Grounding of signal circuits on the field side and control room side
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This post was last edited by S0703078 on 2012-2-2 at 11:31. Regarding the grounding of signal circuits, I have two questions and would appreciate everyone’s guidance. First, the complete equipment on site comes equipped with a PLC control system, and there is signal communication between the PLC and the DCS in the control room; therefore, the grounding of the PLC’s signal circuits should be at the negative pole of the PLC’s power supply. So, how should the circuit grounding of the PLC be handled? Is its grounding electrode driven on-site or grounded in the control room? II. Since the electromagnetic flowmeter needs to be grounded on-site, does the control room side also need to have a loop ground? If loop grounding is also done on the control room side, is a signal isolator needed? With a signal isolator in place, can the electromagnetic flowmeter only be connected to the DCS, and cannot it share a multi-core cable with other signals?
I feel that many cabinets and card assemblies already have internal grounding; it should be sufficient to ground the instruments on site as required. I haven’t encountered a situation where I needed to ground the signal wires separately inside a cabinet. Unless the shield of the shielded cable is grounded to the grounding terminal of the cabinet.
First question. . Your PLC is already connected to the DCS, so there’s no need to worry about circuit grounding; leave that to the DCS. Isn’t that the case with all other instruments as well? The second question. . The on-site grounding of your electromagnetic flowmeter is an anti-static grounding, not a working grounding. . In other words, it’s not the same as what is received by the central control system.
For electromagnetic devices, it’s probably the enclosure that needs to be grounded on-site, right, to prevent electric leakage?
It is not easy to have standard grounding electrodes at the site; therefore, the grounding of signals is carried out in the control room, where grounding electrodes are installed as grounding terminals. It also seems that ensuring proper grounding for the PLCs at the site is not straightforward, so it is better to use the grounding system in the control room. In terms of engineering, the PLC systems at the site are simply complex instrumentation components within the control system. As for the grounding of electromagnetic flowmeters, it is a safety protection ground that is different from the working ground of the instrument.
This post was last edited by S0703078 on 2012-2-6 at 14:16. 1. It is indeed not easy to install a dedicated working ground electrode for instruments on-site. According to what was said on the 5th floor, if the PLC is considered a complex instrument device on site, and if it shares the same power supply as the DCS, does that mean the grounding of the PLC’s circuits is already done at the control room side? ? I am working on a project currently, where the compressor on site is equipped with its own PLC controller; the temperature, pressure, and other parameters of the compressor are controlled by this PLC. There are some dry contact signals and communication signals between the PLC and the DCS. While reviewing the grounding system diagram, the supervisor said that the circuit grounding for the PLC hadn’t been done and that grounding electrodes needed to be installed on site, which left me completely confused; that’s why I have the questions mentioned above. 2. Page 1107 of the 《Petroleum and Chemical Industry Automatic Control Design Manual》 states that ground-type thermocouples, pH meter solutions, electromagnetic flowmeters, etc., all have their circuit grounds installed on-site. And the people upstairs all said it’s due to leakage protection grounding. My understanding is that grounding an electromagnetic flowmeter is done to prevent external signal interference; it serves as a form of shielding grounding, rather than the circuit grounding mentioned in the manual. Could some other expert please give an example of a situation where loop grounding is implemented both at the site and in the control room, with an isolation transformer placed in between? Thank you! !
This post was last edited by zhaohh3211 on 2012-2-7 at 18:57. 1. When a PLC has signals connected to a DCS, it is better to use a unified grounding system, as this can prevent signal errors and failures caused by differences in the reference potential. 2. The grounding terminal of the electromagnetic flowmeter is generally connected to the DCS grounding bus through the shielding layer.
I have the same question as LZ; I’m paying close attention to this post, hoping for a clearer answer.