Resistance (impedance) per unit length of control cables, types and selection of control cables
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The last edit to this post was made by Xiāoyáo Ténglóng on 2018-5-28 at 16:23. SH/T 3019-2016, the Code for Design of Instrumentation Piping Systems in the Petrochemical Industry, and SH/T 3082-2003, the Code for Design of Power Supply Systems for Instruments in the Petrochemical Industry, both specify the maximum voltage drop allowed in the instrument power supply circuits. However, if a company or project aims to reduce costs by using thinner cables, how can the matching impedance per unit length for these common control cables with cross-sectional areas of 0.75mm2/1mm2/1.5mm2/2.5mm2 be calculated? What is the basis for this calculation? We used to use KVVRP2*1.5 or 3*1.5 cables, with the length of cable laid out generally not exceeding 300 meters; there were no issues during use. But... as for how to calculate it exactly, I’m not quite sure. I’d like to ask the experts among you for advice. To standardize responses, it is allowed to cite formulas and **standards; those who provide good answers will be rewarded with Haichuan coins~! Additionally, control cables are divided into two types. We generally use KVVRP 2×1.5 for powering two-wire instruments and as signal lines. So what is the difference between KYJVRP 2×1.5 and KVVRP 2×1.5? It’s just the material of the insulation coating that differs; how significant is that difference? I’ll discuss it from the perspectives of price factors and actual usage; no new post will be created～～ Automatic control instruments
The resistance per meter of cable can be found. The minimum operating voltage for field instruments is specified in their technical parameters (for example, 24V±10%). The voltage drop across the wire at the maximum circuit current (for example, 23mA) can be calculated (current x wire resistance). 24V minus the voltage drop across the wire must be greater than the minimum operating voltage of the field instrument; otherwise, a thicker cable should be used. The strength of the cable also needs to be taken into consideration; generally, a 1.5 m2 signal cable should be used.
You can find out how to call it; how do you find out? ?
A copper-core shielded cable with a cross-sectional area of 0.2 square units, 1000 meters in length, has a resistance of 90Ω – this is the measured value. With cables of this specification, 500 meters per roll, used for RS485 communication and 4–20mA transmission, tests have been conducted multiple times across various batches, and no issues have been found.
This post was last edited by hayoo112 on 2018-5-28 at 11:40. The screenshots are from Tiankang Cables; for computer cables, the DC resistance is no more than 30 ohms per kilometer for 0.75mm2 cables, no more than 20 ohms per kilometer for 1.0mm2 cables, no more than 14 ohms per kilometer for 1.5mm2 cables, and no more than 8 ohms per kilometer for 2.5mm2 cables.
This is exactly what I want... Is there a complete Tiankang cable selection manual? Please send it to me……
This post was last edited by super9K on 2018-10-31 at 14:24. I have one here, but it’s all control cables.
You need to figure out what to do with the sample yourself – haven’t you been to junior high school?
The sample for selecting control cables is also good... It would be great if there were options for both control cables and power cables... But it’s still fine!
You’ve been to junior high school; could you calculate it for me, or tell me how to do it?
The theoretical voltage drop over a distance of 500m is only 0.9V, so it should not cause any problems… But according to national standards, the cross-sectional area of control cables should be at least 0.75 square millimeters; I’ve never seen cables with a cross-sectional area of 0.2 square millimeters……