Cable selection for petrochemical plants
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SHT3019-2003, 7.2.1 Under normal circumstances, wires with multi-strand copper cores and PVC (or polyethylene) insulation are preferred； It is advisable to use flexible cables with multiple copper cores, polyethylene insulation, and PVC sheathing. SHT3019-2016, 6.2.1 Under normal circumstances, flexible wires with multiple copper cores and PVC (or polyethylene) insulation are suitable for use as wires ； Cables should preferably be multi-strand stranded copper-core cables with polyethylene insulation and polyvinyl chloride sheathing ； Soft cables are recommended for indoor use. An analysis of the two versions for cable selection: the word “stranded” is added, while the word “flexible” is omitted. Stranding is understandable, but how should we understand the term \"soft\"? It turns out that one can simply choose conductors of category 5 or higher; now the word “flexible” is missing, and it’s not clear whether to choose category 2 conductors or those of category 5 or higher. Multiple-strand twisted cables generally refer to flexible cables. Experts could please explain the standards in this regard. The standard-setters are fucking awful; you could just say whether to use category 1, category 2, category 5, or higher – why go through all these word games?
Soft, guiding structure. It is not very scientific to use the term “soft” for wires/cables in standards, but everyone knows it refers to the conductive body. Instrument cables generally adopt a Category 2 structure (7 strands), which is determined based on a comprehensive consideration of strength and wiring convenience. However, R (soft) often refers to a 5/6 class conductor structure. But many people think that R refers to the type 2 structure. I agree with you: the standards should specify the structure of the conductor, rather than referring to it as “soft” or “hard”.
Soft cables are simply not suitable for use in device wiring; it seems the experts involved are incompetent as well. For such standards to remain in use for decades is truly laughable.
As I understand it, multiple strands are used for the core; for example, in a 2-core cable, flexible cables consist of multiple copper wires per core. For example, 12*2*1.5: 12 refers to 12 pairs of cables ； 2 refers to 2 cores per pair, with the cores twisted together ； 1.5 refers to a cross-sectional area of 1.5 mm2, consisting of multiple copper wires (i.e., a flexible cable)
Your understanding is incorrect; Category 1 cables consist of a single conductor, while Category 2 cables have 7 twisted conductors. Category 5/6 cables feature multiple twisted conductors. As for the cable section, electrical engineers can simply follow the IEC standards, such as the relevant chapters in the \"Handbook for Industrial and Civil Power Distribution Design.\" Then why can’t automation engineers develop even a single set of standards? What is the purpose of using multiple twisted cables? Indeed, the skin effect can increase the current, but in self-control applications, the current is very small and some of it is already fixed.