The issue with the thickness of the instrument cable.
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The instrument cables I’ve come into contact with before were mostly 1.52 cables. I didn’t delve into the reasons either. For this project, the design institute specified a cable of 1.52 mm in diameter for the connection from the field instruments to the junction box, and a cable of 1.02 mm in diameter for the connection from the junction box to the DCS. Why? If 1.02 can indeed meet the requirements, then why not change it to 1.02 for everything? Why was it always 1.52 before?
Most of the field instrument cables use 1.5mm2 gauge, with 1.0mm2 being used rarely.
Instrument cables (24V) for power and drive systems generally use 2.5 gauge, while control cables usually use 1.5 gauge. Cables of 1.0 gauge are generally used for wiring inside cabinets, or in some factories, to control costs as branch cables from junction boxes to instruments.
If we are to be precise, using 1.0 gauge wire for about 300 meters is more than sufficient for instrument circuits. The prerequisite is that you need to use cables from major manufacturers.
To save costs, of course – who wouldn’t want to cut corners and save some money?
The thickness of the instrument cable needs to take into account the losses in the circuit, especially for 24V instruments. Choose a larger specification, as it results in lower resistance, which is particularly noticeable over long distances. In the original poster’s question, is the distance from the DCS to the junction box relatively short?
Well, if we talk about this in detail, things get complicated. Simply put, according to the HG 20512 standard, for branch cables the core wire diameter should be between 1 and 1.5, while for main cables it can range from 0.75 to 1.5. As long as you choose a value that falls within these ranges, it will be in compliance with the standard. But you know the instrumentation standards as well. . There’s no principle or basis for it at all. . If you want to be precise, according to electrical standards, you can calculate the voltage drop based on length; I’ve done the calculations for 1.5, and the voltage loss over 500 meters isn’t significant. Another factor to consider is cost: after all, version 1.0 is cheaper. There’s also the issue of quality; design firms are worried that version 1.0 may not meet the required standards and could result in high pressure losses, so they opt for version 1.5, so as to avoid problems even if the quality isn’t perfect. We have 1.5 as well; it’s a conservative estimate.
A 1-square millimeter cable, 1000 meters long, has a resistance of approximately 18 ohms. Speaking purely of signal transmission, if I have to say something, a cable with a cross-sectional area of 0.2 square units is sufficient for meeting the needs of most applications. The resistance of a 0.2 square millimeter cable over 1000 meters is approximately 90 ohms.
It’s not close either. About 700 meters. The main consideration is still cost