First let me say that I'm likely showing my bias this month, as most of my career I've worked with intrinsically safe approaches to electrical circuit safety in hazardous areas. There have been, however, instances—particularly in the analyzer world where we had mixed voltages (i.e., 120 VAC analyzers with 4-20 mA outputs)—where explosion-proof was the only viable option.
If you'll recall, explosion protection is based on the fire triangle, with the three sides necessary to support fire or explosion being oxygen (air), fuel and energy. Intrinsic safety works by keeping the energy level below the ignition threshold, while explosion-proof measures work on containment. That is, by keeping the energy of an explosion inside an enclosure from escaping to the outside. Because of the principles on which explosion-proof measures are based, design and especially maintenance are key factors to their success.
I always specify backplanes in any enclosure I design. Not only does this help with being able to mount equipment without worrying about enclosure integrity, it has the added benefit of providing a roughly 1-in. gap between anything being mounted in the enclosure and the side wall. Though a tight fit, it leaves room for most people to get a finger in if they need to access something along the edge. I also prefer a hinged enclosure, more so for larger units and especially so for explosion-proof ones; the doors are quite heavy and the hinges help keep things aligned.
This leads to the next important consideration: space, not only for the equipment and connections but also for maintenance. Space considerations include not just room for components and cables themselves, but for fingers and tools as well as heat dissipation. Heat rises, so the top of the enclosure will normally be warmer than the bottom—keep that in mind when placing components. Also, will the enclosure be in the sun or shade? Remember that solar heating can quickly add several degrees to the inside temperature and, unlike with one’s car, we can't open a window to let the heat out.
Components inside will inevitably fail at some point, and that means they'll need to be replaced. Further, other elements in the enclosure will require maintenance or calibration at regular intervals (fingers and tools again).
Maintenance can introduce risk
Maintenance activities also mean opening the enclosure, and in the case of an explosion-proof one, such actions introduce risk. This is not only because the area may need to be declassifed while it's opened, but there are also many ways in which the integrity on which the containment principle depends can be compromised.
The flanges on an explosion-proof enclosure not only act as radiators to disperse heat, but also as a sort of restriction orifice to dissipate energy. Their integrity is absolutely critical. The flanged surface needs to be kept clean and free of scratches, especially ones that extend across the full surface. The tight seal of the flange contains the energy.
Keeping the seal tight also requires that all the enclosure bolts are properly tightened. I've seen instances where only a few bolts are properly tightened, while others are only finger tight and, in some cases, not even threaded flush—in part because there are so many of them. Such enclosures are no longer explosion-proof.
One suggestion I've seen for putting on enclosure covers of any type, but especially explosion-proof ones, is to use the same concept as you would when bolting on a car tire: tighten and re-tighten bolts in sequence, going back and forth across the enclosure diagonally. After putting the bolts on finger tight, start tightening in one corner then criss-cross the enclosure at least twice to be sure you have them all completely and evenly tightened. Spending a little extra time getting it right now could prevent a catastrophe later.
For my part, I say give me my “plain” NEMA 4X box with intrinsically safe components, where I only have to verify the design, keep out the bugs, prevent condensation, and...
Hey, wait a minute! I guess working with enclosures of any kind is not so simple after all.
