【Daily Question 20090315-16】IO Channel Isolation (also counts as an answer)
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【Daily Question 20090315-16】 1. Under what circumstances is channel isolation required for the IO modules of DCS or PLCs? 2. What are the different types of isolation? 3. What are the characteristics of the isolation methods used by various manufacturers? 4. What are the characteristics defined in terms of voltage levels? 5. Any other related topics? Feel free to add your comments. I’ve noticed that people often talk about isolation from the field side to the IO modules. In fact, isolation between IO channels is also very important, yet many people are not clear about its protective functions. Similar posts have been posted before; you can refer to them at http://bbs.hcbbs.com/thread-388158-1-1.html for further discussion. Many years ago, this was a popular topic – group isolation or channel isolation of IO modules, isolation from ground, isolation from the bus, etc. The reason for this is that if one channel is exposed to high current or voltage, it won’t affect the measurements of other channels, and its voltage won’t interfere with other channels either. Most manufacturers set the isolation voltage based on the measurement signal, ranging from 1500V to 220V. Honeywell and Yokogawa do a good job in this area; you can check their IO manuals. These are my personal opinions. If you’re interested, we can continue to discuss this topic. Thank you all for your participation. Note: Many such detailed issues are no longer given much attention these days, but I’m still thinking about whether they’re still meaningful. Posts that are duplicated or plagiarized will be penalized accordingly; I hope everyone will express themselves freely and actively participate, with those who offer valuable insights receiving proper rewards. This post was last edited by wopale3 on 2009-3-16 21:32]
Our IO modules are generally not isolated from each other; instead, an isolation barrier is used to separate the field transmitters (4-20 milliamps) from the control room. I’m not too sure about the rest; I hope those who have used it can share their thoughts.
1. In what situations is channel isolation required for the IO modules of DCS or PLCs? Classified by signal type: For 4-wire 4‑20mA transmitters, isolation is necessary because the transmitter has its own power supply and may be grounded at the field side. 0‑5V/0-10V signals: The signal source provides its own power supply; it may also be grounded at the field site or connected to other systems, requiring isolation. Field-grounded thermocouples: The hot end is in contact with the enclosure, which is in turn grounded; such thermocouples are rare and require isolation. Non-grounded thermocouples: They make up the vast majority of thermocouples and do not require isolation. Thermal resistor: No isolation is required. Two-wire 4‑20mA transmitter: Its power is supplied by the DCS system, so the internal circuit of the transmitter is not grounded and no isolation is required. 5: Purpose of isolation: To prevent damage to DCS equipment in the event of electrical failure of the field devices ； Prevent on-site equipment from damaging DCS equipment in the event of high voltage between them ； Limit the spread of local failures or damages in DCS ； Cutting off the ground connection of the field equipment prevents the ground loop current that is generated due to the potential difference between the DCS system’s ground connection and other ground connections; this ground loop current results in a high common-mode voltage at the input of AI devices, thereby affecting measurement accuracy. This post was last edited by afei6896 on 2009-3-14 21:47.]
Modern DCS systems possess a certain degree of electrical isolation functionality. But when working on projects, we usually use safety barriers to isolate the interfaces with other specialties (such as electrical engineering). In explosion-proof environments, isolation is also necessary; otherwise, we try to minimize such measures wherever possible.
Overall, DCS offers better isolation compared to PLCs. Additionally, for digital inputs on the 3rd and 4th floors, using intermediate relays for isolation can help prevent electrical interference caused by low voltages, as well as avoid the risk of accidental connection of external high-voltage sources to the 24V DC modules. Siemens modules feature electrical isolation for every two channels, in addition to optical isolation from the backplane bus. The purpose of this isolation, in my opinion, is to prevent interference and avoid damaging the modules due to carelessness
We added a partition to the AI module of the DCS we use; the purpose of this is to prevent damage to the channels in case voltage signals end up being introduced through the signal input cables. At first, those without isolation were used; as a result, 200VAC voltage came in from the field operation panel and damaged two of them, so they were all replaced with those that have isolation. There are no DI or DO signals; last time as well, 220V voltage was applied in series, and all the DI points were under voltage, but this did not affect the normal operation of the system. There was a voltage of 220V with respect to ground on the +24VDC terminal that supplies power to the card module as well; this was caused by voltage from the status signal wires of two motors being connected in series, and the problem was resolved after those wires were removed.
Adding to what was said above, optoelectronic isolation can be used in areas with special I/O requirements. It mainly prevents unexpected situations at the site (such as high voltage, short circuits). . . ) Damaged DCSI/O template.
DI/DO modules can be chosen with electronic fuses, while AI/AO modules can be selected with isolation features; however, the price is quite high. Last edited by lord82 on 2009-3-15 10:26.]
We used to use Zhejiang Zhongkong’s DCS system, in which the various units were isolated from each other; now we’re using Hollyland’s system, which features individual unit isolation. I heard that Zhongkong now also produces cards for individual unit isolation, though at a slightly higher price
I had never considered the issues of IO channel isolation and IO module isolation before; I thought those were things the system supplier should take care of. Now it’s a good opportunity to learn about them. Please ask the moderator to help clarify the concepts: 1. The concept of a channel refers to the circuit through which an I/O module or I/O card connects to field devices. For example, if an AI card (4–20 mADC, active) has 8 points, it can connect to 8 two-wire transmitters; in this case, it can be said that the AI card has 8 channels. 2. IO channel isolation refers to the isolation between the circuits provided by I/O modules or I/O cards ； 3. IO module isolation refers to the isolation between modules and between cards ； I’m not sure if my understanding is correct; please advise, moderator! Thank you! If my understanding is correct, then first, the channels are isolated from one another; this prevents high voltages that might originate from the field device circuits from entering the channels and damaging them. It also prevents high voltages from causing damage to the modules or circuit boards ； If explosion protection is required, an isolation safety barrier must also be added to the intrinsically safe circuit; this barrier serves to limit the amount of energy that the module or card can supply to the field devices. The safety barrier cannot isolate high voltage ； 2. Module isolation or card isolation refers to the separation between different modules or cards, thereby preventing one module or card from being damaged by high voltage impacts and affecting other modules or cards. I don’t understand how the high voltage from high-voltage equipment can get into the control circuit Generally, working ground and protective ground are provided during design. In addition, the connection points through which high-voltage electrical equipment is grounded to the system are all dry contacts, and the DO signals enter the cabinet after being isolated by relays; thus, there is no possibility of high voltage reaching the system modules. So how can there be issues with channel isolation or isolation of IO modules or cards? Has the system supplier come up with another trick to increase profits? This post was last edited by Meihua Piaoxiang on 2009-3-17 at 14:36.]