As discussed in the Typical Architecture of Instrumentation and Automation System, the automation architecture of the plant is having a marshalling cabinet in between the junction box and the system cabinet.
This marshalling cabinet function is to interface the incoming field cable (which is normally a multipair cable) and the I/O (Input/Output) card connection. Without marshalling cabinet, we can sure (for large number of I/O) that there will be a trouble for operation personnel to maintain and operate the cabling.
One of the important interface function of the marshalling cabinet is the cross wiring function. Cross wiring is always necessary since the incoming field signal and the channel quantity of the I/O card is always different.
Image Courtesy : ITEC Engineering
For example, let’s say we have a 24 pair incoming field cable that carry 20 field analogue signal. This field signal will be split at least into two analogues 16 channel I/O card.
The First 16 I/O will be in the first 16 channel I/O card and the other 4 will be in the next 16 channel I/O card. To have this split wiring (cross wiring) in the system cabinet is not a good practice from the operation and maintenance point of view. Thus the marshalling cabinet is needed.
Another cause of the cross wiring is the mixed of I/O signals that coming from the field. The incoming multipair cable can have mixed AI (Analog Input) and AO (Analog Output) signal in the same multipair cable. This mixed signal will be split in the marshalling cabinet so that the AI signal will be terminated in the dedicated AI termination board and so do the AO signal.
In some application such as voting application for safety system (i.e. 2oo3 voting) there is a requirement to have the I/O signal allocated in the different I/O card so that the common fault cause can be avoided. This application ofcourse once again need a cross wiring in the marshalling cabinet.
Image courtesy : srspower
In typical application, the marshalling cabinet is having the following cable routing.
- The multipair field cables enter the marshalling cabinet through the bottom part of the cabinet.
- Then each wire of the incoming multipair field cable is terminated in the surge protection devices or surge arrester. If there is no requirement of such surge protection devices, each wire is terminated in the terminal block.
- From the surge protection devices, in Non IS application, there will be a cross wiring that match the field signal and the I/O address assignment in the termination board. If the system is IS, then before cross wiring, each wire connection is routed first to the IS Barrier.
- The dedicated system cable that has a ‘plug and play’ connection is then routed from the termination board to the I/O card in the system cabinet.
- Some termination board may require a dedicated DC power supply that can be taken from the DC power supply in the marshalling cabinet.