4-20mA Junction Box versus Fieldbus (FF) Junction Box
How will a field JB of conventional 4-20mA connection look different from that of FF JB.
Conventional Junction box
Convectional Junction box contains Terminal Strip and Earth Strip/earth bus bar only.
Instruments field pair / core cable (From Instruments to Junction Box for transmitters, switches, valves, solenoid valve etc.) is terminated at one side of terminal strip and multi-pair cables (From Junction box to Marshalling cabinet/system cabinet/control panel) are terminated at other side of terminal strip.
The shield/screen of cable is terminated on Earth strip or terminals based on project philosophy.
Field Instruments (4-20mA) to Conventional Junction Box Animation :
A typical FF JB arrangement is shown in Fig below. The internal layout of components may vary for different designs and suppliers, however the components remains same mostly.
The cable connected to field instrument (transmitters, switches, valves, solenoid valve etc.) is termed as Spur Cable and cable connected to control system is termed as Trunk Cable.
The example used here is High Power Trunk concept, where Trunk is non IS and Spur is IS (IS – Intrinsically Safe).
As Trunk is non–IS, trunk termination assembly is physically Isolated by means of separator plate section inside JB to provide separation between IS and Non IS signals.
Internal wiring is done by FF-JB supplier. Field Engineer has to terminate Spur and Trunk cables at site on termination boards.
The FF JB design is based on segment design which will provide no of spurs and Trunk connected in single FF JB. Segment design is a separate topic of discussion hence not covered here.
In some cases temperature multiplexers are used to connect temperature Element on FF segment. These multiplexers are installed inside Junction box and this multiplexer JB is connected to FF JB as a spur like any other transmitter spur. A good design engineering practice is to limit 1 to 2 no. Multiplexer in a segment.
Fieldbus Signals Flow Animation : FF Field Instruments to FF JB, then to Host/Control Room
Article by : Nilima singh Instrumentation and control Engineer 13 year experience in Detail engineering Worked on various Refinery, Oil & Gas, Petrochemical and chemical projects
No Words to say, Best Article, Now i got some idea on Fieldbus & 4-20ma Junction boxes. Amazing efforts. Very good animations. thank you
Very nice and good knowledge thanks
I got a very nice information Thank you very much
Best explanation :)
But one question... trunk cable is shown just one .. can we use the spare trunk cable for redundancy purpose ??
Hi Anas, Yes Trunk cable have two pairs, Second one is spare cable. Generally we don't use redundancy, second trunk cable only for spare. Same already mentioned as Note in the animation.
Good job, thanks. However there is a little mistake : in the first picture, you wrote pairs for branch cables but you are showing triads.
hello sir ,
what is cascade loop and feedforward loop plz describe it with diagram
Hi, Please check CONTROL SYSTEM category, Already Available.
hello sir,
Can you give us more detail knowledge on DCS systems functionality more on honeywell TDC 3000.
what is the maximum distance between instrument to FF junction box and junction box to control room.
Can you send me more about Foundation Field bus system like, the distance from instrument to FFJB and from FFJB to control room.
What is segment in terms of FF system And clear wiring diagram from spur cables, internal wiring and to control room.
HOw FF works.. thanks
In the marshaling cabinet also gathering multiple trunk cables from different junction box and combined in one internal cable with one redundant and provide multiple communication to the DCS I/O's cards
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No Words to say, Best Article, Now i got some idea on Fieldbus & 4-20ma Junction boxes. Amazing efforts. Very good animations. thank you
Very nice and good knowledge thanks
I got a very nice information Thank you very much
Best explanation :)
But one question... trunk cable is shown just one .. can we use the spare trunk cable for redundancy purpose ??
Hi Anas, Yes Trunk cable have two pairs, Second one is spare cable. Generally we don't use redundancy, second trunk cable only for spare. Same already mentioned as Note in the animation.
Good job, thanks. However there is a little mistake : in the first picture, you wrote pairs for branch cables but you are showing triads.
hello sir ,
what is cascade loop and feedforward loop plz describe it with diagram
Hi, Please check CONTROL SYSTEM category, Already Available.
hello sir,
Can you give us more detail knowledge on DCS systems functionality more on honeywell TDC 3000.
what is the maximum distance between instrument to FF junction box and junction box to control room.
Can you send me more about Foundation Field bus system like, the distance from instrument to FFJB and from FFJB to control room.
What is segment in terms of FF system And clear wiring diagram from spur cables, internal wiring and to control room.
HOw FF works.. thanks
In the marshaling cabinet also gathering multiple trunk cables from different junction box and combined in one internal cable with one redundant and provide multiple communication to the DCS I/O's cards
It's a great learning portal.