Regarding the design of instrument cables

See which area you are in. In explosion-proof areas, different cables must be selected depending on the zone: in Zone 2, flame-retardant cables are used in conjunction with explosion-proof junction boxes; whereas in Zones 0 or 1, intrinsically safe cables are chosen as required

1. A junction box is installed on site. From the instrument itself to the junction box, and then from the junction box to the cabinet area, what type of instrument cable is generally used, and what are the requirements? I checked the specifications related to instrument piping and wiring, but could not find any relevant requirements. From the instrument body to the junction box: In general, intrinsically safe/exploded-proof types are used, with copper mesh shielding made of polyethylene-insulated polyvinyl chloride sheathing; the wires are stranded together, depending on the design preferences of the design institute and the wiring layout. Some choose armored, while others choose unarmored. From the junction box to the cabinet: Multi-core cables are generally used, which are intrinsically safe/exploded-proof, with polyethylene insulation, polyvinyl chloride sheathing, and a copper mesh shielding; they consist of multiple twisted strands and are usually unarmored. 2. A junction box is installed on-site; between the junction box and the cabinets, multi-core cables are generally used. If signals from the same multi-core cable are distributed to several cabinets, what is the best way to address the issue of splitting such a multi-core cable? In such cases, it is possible to use terminals for bridging within the marshalling cabinet; the advantage is an aesthetically pleasing wiring layout, while the disadvantage is an additional fault point as well as an increased number of terminals. Or, plan the layout as thoroughly as possible during the initial planning stage to avoid such issues. Or, 2-3 outlets can be provided at the outlet of the junction box to connect to different cabinets.

Answer to the first question: Regarding the selection of cables between the field equipment, the junction box, and the cabinet, the first thing to consider is the type of signal – intrinsically safe/exploded-proof. Non-intrinsically safe cables are also used, but less frequently; they are more common in air separation units, where designs based on intrinsically safe/exploded-proof circuits are required. For cables running from the field instruments to the junction box, most traditionally used conduit, but this makes maintenance difficult and also raises issues related to water ingress and condensation. Nowadays, armored cables are preferred in projects, which is much better. Cables running from the junction box to the cabinet are usually multi-core cables, and unarmored ones are sufficient； Answer to the second question: Regarding the issue of multi-core cables, it was mentioned above that excessive terminal cabinets can be used to address this problem. In fact, the signals in question here relate mainly to DI/DO signals; in larger control systems, DI/DO signals are usually handled in separate cabinets, and the circuit design approaches differ as well, with certain isolation requirements in place. As for AI/AO signals, especially those that pass through safety gates, they can be arranged in the same cabinet, while it is rare for such signals to cross cabinets. Therefore, it is uncommon for analog cable signals to cross cabinets, and this aspect is generally not taken into consideration.

If the cable running from the instrument itself to the junction box is armored, is it still necessary to use raceways or conduit?

Currently, terminal cabinets are installed in the middle for the wiring of large-scale installations. However, it is not possible to use multi-core cables for all the wiring between the terminal cabinets and the control system cabinets, as the signals need to be sent to different system cabinets, which makes things rather complicated.

It’s basically not necessary anymore; you use conduit to secure and protect the cable, and the outer armor on armored cables serves the same purpose

If the layout is arranged in the form of terminal cabinets, safety barrier cabinets, and control cabinets, cross-connections between these various cabinets will inevitably occur. Personally, I think it’s still better than having cables run from the field directly to the terminal cabinet across different cabinets. Alternatively, an integrated controller, safety barrier, and terminal block in a single cabinet can be used to reduce interconnections, but proper planning is required in the initial stage. It is difficult to completely avoid the situation where multi-conductor cables cross cabinet racks. This requires the owners’ staff to have an understanding of both the on-site wiring layout and the arrangement of the system cabinets. This is feasible for small projects, but for large projects, neither the systems nor the on-site setup is currently managed by a single person. Additionally, it is necessary for the personnel from the design firm to take responsibility for planning and to coordinate with the owners and system manufacturers. It’s hard to achieve perfection in comparison.

Why not go directly to the DCS or safety barrier?