There is obviously more to an enclosure than “just a box with a door” to mount common equipment inside. Unfortunately, like power supplies that we discussed earlier this year, enclosures are often an overlooked element of the engineering design that can have unfortunate consequences.
Like power supplies, they and their associated glands/seals protect the electronics from the surrounding environment. In an operating facility, this environment is usually more than just weather, though that in itself can be a challenge. It also includes potentially hazardous or corrosive gases as well as the occasional bath by an overexuberant operator’s flushing operations.
The most widely used enclosures are typically made of metal; either epoxy-coated or perhaps a form of stainless steel, with appropriate gasket materials and closures. Unfortunately, putting a wireless device inside a steel box is the same as putting it in a Faraday cage, and is one of the better ways to prevent signal transmission. Many installations still use metal enclosures because they have proven themselves over time, and of course you can always put the antenna outside the enclosure. Unfortunately, doing so also has some consequences as it introduces a number of additional connections: cable from transmitter to enclosure gland, gland to antenna cable, and antenna cable to antenna. Every connection introduces small losses, and if you are starting with a small signal, that can make a difference. And connections can be improperly made or become loose over time, especially in an operating facility where they may be exposed to vibration.
A logical choice, then, is to use a non-metallic enclosure. Fortunately, there are a number of options on the market that have been used quite successfully for many years in environments not friendly to metal. The three most common options are polycarbonate, polyvinylchloride (PVC), and polymer-fiberglass composites. All of them allow you to install your wireless device inside the enclosure without noticeable impact to the RFI/EMI signals.
Factors to consider when choosing which of the materials to use include: price, effect of temperature on the integrity and impact resistance of the materials, effect of sunlight (especially ultraviolet rays), as well as weight and strength.
In addition to the enclosure materials, choosing the correct seals and closures to secure the cabinet and ensure a proper seal are other important factors impacting the integrity of the system. Many of the same types of latching mechanisms used on metal enclosures can be used with non-metallic systems to prevent unauthorized access with dual latching/lockable lids.
Another item to consider when designing your enclosure, or for that matter any design: Don’t forget that the equipment inside will need to be maintained someday, so be sure to leave sufficient room for hands and tools. You may not be thanked for doing so, but you will be cursed for not remembering the other half of the automation team.
Composite enclosures can be manufactured of multiple layers for such purposes as insulation against the cold here in Canada winters, or conversely the heat in warmer climates. Of course, the insulation itself is not sufficient to maintain conditions inside the cabinet, so accessories such as heaters, coolers, windows to view inside, mounting plates, etc. need to be incorporated into the design to make it suitable for the application and location in which it is to be installed.
Regardless of its rating, if it is not properly maintained, especially items like the seals, or secured by properly tightening all the latches or screws around the perimeter, any enclosure can quickly become “NEMA nothing” or “IP 00.” In fact, the argument could be made that because you falsely believe the enclosure is protecting the equipment inside, the risk is actually greater than it would be otherwise. Therefore, next time you are walking through your facility, take a look around and verify that your enclosures are providing the protection for which they are intended.