What are Instrument Junction Box?

What is a Junction box?

Instrument junction boxes are an integral part of every electrical, control and instrumentation installations. The junction box is an enclosure used for interconnection between field devices. As the junction box is outdoor equipment, it designed or selected according to the environmental conditions.

Simplyjunction box gives protection to the cable between instrument and control panel. Junction boxes are required at many different work locations and systems (wet, dry, onshore, offshore conditions etc…,), it recommends the need for proper maintenance and standardizations of the junction box.

Specification for an Instrumentation Junction Box:

  • Material used
  • Degree of protection
  • Types of terminal
  • Cable entry direction and devices
  • Earthing
  • Temperature class
  • Breather/drain plug
  • Enclosure size
  • Labels
  • Door
  • Terminal block backplate
  • Ambient temperature

Components of Junction box:

  • Terminal strip
  • Gland plate
  • Breather
  • Insulated earth bus bar
  • Mounting Rail



Majority of instrument junction boxes are made of 316L grade stainless steel which allows more environmental protection than another type 303 & 304 which are subjected to corrosion.

For outdoor operations, the material should be selected with good corrosion-resistant, high yield, tolerance to temperature and high creep strength.

Type of Terminal:

On offshore structures and units there are two types of terminals: screw and cage. The connection of the screw type is time consuming and the screw mechanism limits a terminal’s minimal size, making screw terminals less useful where a large number of connections are most commonly used with Spring cage or pin-type terminals.

Drain or Breather Plug:

All external instrument junction boxes should be equipped with breather / drain plugs for condensation or drainage and allow air from the enclosure to flow into the surrounding area to minimize the build-up of moisture. All breather / drain assemblies are SS316/SS316L type and the corresponding junction box shou IP rating.


All equipment should be marked with protection level, gas group, sta class applied, and certificate number. Labels and signs should use words and staff abbre. Labels should be made from materials and produced through a process that will satisfy all expected operating environment conditions.