO induction electric treatment

This post was last edited by HEJIYUER on July 16, 2015, at 00:20. Let me share my lessons and methods. 1. Instrument cables should have at least overall shielding; for example, for solenoid valve power supply, KVV multi-core cables can be used, but KVVP cables with overall shielding are preferred. It’s even more necessary to go to MCC. There is no blocking on the original poster’s end; I guess the cables were designed/procured by electrical specialists – that’s the common approach in China. The cables here are also purchased from instrument suppliers, but the number of conductors is determined by electrical requirements; therefore, they all come with a total shield. The original poster encountered similar problems. 2. Both DO and DI are said to be \"passive contact signals,\" but the voltage level of the circuit is determined by the power supply for that circuit. The DI is rated for 24VDC instrument power supply, but the DO contacts are connected to the electrical secondary circuit, which operates at 220VAC; therefore, the instrument terminals of DO are definitely exposed to high voltage, with the voltage level determined by whether the contactor is on or off. The motor stopped, and the secondary circuit lost the load of the contactor coil; as a result, the instrument terminals were at 220VAC. Are the DI/DO instrument terminals arranged together there as well? Will maintenance personnel be able to tell which one is 220V in the future? 3. I recommend separating the DI/DO cables first, as they operate at different voltage levels. However, if you don’t have a junction cabinet in the electrical room, these signals cannot be separated. You should know that in MCCs, cables are routed through drawer units; for each motor, there’s one cable that serves both electrical and instrumentation purposes. The purpose of installing junction boxes is not only to save on cables (though this doesn’t save much money if the cables are short); the main reason is to reorganize the DI/DO signals and separate the main cables for DO/DI. I hope you have a junction box to rewire the cables and arrange the main DO cable separately. 4. Then, place the instrument cables in a separate, sealed trunking that is properly grounded; this trunking serves as the overall shielding for the cables. You may need to repurchase some trough boxes. 5. Regarding your first measure, \"to change the DO output of the DCS to use a relay on the electrical cabinet side for signal isolation, with the relay being powered by 24V DC\" – how did you come up with that idea? This is a one-time solution to completely eliminate 220VAC. Is this solution electrically acceptable? I once posted a thread to see the replies from Haiyoumen, and there was also your reply. http://bbs.hcbbs.com/thread-1392825-1-1.html 6. Equipotential grounding is required; it is stipulated by the standards. Also, check the gauge of the grounding wires from each cabinet to the \"signal ground\" busbar; in my design, it should not be less than 6 mm2. This is especially true for the grounding wires connected to the 24VDC power supply – after they are combined using the small busbars inside the cabinet, they are then connected using 16 mm2 grounding wires. @Jiaguoyun, do you have any opinions? Let’s exchange ideas.

This post was last edited by jiaguoyun on 2015-7-16 at 09:56. The issue of converting 220V electrical signals to those used in instruments is one that everyone working in the field of instrumentation has encountered. Here are a few of my own experiences: 1. In a new desulfurization project that had been on hold for a year, I took over its management after joining the technical improvement team. During the commissioning phase, it was found that the electric door switches wouldn’t operate. When trying to check the continuity of DO/DI signals on the DCS side, I used the voltage measurement function by mistake, and as a result, 220V was read. I was terrified at that moment; fortunately, the DCS cards were quite durable, and I didn’t touch the wire ends with my bare hands… Later, it was discovered that the construction team hadn’t laid enough cable, had made connections along the way, and even changed the color of the wire cores, which caused all sorts of problems. 2. For the renovation of an existing project, it was necessary to display the start/stop status of the compressor as well as its current levels on the DCS. Control cables (with shielding) were then laid from the DCS side to the power distribution room (which contained both high-voltage and low-voltage systems). Since no proper design had been done, I decided to handle this task myself. At that time, the high-voltage power distribution room was under maintenance and disconnected from power. Some of the cables ended up resting against the doors of the distribution cabinets, while others fell to the ground. After the cables were laid, power was restored. Later, someone straightened out the cables on the ground, and in doing so, they touched the cover of a utility trench, resulting in sparks flying everywhere. The electrical technician checked the voltage and found it to be over 100 volts; even the shielding layer carried electricity when tested with a voltage tester! After multiple attempts of touching the wire end to the manhole cover, the voltage disappeared... 3. In a newly implemented monitoring project, optical fibers were run through instrument cable trays to locations near the cameras; from there, they were connected to the camera units via optical terminal boxes. After completion of the construction work, when the system was powered on for testing, all camera feeds appeared clear. However, as soon as a certain high-pressure pump was activated, the video feeds from the four cameras linked by one fiber optic cable became unviewable. This high-pressure pump was actually quite far away from the cameras... Later it turned out that during installation, there was a section where the instrument cable tray couldn’t be used; to save time, the construction crew simply ran the cables across an electrical cable tray instead... 4. In a recent project during the individual unit testing phase, the solenoid valve in one packaged unit failed to open despite everything else functioning normally. Once this solenoid valve was activated, all other operations proceeded without issues; however, two DCS cards got damaged in the process. Upon investigation, it was found that no relay isolation had been applied to the DCS-side DO outputs. I pointed out that 220V shouldn’t have reached those points from the electrical side, so we asked the electrical team to investigate. Over a dozen personnel from the electrical, instrumentation, and construction teams spent hours trying to figure it out; ultimately they concluded that there was nothing wrong with the 220V supply coming from the electrical side—the design documents never specified whether that side operated on active or passive signals... The final solution adopted was adding relay isolation to all DCS DI/DO circuits. Since then, I haven’t paid much attention to this matter; I wonder if any further problems have arisen after implementing this measure... @HEJIYUER claimed the “sofa seat” here and provided such a comprehensive response that there’s hardly any room left for others to contribute :lol Let me now discuss the three solutions proposed by the OP: The first two approaches should be fine; after completing the second measure, it would also be advisable to test the grounding resistance... As replied by expert H, I too believe there’s no need to modify the separate grounding setup; instead, we should look into possible causes related to the design and construction quality!

This post was last edited by HEJIYUER on 2015-7-16 at 14:42. 1. When checking the ON/OFF status of DO/DI on the DCS side, the voltage setting was used by mistake, resulting in a reading of 220V. The electric actuator contains a separate 24VDC DC power supply (with a common negative terminal); the passive DOs of the DCS can be connected to this circuit for on/off control. But your problem is that “the wires are connected incorrectly”. Is that a split-type electric head? The construction team laid the wires for insufficient length, created joints in between, and even changed the color of the wire cores, which caused a complete mess… The openings in the electric connectors are generally not very large; KVVP cables are sufficient for DI/DO connections. If using \"twisted pair split-screen + master screen\", it’s very difficult to fit five signal cables into the ports. Apart from the electrical power supply, the electric actuator is also equipped with instrument signals (all of which are low-voltage). The main cables for the instrumentation/electrical systems are separate; how could there be a connection in the middle? 2. The electrician checked it, and the voltage was over 100 volts; there was electricity present on the shielding layer as well when tested with a test pen! After touching the wire end against the manhole cover several times and then taking another measurement, the voltage disappeared… The 24V DI motor’s feedback signal was affected by this; the shielding layer wasn’t properly grounded, which led to interference. The motor stopped working, but the DCS continued to show it as being in \"operating\" mode – quite funny. Later, fixing the grounding properly solved the problem. 3. For a newly installed surveillance system, fiber optics were laid through the instrument tray to the vicinity of the cameras, and then optical transceivers were used to connect to those cameras. After the installation was completed and the system was tested, all camera images were clear. However, as soon as a certain pump (high-pressure) was started, the images from the four cameras connected by the same fiber optic cable became unviewable. Can fiber optics be interfered with in this way? I really didn’t encounter any problems. How was it resolved later on? Here, the optical fibers are in separate trunking boxes; otherwise, how will we be able to check them in the future? Besides, it gets damaged when pressed by other cables. 4. In a recent project, during the individual unit testing phase, one of the solenoids in a complete set of equipment would not operate, while all the others functioned normally. Once that solenoid was activated, all other devices started and stopped properly. Two DCS cards were damaged as a result. Upon investigation, it was found that there was no relay isolation for the DO signals on the DCS side. I said that 220V should not reach the electrical side, and asked the electrical team to investigate the cause. Dozens of people from the electrical, instrumentation, and construction teams spent time checking the issue, but ultimately it was determined that there was no problem with 220V reaching the electrical side, as the design did not specify whether the electrical side should be active or passive. The solution adopted was to add relays for isolating all DCS DI/DO signals. After that, no further attention was paid to this issue; it’s not known whether any other problems arose as a result of this approach. Even for the solenoids in complete sets of equipment, I insist on using 24VDC, as lower voltages are safer for people. The 24VDC power supply units are configured in pairs for redundancy, which makes them more reliable than 220V. I would prefer to thicken the cable once and for all, but I try to choose solenoids with low power consumption. Both the DI and DO instrument sides are isolated by relays to provide self-protection first. For space-saving purposes, use terminal relays. @Jiaguoyun, you’ve taken the bench seat; I’ll continue to sleep on the floor. You can go and dig another hole then.

In the first issue, it was a project from 2007; I took on the work in 2009. The instrumentation/electrical cables were not separated – 380V and DI/DO signals were carried along a single seven-core cable, and there was no separate shielding. What’s most absurd is that the connections on the cable were made before it entered the drawer cabinet; from the terminal blocks in that cabinet, DI/DO signals were sent to the DCS. The drawer cabinet and the DCS cabinet were in the same room. Since we were in a hurry to get everything up and running, we couldn’t identify the problem, so we even removed the cover plates… It was almost as if we were about to dig a hole, haha… In the third issue, I had the construction team install galvanized pipes for the section that ran across the electrical tray. Fortunately, no further problems occurred with the cameras due to interference. Since cameras are being added on top of the existing setup, it’s not practical to install cable trays separately. I seem to have missed mentioning the fourth issue; there was also a fault related to the common terminals – the DCS cabinet connected things that shouldn’t have been connected together, which caused 220V to be applied to certain components. Before I could notice it, the guy secretly fixed it… The solenoid valves, on the other hand, operate at 24V. So you’re not just staying up late to post messages after all – hurry up and seize the opportunities to make money! Maybe we can even make a water bottle: lol@HEJIYUER

It is recommended to use the relay isolation of the first method, with the instruments isolated separately. We have encountered similar situations before, but we used shielded cables in those cases.

First of all, thank you all for your replies; they have been very helpful. Thank you! Currently, in our MCC cabinets, the input and output connections are not separated – it’s a result of poor design by that idiotic design team, and there’s nothing we can do about it. To this day, they still insist stubbornly that their design institute is right, claiming it’s fine even though the design changes are thicker than the original drawings. I’m really impressed. It has come to this point now; the management says that as long as the project is completed on time, it’s fine to start working on it first, without caring about the consequences later. Ah, typical of Chinese style of leadership – making decisions after much hesitation. It’s like being under constant pressure every day; there’s nothing I can do about it. I made a few changes today and the results are pretty good; the voltage has dropped to around 2 volts. The main issue is that the large equipment isn’t turned on yet, and I’m worried that this unshielded cable might cause voltage fluctuations once the equipment starts operating. This has been a persistent problem for me. I’ll study this further today, and if that doesn’t work, I’ll see if using a capacitor could be a solution. It would act as a filter, but I don’t know what value of capacitor would be appropriate – I’ve forgotten the formulas from school. :Lol, also, what if we create some isolated security zones? I’m not sure if that would be useful

It is truly a lesson in blood. One marine friend said that the experience here can help us accelerate our growth, and I completely agree; however, in engineering, it is the results that matter. In the long run, it’s better to prepare to replace these cables; look for an opportunity to park the vehicle, and treat this as a project for addressing potential hazards. For DCS MCC signals, it is best to use a separate cabinet, with DI/DO on separate sides, so that maintenance can be carried out more easily by separating them according to voltage levels. At the electrical section, handover cabinets (or distribution boxes) are also installed, allowing DCS-MCC to run multi-core cables point-to-point. Design is a bit more troublesome, but it makes things much easier for users. Adding a relay/safety barrier for isolation is definitely beneficial; sometimes I use the safety barrier as an isolator. Since it’s not used in an intrinsically safe circuit, as long as it can serve that purpose, it’s good enough – at least it provides isolation from ground. I remember that capacitors are used to eliminate high-frequency interference; however, I’m not sure if they’re effective against induced voltages. Once, I used a 24V DC power supply of poor quality with high ripple voltage; as a result, the 4-wire meter on site wouldn’t light up, even though the voltage was sufficient. Upon further inspection, it was found that the meter on site could switch automatically between 24V and 220V, with the switching mechanism relying on voltage and frequency. The DC power supply was of very poor quality; as a result, the meter switched to the 220V mode, and it’s impossible for the lights to work at 24V. Later, a filtering capacitor was added to the header, and it worked just fine. The original poster said “a few have been fixed”; what method was used? Please share it.

I tried three times, didn’t hit anything, so I gave up.

If you draw a circuit diagram showing the signal flow between the instruments and the electrical systems, it will become clear: DI signals can go directly into the card, while DO signals must be isolated. DI and DO are separated

There were few people around in the middle of the night, so I tried to sneak in, but it didn’t work anyway. . . . :'( @jiaguoyun