PLC, Remote I/O and Marshalling Panels

Marshalling is a word we often use to describe the organising, or gathering together, of a group of people or things. Think of “the general marshalled his troops,” for example. In computer terminology, marshalling is the process of gathering data and transforming it into a standard format before it is transmitted over a network.

The common meaning in both examples is “to gather together.” The world of electrical switchgear is no different. Marshalling panels gather together multiple wires and cables, provide cross wiring functionality between the control room cabinet and field instruments. In other words, they connect signals from thousands of different field devices to a control system in a way that’s both logical and reliable. The main advantage of this type of interface is that any issues with input or output can quickly be identified and repaired. Routine maintenance can also be carried out in the field without jeopardising the operation of the entire control system.

We typically find this type of marshalling in large DCS systems, such as those found in chemical plants and refineries, for example.

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How Do Marshalling Panels Work?

The process of connecting field signals to process controllers is pretty standard, no matter what industry you’re dealing with. Firstly, the field signal wiring is brought to local junction boxes. These are similar to the junction boxes we’re all familiar with in our own homes. These boxes are then consolidated into more centralised junction boxes, which are in turn grouped into multi-core cables. The grouping is often done according to common characteristics, such as analogue or discrete, for example, or high or low voltage. Finally, these cables are brought into the marshalling cabinets. The output of these cabinets is then connected to the control system.

The main idea behind this marshalling process is to bring all the relevant instruments into the control room in a way that’s not only cost-effective, robust and durable, but which is also well ordered and easily maintainable.

Marshalling is usually carried out in one of three ways: cross-wire, fan-out and half-knit.

Cross-wire Marshalling

Cross-wire marshalling is a method whereby field cables are terminated in pair order. The disconnects, fuses, or termination assemblies are scrambled, and prefabricated cables are used between the disconnects and system cabinets.

This method works best when separate marshalling and system cabinets are used. Because the prefabricated cables are available in different lengths, it’s not necessary to know the final room layout until much later in the process. In addition, relays, fuses or isolators can be installed between the field signals and the input/output (I/O) terminals. Cross-wire marshalling is quick to install and easy to maintain. It does, however, need additional cabinet space.

Fan-out Marshalling

In this arrangement, unlike with cross-wire marshalling, field wiring is not terminated in pair order. Instead, a custom termination per cable is required. Fan-out marshalling is best suited for situations in which cabinets are connected for testing and then disconnected for shipping. This happens when they are shipped separately due to the construction schedule – marshalling cabinets are often needed on site several months ahead of the system cabinets. Once on site, the cables are reconnected to the system cabinets.

Half-knit Marshalling

With half-knit marshalling, the field cables are terminated, in pair order, on marshalling blocks. This is a very efficient method which makes for easy maintenance. This type of marshalling is suitable for the following situations:

  • Where marshalling and I/O terminals are in different system cabinets, but the cabinets are permanently joined together.
  • Where marshalling and I/O terminals are in the same system cabinet.
  • Where cross-wiring is done in a remote instrument enclosure (RIE) or on site.

Half-knit marshalling is not as suitable in situations where the marshalling and I/O terminals are in separate cabinets that are assembled in different places.

Each of the types of marshalling has advantages and disadvantages. Cross-wire marshalling, for example, offers more flexible scheduling, and it comes with better lifecycle benefits than half-knit or fan-out marshalling. It is, however, the most costly option. They all, however, have the same end goal, which is to connect field devices to the control system. The only real difference is how the multi-core cables are treated in the marshalling cabinets, and how they are connected to the control system.

JB Switchgear believes in solutions through focus and commitment, and we strive for excellence in everything we do. All our products meet the most stringent national and international safety and performance standards. We have a comprehensive range of floor standing PLC, remote I/O and marshalling panels for automation and related applications, so please contact us today to talk to us about how we can help you with your next project.