Every component of a control system, from sensors to cables to controllers, needs to be protected from its environment. The most common way of providing this protection is by placing the equipment in some form of enclosure.
Sensors and associated electronics are typically packaged in a metal transmitter housing. Cables have insulation, of course, and in many cases, the additional protection of conduit or a cable tray. Controllers and other larger electronics or combinations of equipment are normally mounted in some form of cabinet.
The most common form of enclosure is metal, typically painted steel with stainless steel closing mechanisms and a suitable gasket or sealing assembly to provide the necessary level of protection from ambient elements, and the associated required NEMA or IP rating.
However, once we make an enclosure moisture-resistant, that means we're not only keeping the moisture out, but also keeping moisture in. There's always going to be moisture inside the enclosure because it contains air, and when the temperature drops this can lead to condensation. Water and electricity do not mix well, and water also contributes to accelerated corrosion. It's therefore always a good idea to include a breather/drain as part of the enclosure design, especially for explosion-proof enclosures placed outside.
Breather/drains allow the enclosure to equilibrate with ambient conditions, and as a result, prevent condensation when installations are subjected to fluctuations in temperature, while also effectively draining any water in the enclosure. Because water collects in low spots, these units should be placed at the lowest point of an enclosure. Once installed, the breather/drains should be maintained because, if spiders decide to spin a web or nest in this nice cozy location, the unit will become plugged and cease to work.
Put unions between the enclosure and cable seals, so if it's necessary to remove the cable from the enclosure (perhaps because you forgot the breather drain and the enclosure corroded), you won't have to replace the entire cable. If there's no union, it will be necessary to break the seal, thus damaging the integrity of the cable.
Also, remember that real people have to work in these enclosures, often wearing gloves, which in addition to making their fingers “bigger” also results in some loss of dexterity. Therefore, don't forget to leave sufficient working room around the installed equipment, not just for the workers but for such things as bend radius of cables, cable tags, etc. I've often seen a nice, small, 6 x 6-in. enclosure that looks beautiful on paper get tossed by the field crew because they simply couldn't terminate to what was inside it.
For the price of a slightly larger enclosure, the amount of time saved in the field will more than pay for the incremental cost of the box. My personal rule of thumb is a minimum of 2 in. on either side of any equipment (terminal strip and cable management duct are two pieces of equipment), which means 4 in. and preferably 6 in. between two terminal strips without ducts.
If you're planning to mount an access point or other wireless device inside a metal enclosure, remember that a Faraday cage is made of metal, so your signal attenuation will be atrocious. Consider using one of the many fiberglass or polycarbonate enclosures on the market instead. These also have the option of insulated walls if required.
Should you need to use a metal enclosure for your wireless device, it will likely be necessary to place the antenna outside. An external antenna normally means a number of connections to transition across the enclosure boundary, so consider the signal attenuation across each connection as part of your design.
Though many practitioners think "it's only an enclosure,” it's critical that it be done right because your work will be in use for a long time. A field junction box might be protecting something as simple as terminal strips or an access point, but the infrastructure equipment inside is expected to last life of plant, and is very difficult as well costly to replace.