Instrument Junction Box

Junction box is an enclosure used for cables interconnection between field devices and control room. It encloses terminal strips for cable termination.

Instrument Junction Box

Junction box shall be designed to suit environmental condition where the box will be installed and shall have certification of Ingress Protection code and hazardous area protection which conform to the classified area.

Junction Box consists of the following parts, but not limited to:

  • Terminal block which comprises terminal strips for cable connections
  • Gland plate (for non metallic junction box) for cable gland earthing
  • An insulated earth bus bar for overall cable screen termination (optional, drain wire could be connected to terminal strip)
  • Breather/drain plug
  • Mounting Rail complete with end bracket

Instrument Junction box typical schematic

Below is picture of instrument junction box:


Junction Box for instrument digital signal should have bus bar for connecting screen of individual cables and overall screen of multipair cable.

In common practice, junction box shall be separated for the following purposes:

  • PCS signals and SIS signals
  • Analog signals and Digital signals
  • Signal with different voltage level