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. 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 (Analaogue Input) and AO (Analogue 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.
In typical application, the marshalling cabinet is
having the following cable routing.
1.
The
multipair field cables enter the marshalling cabinet through the bottom part of
the cabinet.
2.
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.
3.
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.
4.
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.
5.
Some termination board may require a dedicated DC
power supply that can be taken from the DC power supply in the marshalling
cabinet.
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