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 :
Junction Box Parts
Junction Box External View
Junction Box Internal View
FF (Foundation Fieldbus) Junction box
FF Junction box generally contains
- Spur termination board and Field Barrier
- Trunk Termination Assembly
- Surge Protector
- Fieldbus terminator
- 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 FFJB 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
FF Junction Box
Also See : DCS/PLC Signal Flow Animation