What is a Junction box, and its applications?

What is a Junction box?

Junction boxes are used in most of the electrical and instrumentation installations. A junction box in the instrumentation field is a device that would act as an interconnecting medium between the process field instruments and the equipment which is used to control and monitor the field instruments, this equipment would be located in the control room. Mostly the junction boxes are used to terminate the multiple field devices of common systems in a general area. We should choose the instrument junction box according to the environmental condition in which it is going to be installed.

What is the importance of the junction box in an instrument control system?

A junction box will be useful to simplify the wiring of the field instruments with the control room. There could be a lot of field instruments in a process plant, to do the individual cable connection of field transmitters to the control room is not possible. In this case, we can use a junction box to solve this problem, the field transmitters can be wired and also terminated in the field junction box. This connection will be continued to the main junction box and from this main junction box it will be wired to the marshaling cabinet and from there these wires will be wired to the controller IO cards

There is fireproofing in the junction box cover so that these boxes would be able to prevent short circuits in case of fire. This box would enclose the terminal strips for cable termination.

Process control by the instrument control system

The signal will be transmitted from the field transmitters to the local field junction box and from there it will be transmitted through a multi-core cable to the main junction box. The signal will travel from the main junction box to the marshaling cabinet and from there to the control panel. So the signal would enter into the input card in the controller. This controller would process the data and the output data will be transmitted to the final control element through the output cards and the transmitting path would be the same as we discussed earlier.

What are the major parts of the junction block?

parts of the junction box

How field instruments are wired to the junction box?

The instrument cable entries to the field junction box must be done through the bottom. The twisted multi-core cable must be cut to a proper length to reduce the looping and flexing of the cable within the junction box. The outer jacket of the shielded twisted pair cable must be left intact at the point of termination. The shield drain wire must be insulated from the jacket end to the terminal. Spare pairs in the multi-pair cables must be labeled spare in addition to the source and the destination terminal numbers. In the case of the individually shielded multi-pair cables, each cable should be insulated from the cable jacket end, up to the point of the termination. So that the foil shielding would be intact and also free from accidental grounds. The shield drain wire must be insulated from the foil end to the terminal.

All the wiring should be tagged at each end and each wire tag should have two labels. The first label would represent the terminal number to which the wire would be connected and the other label would show the opposite end of the wire connection.

What are the various IP ratings that must be considered while selecting a junction box?

The electrical junction box for the outdoor instrument signal wiring must be installed complete with the mounting hardware, nameplates, terminal blocks, and wiring.

  • For outdoor electrical junction boxes, NEMA 4X enclosures are normally preferred
  • The explosion-proof junction box must have the NEMA 7 specification
  • The entrance to the junction box must be from the side or from the bottom and there shouldn’t be any hole on the top of the junction box
  • High-density terminal strips must not be used in the junction box
  • The junction box should have IP-66 ingress protection

For more detail refer- IP ratings of the panel

  • All the explosion-proof and flame-proof junction box enclosures should have spare entries sealed with the suitably certified 316 SS plugs.
  • The cable gland which is used in the junction box enclosure must have IP 66 protection
  • All the glands, adaptors, reducers, and plugs that are used in the junction box must be Class 1 certified.

How marshalling panels are wired to the junction box?

The cables between the field junction box and the marshaling panel must be routed in conduits or trays or they should be directly buried. The major purpose of the marshaling cabinet is to do the main cable termination and then to redistribute the field device to its I/O cards with the help of internal wiring. The multi-paired wires from the junction box would reach the marshaling cabinet. Then in the surge arrestor section, each wire would be terminated in case if there is no surge arrestor then it will be terminated in the terminal block.

What are the types of junction boxes?

Increased safety box

The junction construction would be 1.5mm 316 SS, boxes would have a bottom entry with a removable gland plate. The gland plate would be bolted to the box with 316SS bolts., sealed with neoprene gaskets, and fitted with an earthing stud. The incoming cable would be entered to the side of the junction box and the outgoing cable would be from the bottom of the junction box. These boxes would have external mounting lugs. These boxes would have door hinged at two points with neoprene seal fitted all around,

Explosion-proof box

This type of junction box will be created from 316 SS epoxy coated copper-free die-cast aluminum. These boxes would have mostly four mounting lugs, drilled with 10mm fixing holes and an 8 mm external earth lug. Cables would be entered from the bottom. These box lids would have cast hinges integral to the box with the stainless-steel pins.

What is the difference between a Utility box and the Junction box?

A utility box can also be called an electrical box and it would be composed of switches, electrical sockets, and also fixtures. The major difference between a junction box and a utility box is that a junction box would house the electrical connections while the utility boxes are used to install switches, fixtures, etc.

Some useful information about Junction boxes

Instrument junction boxes are used in many systems, and they would be used in different kinds of work locations like wet or dry conditions. These boxes can also be seen in safe and hazardous areas too. We should maintain the instrument junction box in safe and efficient conditions. We can also see the instrument junction box in offshore structures and units. All the cable entries to the instrument junction box must be fitted for the certified sub-components and with the proper IP ratings. All the enclosures must be lockable and must use IP65 304 stainless steel door locks. All the enclosures should have hinged doors that would open outward.

Junction box

Where are junction boxes located?

  • Junction boxes are located at the intersections of the duct runs
  • junction boxes are generally spread across the plant area to interconnect various field instruments with the control system
  • It should be located along with the horizontal baseline the points where the junction box would be centered
  • All the junction boxes in any one interconnecting area must be placed with the arrow pointing in the same direction, and this would create a consistent pattern for all interconnecting duct runs.
  • The junction box for the outdoor instrument signal wiring must be installed complete with mounting, hardware, nameplates, terminal blocks, and wiring

Which materials are used to construct junction boxes?

Junction boxes are available which are mostly made up of steel, stainless steel, aluminum, and fiberglass-reinforced plastic material.

Instrument junction box which is used offshore would be made up of stainless steel. The stainless steel which is used for the manufacturing would be 316L grade stainless steel.

Instrument box which is used in outdoor locations would be mostly made up of glass fiber reinforced polyester. So that it can handle the varying environmental conditions and by choosing this material we can prevent corrosion. These boxes would have a high yield tolerance to high ambient temperatures. The glass fiber reinforced polyester provides high resistance to contamination from oils and it also has a good life expectancy and excellent mechanical properties.

Things to be considered while selecting a location for a junction box?

Instrument junction boxes could be located indoors and outdoor, mostly these boxes would be located between the field device and the control system. So we must choose the instrument junction box according to the location where it is going to be installed. If the instrument junction box is used offshore then it must be made up of stainless steel. If the instrument junction box is going to be installed in an outdoor location, then it must be made up of fiber-reinforced polyester.

We should also consider the ingress protection (IP) ratings while selecting a junction box for a specified location. So while choosing the junction box according to this rating, we could protect the equipment from intrusions, dust, water, and accidental contact. We should choose the junction box’s IP rating according to the location where it is to be installed.

IP ratings are based on different locations and it is divided into four categories and they are A, B, C, and D. The IP ratings for indoor is defined with low severity levels and the outdoor IP ratings are defined with high severity levels.

CategoryJunction box locationIngress protection
AIndoor with HVACIP 21
BIndoor machinery spaceIP 44
CIndoor with water mistIP 54
DOutdoorIP 56

What are the factors to be considered while selecting a junction box?

  • We must consider the material used for the junction box
  • Protection
  • Terminal type
  • Size of the enclosure
  • We must consider the temperature class while selecting the junction box
  • Earthing
  • The direction of the cable entry
  • Breather drain/plug

How to properly ground a metal junction box?

All the components of a junction box such as door, terminal plate, gland plate, enclosure, and earth bar must be individually star terminated to the earth stud. The earth stud should have all the proper nuts, washers, and lock washers for proper installation.

Junction box earthing scheme


post-graduate in Electronics & communication.

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