Does grounding one of the wires in a two-wire field instrument affect the signal (with an isolated safety barrier on the cabinet side)?
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As the title suggests, I would like to discuss this issue with the experts in the industry: when an isolated safety barrier is used, one of the wires of the field instruments (two-wire system) becomes grounded. Does this have any impact on the 4–20 mA current signal? I tested it myself and found no impact: that is, with the cabinet located before the safety barrier (with the area behind the safety barrier being the side where the input cards are located), a wire was used to directly connect the + or – terminal of the AI input to the ground terminal. Then, a multimeter was used to measure the input current, and the process signals were also checked on the DCS screen; the current remained unchanged and the signals were normal. I heard some people say that the signal can have an impact on ground, but I’m not sure which one is correct! ? Theoretically, since the output side of the isolated safety barrier is already floating with respect to ground, a single wire connected to ground does not form a circuit; it merely pulls the voltage at the grounding point to the ground level. As long as no circuit is formed, there should be no current diversion... Looking forward to experts coming forward! ! !
Two-wire instrument, two-core cable; the power supply is provided by the safety barrier, right? If the + terminal is directly grounded, electrical energy is directed straight into the ground – will it still pass through the meter? ； Two-wire instrument transmitters have resistance, and grounding them directly has no effect in this case, which is strange; if it’s a passive constant current source, there might be no impact. Normally, the cables in the cable tray get a bit damaged, and when they come into contact with metal, it has a significant impact.
In my opinion, due to the low voltage (DC24V), the loop resistance to ground is high; for the instrument circuit, the current division caused by this can be ignored, so there is no change in the measurement.
This is the topic of anti-interference. Cables are like antennas, \"coupling/receiving\" interference signals, especially during long-distance transmission or those coming from electrical MCC areas. Theoretically, floating signals are most effective against common-mode interference. We add safety barriers/isolators (using transformer isolation) precisely to achieve the above objectives. If the common-mode voltage at the card tray is not high, for example, less than 10V, then a single-ended grounding approach has little impact on AI; otherwise, it is recommended to add isolation at the site. You can refer to the Yokogawa IO installation manual. Grounding the negative terminal at the site has little impact on the signal, but it reduces the circuit’s resistance to interference. I remember that for all AO cardboard outputs, the design was floating, and that was also the purpose.
On the second floor, the power supply is provided by a safety barrier. I’ve tested it myself; I don’t know why, but you said that \"if the + terminal is connected directly to ground, electricity will flow directly into the ground\". Is that really the case? Since the current enters the ground from the positive terminal, it must flow out from the negative terminal. As the negative terminal is insulated from the ground, theoretically there should be no issue either; I’m not sure why though
Fourth floor. The negative terminal at the site is grounded, so it has little impact on the signal. Will grounding the positive terminal have an effect? If it is a ground-insulated system
It will have an impact. I’ve encountered situations where the wiring ends came into contact with the instrument housing, and since the instrument housing was grounded through conduit, it had a significant effect on the signals (I wasn’t using a safety barrier in my case)
If a safety barrier is not used, it will definitely have an impact, because it’s not an isolated system; what I’m discussing here is the situation after using an isolated safety barrier
First of all, you need to understand that DCS systems are very intelligent these days; there is an option that allows you to choose whether the output should be set at the lowest level, the highest level, or kept unchanged in case of an I/O disconnection. Have you checked if there are any alarms in case of hardware system failures? The safety barrier serves to prevent large currents from passing through and reaching the cards. Its function of isolation is essentially that of an isolation transformer. So, if there is a disconnection, where does the current come from for the coil on the field side of the safety barrier?