A marshalling cabinet is a type of electrical enclosure that is used to organize and terminate field wiring in an industrial control system. It typically contains a series of terminal blocks or connectors that are used to connect field wiring to the control system.
Here is a diagram of a typical marshalling cabinet:
What is the Marshalling Cabinet?
The marshalling cabinet is an interface between the system cabinet (DCS/PLC control panel) and the field junction boxes. The primary cables are laid between the field junction boxes and the marshalling cabinet. Additionally, the prefabricated cables from the system cabinet IO cards terminate in the marshalling cabinet. It also separates the control system panel from the field control panel. A typical architecture for an instrumentation and automation system has a marshalling cabinet between the junction box and the system cabinet.
Marshalling cabinets are used to organize and protect the field wiring in a control system, making it easier to maintain and troubleshoot the system. They are commonly used in industrial control systems, building automation systems, and other applications where field wiring needs to be organized and protected.
What is the function of the marshalling cabinet?
The main functions of the marshalling cabinet are listed below.
- Signal conditioning for analogue signals
- cable termination
- signal isolation
- cold junction compensation of thermocouple signals
- power supplies for field devices
Salient features of a marshalling cabinet:
- Marshalling panel is used for grouping I/Os.
- A marshalling panel is placed between the DCS system panel and the field junction box where the field devices are connected.
- I/Os like analog input, analog output, digital input, digital output, and pulse I/O are grouped together in the marshalling panel.
- Isolation and signal conditioners send I/O signals from the marshalling panel to the CPU or controllers of the system.
- Marshalling panels facilitate cross-wiring between field instruments and control systems.
- With this type of interface, problems with inputs and outputs can be found quickly, and maintenance workers can do routine tasks in the field without putting the core of the control system at risk.
What are the benefits of marshalling panels?
The precise location of the marshalling cabinets will reduce cable costs and make it simple for maintenance personnel to trace wire routing and easy troubleshooting.
In a marshalling panel, inputs and outputs can be easily identified, and faults can be diagnosed with minimal effort.
What is cross-wiring in marshalling cabinet?
A large number of analogue and digital sensors and actuators may be terminated into each field junction box. Following that, 16 pair or 24-pair multi core cables are used to connect these devices to the central control panel. Therefore, the cable connecting the field junction box to the central control system may contain both digital and analogue signals, as well as input and output signals. This causes an issue within the control panel.
How does an engineer redistribute these signals so that all signals of a particular type are routed to the appropriate digital or analog I/O module of the controller?
To answer this question, a marshalling cabinet or marshalling control panel is used. The field wires are switched to the right I/O module and channel on the controller using either one or two banks of terminal blocks. This method is called cross-marshalling.
Why we are using Marshalling Panel?
The main purpose of using Marshalling cabinet is to do signal segregation properly from field junction box to the control system by doing cross wiring and also to do necessary signal conditioning.
For example, let’s say we have a 20-field analog signal coming in on a 24-pair incoming field cable. At least two 16-channel analogue I/O cards will be made from this field signal. The first 16 I/O will be on the first 16-channel I/O card, and the next 16-channel I/O card will have the other four. From an operation and maintenance point of view, it is not a good idea to have this split wiring (cross wiring) in the system cabinet. So, we need the marshalling cabinet.
Mixed I/O signals coming from the field are another cause of cross-wiring. There can be a mix of AI (analog input) and AO (analog output) signals in the incoming multi-pair cable. This mixed signal will be separated in the marshalling cabinet so that the AI signal will end at the AI termination board and the AO signal will end at the AO termination board.
In a normal scenario, the cables go through the marshalling cabinet in the following way:
1. The multi pair field cables go into the cabinet through the bottom.
2. Then, each wire of the incoming multi pair field cable is connected to the surge protection device or surge arrester. If there is no need for these surge protection devices, each wire is connected to the terminal block.
3. In a non-intrinsically safe application, the surge protection devices will have cross-wiring that matches the field signal and the I/O address assignment in the termination board. If the system is IS, each wire connection must first go to the IS barrier before it can be used for cross-wiring.
4. The dedicated system cable with a “plug and play” connection is then routed from the termination board to the I/O card in the system cabinet.
5. Some termination boards may need a dedicated DC power supply that can be taken from the DC power supply in the marshalling cabinet.
Marshalling Panel Vs Junction Box – What’s the difference:
| S. No | JUNCTION BOX | MARSHALLING CABINET |
|---|---|---|
| 1 | Instrument Junction Boxes, often known as JBs, are a crucial component of every installation for controls and instrumentation. An instrument junction box is an enclosure containing terminals that permits connections between field devices (i.e., instruments, switches, etc.) in the process/production zones and control or monitoring equipment, which is normally located in the control room. | The marshalling cabinet is an interface between the system cabinet (DCS/PLC control panel) and the field junction boxes. The primary cables are laid between the field junction boxes and the marshalling cabinet. Additionally, the prefabricated cables from the system cabinet IO cards terminate in the marshalling cabinet. It also separates the control system panel from the field control panel. |
| 2 | They keep the weather out of electrical connections, keep operators and technicians from getting electric shocks by accident, and make it easy to get into a circuit for maintenance and troubleshooting. | Marshalling Panels allow field instruments and the control system to be wired together in different ways. With this type of interface, problems with inputs and outputs can be found quickly, and maintenance workers can do routine tasks in the field without putting the heart of the control system at risk. |
| 3 | Most of the time, a junction box is used to connect multiple field devices from the same system in the same area. Instrument junction boxes should be made to fit the conditions of the places where they will be used. It needs to be certified to show that it has the right ingress protection (IP) and hazardous area protection. | The marshalling cabinet’s key duties are signal conditioning for analogue signals, cable termination, signal isolation, and cold junction correction for thermocouple signals. It also gives power to devices in the field. The majority of the marshalling cabinets were installed in safe locations (controlled environment) along with the MCC, electronics room, and control room. |
