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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 Junction Box

Field Instruments (4-20mA) to Conventional Junction Box Animation :

Field Instruments Junction Box Animation

Junction Box Parts

Junction Box

Junction Box External View

Junction Box External Parts

Junction Box Internal View

Junction Box Wiring

FF (Foundation Fieldbus) Junction box

FF Junction box generally contains

  1. Spur termination board and Field Barrier
  2. Trunk Termination Assembly
  3. Surge Protector
  4. Fieldbus terminator
  5. Trunk Termination board
  • 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 Junction Box

Fieldbus Signals Flow Animation : FF Field Instruments to FF JB, then to Host/Control Room

Fieldbus Signals Flow Animation

FF Junction Box

FF Junction Box

Also See : DCS/PLC Signal Flow Animation

Article by :
Nilima singh
Instrumentation and control Engineer
13 year experience in Detail engineering
Worked on various Refinery, Oil & Gas, Petrochemical and chemical projects

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Karthikeyan M September 16, 2017 at 11:48 am

No Words to say, Best Article, Now i got some idea on Fieldbus & 4-20ma Junction boxes. Amazing efforts. Very good animations. thank you

Krishan lal September 19, 2017 at 4:02 am

Very nice and good knowledge thanks

Baba September 22, 2017 at 6:00 am

I got a very nice information Thank you very much

Anas Kadri October 11, 2017 at 7:05 am

Best explanation 🙂

But one question… trunk cable is shown just one .. can we use the spare trunk cable for redundancy purpose ??

S Bharadwaj Reddy October 11, 2017 at 12:59 pm

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.

azetou October 11, 2017 at 3:03 pm

Good job, thanks. However there is a little mistake : in the first picture, you wrote pairs for branch cables but you are showing triads.

Jigar Rana October 12, 2017 at 5:18 am

hello sir ,

what is cascade loop and feedforward loop plz describe it with diagram

S Bharadwaj Reddy October 12, 2017 at 5:23 am

Hi, Please check CONTROL SYSTEM category, Already Available.

JUSTIN October 12, 2017 at 6:53 am

hello sir,

Can you give us more detail knowledge on DCS systems functionality more on honeywell TDC 3000.

Deepakkumar December 12, 2017 at 7:17 am

what is the maximum distance between instrument to FF junction box and junction box to control room.

Dodz March 24, 2018 at 4:12 pm

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

Mk Chaudhary October 12, 2018 at 9:21 am

It’s a great learning portal.

Moses Kashimba December 20, 2018 at 9:54 am

The information is so helpful, thank you.

Ramprasad June 12, 2020 at 8:46 am

we know that the general procedure for conventional wiring are shields are not terminated at the field side, whereas what is procedure do we follow in the FF concept. does shield termination at the field will be there or not.

Aspak July 28, 2020 at 8:53 pm

Thanks for the information.


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4-20mA Junction Box versus Fieldbus (FF) Junction Box

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