Issues with the conductors in instrument cable installation
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We have internal disagreements regarding the selection of cable cores and the installation methods. In the past, we have always used a point-to-point single-cable wiring method, from the field instruments to the DCS system cabinets (mainly 2-core or 6-core cables) ； Now, it has been proposed that multi-core cables (such as 36-core cables) can be used, explosion-proof junction boxes can be employed on-site, corresponding terminal cabinets can also be installed between the cabinets, and then connections can be made from these terminal cabinets to the DCS system cabinets. It is said that this method is used quite often in power plants and the petrochemical industry. My personal comparison of these two methods is as follows: the first method involves fewer connection points and less work required. However, for the same number of connections, the total length of the cable increases significantly compared to multi-core cables, which in turn leads to higher installation costs ； The second method involves more connection points, which triples the number of potential failure points; this creates significant challenges for future maintenance. Additionally, signal interference may increase, and there could also be issues related to equal potential across multiple junction boxes. The advantage, however, is that the installation cost is considerably reduced. I wonder what suggestions you senior colleagues and experts have? How does your company do it? I hope everyone will feel free to share their insights and offer reasonable suggestions. Thank you
If you are the manufacturer, that is, Party A, it is best to have a single cable running from the instrument to the cabinet, without any junction boxes in between. Firstly, additional connectors mean more potential failure points; secondly, when an explosion-proof junction box on site is opened, power must be cut off to all the components inside it. Moreover, over time, factors such as aging and inadequate maintenance can lead to a decline in the explosion-proof performance of the junction box. This is also a potential risk point. If it is an installation company, using an intermediate junction box plus multi-core cables can indeed save a lot of work. For Party A, using intermediate junction boxes and multi-core cables doesn’t save much in terms of costs
The choice of how to connect the wires is fundamentally based on cost considerations. In practical terms, it depends on the number of connection points at the site; if there are over 100 points on one floor, it’s appropriate to use junction boxes to connect multi-core cables, as this offers advantages in terms of cable costs, installation expenses, and other aspects related to cable trays. But if you have only about 10 points in one layer, and they are fairly spread out from each other, then there’s not much need for junction boxes or multi-core cables; using multi-core cables in such cases would actually increase costs. That’s how it is. Therefore, one must analyze specific problems on a case-by-case basis; it’s not possible to make generalizations. Each of the two methods has its own characteristics and uses, and there is no such thing as a purely better or worse option.
Multi-core cable + on-site explosion-proof junction box is better! A good design institute is needed, as based on the factory layout designed, signals of the same type should be gathered in a single junction box in nearby locations; moreover, it is necessary to take into account the layout of the factory area and the production lines. Construction and costs have been significantly reduced. As for increasing the number of failure points, if the wiring in the on-site junction box is done properly during construction, there are basically no issues, and subsequent maintenance becomes much easier. Signals can be grouped together in one junction box, which prevents interference; 18-core or 36-core cables can be used for this purpose.
Yes, the number of connection points on each floor in modern enterprises is definitely more than what you mentioned, which means that junction boxes are necessary. I’m not sure what method your company uses? How is it being used?
You first need to figure out one thing: what kind of cable should be used. If you use a junction box, you basically need to use dual-screen twisted pair cable, measured in pairs rather than in the number of cores. So how can there be a problem with interference? Whether to use a junction box depends on the specific conditions on site; it’s not the case that it’s necessary in petrochemical industries but not required in power plants – multiple factors need to be taken into consideration. But in terms of cost, the two are almost identical.
Depending on the size of the project, for smaller projects, a straight cable is sufficient. If the project is on a large scale, use branch cables + junction boxes.
The approach using multi-core cables + on-site junction boxes + terminal cabinets in the control room is good. The reasons are: 1. The cost of cable laying is low ； 2. The control terminal cabinet is connected to the DCS, which prevents direct wiring inside the DCS cabinet, reduces wear on the screws used for fixing components within that cabinet, and also decreases the need to open and close the doors of the DCS cabinet ； 3. Multi-core cables can be equipped with either grouped shielding + overall shielding, which facilitates grounding and interference resistance ； 4. Adding on-site junction boxes increases the number of connection points, but by adding inspection points along the circuits, it is not necessary to climb up to the sensors/transmitters in order to conduct inspections ； 5. An appropriate amount of slack (spare core) can be left in multi-conductor cables, allowing connection to the spare core wire in case of issues with the main conductors ； 6. It facilitates the management and transmission of different types of signals at the site; different on-site junction boxes can be used depending on the spatial location and signal type.
Both of these are traditional methods; we usually use them in combination, deciding based on the actual circumstances. There’s nothing special to be concerned about. You might be worried because you haven’t used multi-core cables before, but in practice they work just the same – there’s no difference at all. Don’t exaggerate the issue; both are proven and reliable methods, so feel free to use them without hesitation.