Is there a difference between measuring the loop current at the positive terminal of a four-wire instrument and measuring the loop current at the negative terminal?
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Use a multimeter current range in series with a four-wire instrument to measure the loop current at the positive end and the loop current at the negative end. The positive cutoff is 4.03mA and the negative cutoff is 5.1mA. The meter provides 24Vdc for the system alone. The negative potential of the negative sum signal of the meter is 0. But the DCS system is still calculated according to the positive end. Can anyone explain?
Is the reference place of dcs different from the reference place of table?
When measuring the negative terminal potential of the dcs, it is 0. The dcs system only has a 24Vdc power supply, but the dcs is still converted according to the positive terminal current. This post was last edited by faner on 2009-3-7 22:55 ]
This is unlikely to happen. Please indicate the DCS manufacturer and module model.
How do you use a multimeter to measure the current of a four-wire instrument? Why can't I detect it? Is it the same as testing the second-wire system?
The four-wire system means that the detector is powered by an external power supply instead of the card. However, there are only two wires on the card terminal, so the current measured at the positive and negative ends is the same. This has nothing to do with the system. If the detected currents are different, there are only two situations: 1. The system insulation of a certain line of the signal line is reduced, causing partial current bypass, so the measured current will be different. 2. Use a non-isolated card and the grounding of the system is different from the ground potential of the detection equipment. Install an isolation converter in front of the card to test. If the current on the positive and negative lines before the input converter is still different, it is the first case, otherwise it is the second case. The second type generally occurs when PLC is used without a converter. This situation generally does not occur with DCS. The first type generally occurs when the terminals of the site or intermediate junction box are damp. As a weak current signal, the possibility of current bypass due to the degradation of cable insulation is extremely small.
Can someone upstairs please make it clearer?: Why does this happen when non-isolated cards are used and the system ground and instrument ground potentials are different? It would be better to have a diagram to make it more intuitive. Thank you~! In addition, according to this statement, any instrument powered by a card (two-wire system) is an active instrument, and any instrument powered by another circuit (such as a four-wire system) is a passive instrument. Can you say that?
Just disconnect one of the load signal wires and connect the output signal terminals in parallel for measurement.
Please show the diagram of the terminals for instrument terminal wiring and DCS terminal wiring, and explain how the terminals of the DCS terminal card are allocated. I feel like the original poster may have made a mistake in wiring the A/I card between the loop-powered and non-powered DCS.