Thoughts on the design of intrinsically safe junction boxes
Junction boxes are ‘simple apparatus’ when used in intrinsically safe circuits, and given a little thought they can make life simpler for the installation and maintenance engineer.
The first rule is where at all possible only include intrinsically safe circuits in the junction box. It is possible to design a box for use with both intrinsically safe and other types of circuit but there are considerable complications and questions such as segregation, maintenance procedures and certification have to be addressed. [a subject for a future blog?]. A positive effect of the introduction of the ‘ic’ concept is that circuits, which were previously considered to be ‘nL’, can now be included in the same cable and junction boxes as other intrinsically safe circuits.
The choice of type of box is relatively simple. The way to avoid any discussion as to its adequacy from an ‘explosion- proof’ viewpoint is to use an enclosure which is already component certified ‘Exe’. The fundamental question is however is it suitable for its intended environment? In almost all circumstances the choice of a stainless steel box solves the problem and is not too painful from an economic viewpoint. Plastic enclosures are acceptable but in this case the ‘Exe’ component route is almost unavoidable to satisfy the anti-static and other requirements.
The ingress protection [IP] rating of the enclosure is theoretically determined by the application but the majority of available enclosures claim IP 65 and this has become the norm. In some circumstances additional protection is necessary. For example where a junction box is subject to intense solar radiation some form of shade is desirable or the temperature inside the enclosure can become too high for the wire insulation. All enclosures breath when subjected to variations in ambient temperature and where the air is humid condensation within the enclosure occurs. Consideration to fitting a suitable breather/drain plug should be given and if there is any doubt one should be fitted. The other situation where a breather is desirable is if there is any possibility of process gas feeding back along the interconnecting spur cable. Ensuring that the inside of the enclosure is at atmospheric pressure ensures that the leaking gas does not continue down the multicore. The enclosure needs to be clearly identified so provision for a plant identity label is essential and a warning label such as ‘Intrinsically safe circuits – Take care’ desirable. There is a tendency to paint all things intrinsically safe blue but this is invariably badly done and should be avoided, particularly if the box is plastic or stainless steel.
The choice of size of enclosure is determined by a number of factors. Obviously it has to be big enough to accommodate the number of terminals required. A separate terminal so that each screen can be properly terminated is very desirable. Less obviously and more difficult to determine is that there has to be space to bend the cables to line up with the terminals without putting stress on the connections. It is also desirable to be able to read the cable marking without having to lever apart the other cables. The gland plate on the box has to be big enough to accommodate the large number of glands required and if fitted a drain plug. Some designs require special spanners to tighten the glands. This is preferably avoided since these spanners are never available when they are needed. For example a sixteen pair multicore with individually screened pairs requires 48 terminal blocks, 1 large and 16 other glands and one drain plug and the enclosure needs to be twice as big as was first anticipated. The remaining golden rule is if some of the cores in the multicore are spare then fit all the terminals so that all the cores can be properly terminated. Also drill all the holes required for the maximum number of glands and close off the unused holes with an appropriate stopper plug. Without these precautions using the spare cores at a later date becomes extremely difficult.
It is common practice to use terminals of the type used in low powered ‘Exe’ circuits [sometimes blue] because these satisfy the requirements of segregation between circuits and clearance to ground. Their construction ensures a degree of operational reliability, which is desirable, and there is usually an adequate provision for marking. Terminals carrying intrinsically safe levels of current are allocated a temperature classification of T6 [850C from 400C ambient] and consequently the temperature classification of intrinsically safe junction boxes is never a problem.
The choice of glands is determined by the cables used and the requirement to maintain the integrity of the enclosure. In practice the use of a large number of glands frequently lowers the enclosure integrity from IP 65 to IP54. The use of anything other than round cables should be avoided
The siting of junction boxes is a long neglected art. They should be mounted in an easily accessible position at a reasonable height and in a not too exposed location since junction boxes are frequently the most convenient point at which to do maintenance checks. Avoiding situations where it is necessary to erect scaffolding or hang down from elevated walkways to gain access is desirable. Usually cables are supported on cable trays and the cables are supported where they leave the cable tray so as to limit the stress on the gland. The junction box should be positioned so that the cables can leave the tray in a smooth curve and sharp bends avoided.
Use a large [twice as big as you first thought] stainless steel IP65 enclosure with a sufficiency of ‘Exe’ terminals and glands, position it carefully and all will be well.