Instrument Junction Boxes
Instrument Junction Boxes (commonly referred to as JBs) are an integral part of every control and instrumentation installation. They protect electrical connections from the weather, help prevent operators and technicians from suffering accidental electric shocks, and offer a convenient entry into a circuit for maintenance and fault finding.
In this article we will look at the considerations and options available when specifying a junction box, and also at some of the documentation used by instrument designers and technicians relating to Junction Boxes.
What is a Junction Box?
An instrument junction box is an enclosure housing terminals that allows interconnection between field devices (i.e. instruments, switches etc) in the process/production areas, and control or monitoring equipment typically located in the control room.
Typically, numerous field cables of a common system are joined, via the terminal blocks within the JB, to a multicore cable. Examples could be analogue signals to the DCS, switch signals to DCS, analogue signals from DCS, analogue signals to ESD etc.
Specifying a Junction Box
Like every part of an instrument loop, it is essential that the JB selected is suitable for the required application. Consideration needs to be given to:
- Materials of construction,
- Suitbility for use in hazardous areas,
- Degree of ingress protection,
- Type and quantity of terminals to be housed within the JB,
- Number of cable entries and their direction,
- Requirement for breather or drain plug,
- Junction box size and mounting,
Below, we will look at each of these considerations in turn.
Junction Box Materials of Construction
Junction boxes can be manufactured in a variety of materials including stainless steel, mild steel, glass reinforced polyester, aluminum, polycarbonate and ABS (Acrylonitrile butadiene styrene - a thermoplastic polymer). These are all suitable for a wide range of industrial and OEM applications.
For outdoor areas that are exposed to changing environmental conditions like those encountered in many process and petrochemical plants, and Oil & Gas installations, the use of materials with good corrosion resistance, the ability to tolerate high ambient temperatures and high creep strength, is important. That is why stainless steel is most often selected, with GRP being a popular second choice.
The preferred grade of stainless steel is 316L, as this offers superior protection against pitting-type corrosion than other grades like types 303 and 304.
Glass Reinforced Polyester (GRP) has a high resistance to contamination from oils, has excellent mechanical properties (e.g. strength), and offers a long life expectancy.
Junction Boxes in Hazardous Areas
Junction boxes, certified suitable for use in potentially Hazardous Areas are available from many manufacturers. Common certification standards include the ATEX Standard, and the IEC 60079-series of explosion prevention standards. For JBs, the most common types of protection used are Ex d "flameproof" and Ex e "increased safety". Though it is not uncommon to see Ex"N", or indeed Ex"P". Ex e is often used for junction boxes in intrinsically safe circuits.
For Ex e certified JBs there are two main criteria to consider;
- Are the internal components e.g. terminals etc acceptable for use in the JB i.e. only terminals or other components which are specifically allowed for in the JB's certificate of compliance, and
- Will any internal components, or wiring, be hotter than the temperature classification of the JB allows.
In all Ex certified enclosures it is important that an earth facility is provided. For metallic enclosures the earth facility must earth the enclosure body and can be provided by earth terminals connected to the body through the terminal mounting rail and/or by means of an internal/external earth stud.
A major secondary form of protection for the internals of a JB is its IP rating. Moisture or dust, if allowed to come into contact with the JB internals, could lead to either sparking or physical breakdown of the components and interfere with the explosion protection method being used. It is for this reason that IP56 is usually considered the minimum rating that should be used for junction boxes, especially if they are located outside.
If the junction box could be subject to deluge conditions, as is common in many offshore oil and gas platforms, then greater ingress protection will be required. For deluge service consider specifying that the equipment should meet the requirements of Shell DTS:01.
As already mentioned above, only terminals which are specifically allowed for in the JB's certificate of compliance should be used. Most reputable manufacturers offer junction boxes that can accept terminals from the a wide range of brands e.g. Weidmüller, Pheonix Contact, ABB, Legrand, Rockwell automation etc.
The terminal should be matched to the type and size of cable being used and attention should be paid to the current and voltage ratings of both the terminal and cable.
A removable "gear plate" (or component mounting plate) is usually provided. DIN standard terminal rails can be fixed to the gear plate, before the plate is mounted in the box. This makes for easy and quicker assembly of the box.
The number of terminals, and width of each one, will determine the size of box required. It is good practice to consider future expansion by leaving two or more unused pairs in the main multicore cable.
Cable glands should be selected according to the cable type, screen or armour earthing requirements and the IP rating required. For junction boxes used in hazardous areas the gland must must have the same criteria as the enclosure to which they are connected.
Ex e junction boxes are usually supplied with gland plates which are drilled to accept cable glands which allow cable entry. The gland plate is bolted to the JB and supplied with a gasket to maintian IP rating. It is important to always allow enough clearance around multiple gland entries to allow for fixing nuts etc. Note, the choice of gland, and by implication its size, may limit the number of entries possible in any given gland plate.
Ex d junction boxes are not usually supplied with gland plates. Instead, the housing is drilled and tapped to accept the required glands. Unused ways are fitted with certified blanking plugs.
There is no hard and fast rule about where cables should enter a box. It is often the case however that incoming cables enter the box from the side, and the multicore exits from the bottom of the box. Different configurations of cable entry may impact on the size of box required.
As mentioned above, all Ex certified enclosures must be provided with an earth facility. These are usually an M6 internal to M10 external SS316L earth stud. It is common for Earth studs to be welded to the enclosure wall to maintain integrity of the IP rating.
Bonding wire (usually Green and Yellow) should be fitted inside the JB between the door and body, and between the componenet mounting plate and the body.
Breather and Drain Plugs
The Breather part of a Breather Drain plug provides pressure compensation between the inside of the junction box and the external atmosphere. This minimises moisture build up caused by temperature fluctuations and humid environments.
The Drain part of the plug allows any condensed water present within the junction box to be drained before it accumulates and causes problems.
If fitted, and it is good practice to fit these on JBs mounted externally, breather drains must have the same Hazardous Area certification and IP rating as the junction box to which they are fitted.
Breather Drains are sometimes referred to as Breather Vent Plugs or Conduit Drains.
Junction Box Size & Mounting
The size of a JB is dependant on the number and type of cables being terminated within it. The more cables the more cable entries, and cables with screens require more terminals. The more cable entries, and the more terminals the bigger a JB is required. Further, ergonomics and ease of access for wiring mean that when considering JB size a larger rather than smaller box should be selected. Generally, boxes in the range of 200 to 500mm wide by 200 to 500mm tall should be considered.
Junction boxes should be located in well lit, easily accesible areas. Boxes should be specified with at least four external mounting lugs, two top and two bottom. Due to their weight, especially Ex d boxes, 10 mm diameter or larger fixing holes should be drilled in the mounting lugs.
All JBs should be labelled externally with their unique JB number. And if the box contains Intrinsically Safe signals, an additional external tag, coloured blue with white lettering stating "Intrinsically Safe Circuits" should be fitted.
Lable material choice will usually be either stainless steel, or traffolyte (also written as traffolite). Traffolyte is multi-layered phenolic plastic sheets where each layer is a different colour so engraved letters are a different colour from the unengraved portions. Labels should be fixed to the box ensuring that IP rating and hazardous area protection is not compromised.
Junction Box Doors
Doors should be capable of full 180 degree opening. Increased ease of access can be obtained by specyfing removable doors.
Ensure doors are fitted with seals or gaskets that allow the IP rating of the box to be maintained.
Prevention of unauthorised access to the box needs to be considered. Simple quarter turn door latches may be adequate, though in some applications a padlockable door may be prefered.
A pocket on the inside of the door is useful for storing JB wiring diagrams - see below for further details.
Junction Box Documentation
Junction Box Diagram
A junction box diagram, or as sometimes called - a JB drawing, as a minimum will show:
- the junction box number,
- field instruments and their cable number,
- the terminal numbers to which the instrument is terminated,
- the multicore number and its destination,
- which multicore pair each field instrument is connected to.
Additional information may also be included to help with maintenance and fault finding.
Junction Box Schedule
The junction box schedule is a document, likely to be derived from a database or in the form of a spreadsheet, listing all the junction boxes that have been installed on a plant. It is used extensively during plant design as an aid to procurement as it provides an easy source for material take off. Most operators As-Build the schedule following construction to allow for easily locating junction boxes, and to allow new boxes to follow the established numbering system.
A typical JB Schedule will contain the following fields:
- JB Number,
- JB Type / Specification e.g. GRP, Ex e etc,
- JB Size,
- JB Location/Area,
- Signal type e.g. analogue, digital etc,
- Number and size of incoming pairs,
- Size of multicore leaving box e.g. number of pairs, outside diameter etc,
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