In this episode of Control Intelligence, written by contributing editor Rick Rice, editor-in-chief Mike Bacidore discusses industrial enclosures, their design and the protection they offer.
Transcript
When designing a control system, one of the things that we cannot get away from is the need for an industrial enclosure. Often forgotten in a world where hardware evolves at lightning speed, enclosures take a back seat but fill a necessary role in our systems.
The general design of an enclosure really has not changed in the past 40 years or so, but let us take a stroll through the showroom, as it were, and review what is out there and why.
The physical design of an enclosure serves multiple purposes. First, it provides a physical barrier between the user of the equipment and the electrical components used to control it. Second, it provides protection of the components from contaminants in the environment that the machine or process reside. This is the primary purpose of an enclosure.
Generally, enclosures are square- or rectangular-shaped. They may or may not have a back panel to which components are mounted so that the integrity of the outer enclosure is not breached by drilling through the structure. Multiple interior panels may be mounted to make use of the sides, top and bottom in addition to the larger back surface.
The choice to use side panels must include an understanding of the restriction on which components can be mounted on the adjacent surfaces, so as to not have interference.
While the size and layout of a panel is generally designed to mount all of the components that will reside in the enclosure, additional consideration needs to be applied to consider the routing of wires within the enclosure. High-voltage wires generally should not be routed with low-voltage or communications cables to protect against electrical noise from the high-voltage conductors.
Further consideration should be applied to the flow of the circuits. For example, a fuse or breaker will normally have in and out terminals that are on the opposite end of the device.
It would make sense to mount the contactor, relay or drive below the protection device, but that depends on the configuration of the input and output terminals on the controlled device. Relays and contactors generally have the input and output terminals located to the top and bottom, like the protective device, so locating them below the protective device would make sense and reduce the length of wire needed to accomplish this.
Speaking of protective devices, most manufacturers have a group of auxiliary devices that can be used to daisy-chain the wiring path to reduce wiring. To use these shortcuts, the devices must be mounted adjacent to each other as the jumper components are specifically designed based on this assumption. The same is true of some relays and contactors. The same shortcut assemblies are available to reduce the wiring for these devices, as well. While this helps with panel building and footprint, it has another challenge.
Older variable-frequency drives, for example, would have the input terminals at the top of the drive and the output terminals on the bottom. However, newer VFDs often have all of the terminations on the bottom of the device. This means that power and control wiring will be adjacent to each other, and the routing of wires from the protection device may not be as advisable as, perhaps, mounting the protection device immediately adjacent to the drive.
The environment in which the enclosure resides determines some of the fine details of the physical construction of the enclosure, as well as the materials used in the construction. For example, mild steel is a common material used to make a control box. It is rigid and resists a fair bit of abuse due to normal operation activity.
However, if that control system is in an environment where water or water vapor is present, mild steel would likely rust or corrode quickly. Stainless steel would be a better construction material in this case.
What about corrosive environments? Stainless steel would also be a good choice, but powder-coated aluminum would also provide ample protection. While less resistive to physical abuse, polycarbonate would also be a good choice for an environment where corrosion exists. A consideration during the construction phase, however, stainless steel is not easy to work with compared to mild steel, aluminum and polycarbonate.
Tools used in the construction will tend to wear out more quickly when working with stainless, so extra cost considerations are part of the decision process, as well.
While the general construction of an enclosure has remained the same over the years, some recent improvements have tweaked the performance, as it were. For example, enclosures in wet environments are built to NEMA 4 or 4X standards. The primary element here is the use of stainless steel for the cabinet.
One of the main concerns with enclosures in wet environments is that water or other liquid can lay in the gap between the cabinet door and the cabinet itself. There is a gasket there to ostensibly keep moisture and dust out, but it also creates a catch point where those elements will sit for long periods.
Liquid and debris, of course, will mix over time and create a substance that is difficult to clean. It can also work on the gasket over time and break down the base material. Vendors have come up with some ways to deal with this issue.
One method is to change the design of the door to bring the flange of the cabinet out to the inner edges of the door profile. By doing this, the gasket is protected from prolonged exposure to the liquid and dust. If the liquid lays in the crevice between the body of the enclosure and the extended flange, then it is minimal and has an opportunity to move away better.
Another way that helps with liquid removal is to slope the top of the enclosure. While this usually means that the inner panel will not be as big as the back surface of the enclosure, it does improve the ability to shed water off the top. The door itself may be sloped to match the slope of the enclosure to further help with this feature.
Yet another way to improve the removal of water is to extend the top of the enclosure over the top edge of the door. Imagine the overhang on your house to push water away from the front door before dropping off.
A combination of all three methods greatly improves the ability to keep water off the enclosure and improves both the integrity of the seal and the cleanliness of the top of the enclosure.
Gaskets perform the important function of providing a semi-rigid barrier that takes up any gaps between the door and the enclosure. Two common materials from which gaskets are made are silicone and ethylene propylene diene monomer. Silicone is a synthetic elastomer that stands up well to temperature and has low reactivity with chemicals. They come in a variety of durometers, or hardness, and colors. EPDM is also a synthetic polymer that is resistant to sunlight, ozone, abrasion, cutting and tearing, so it is more resilient than silicone. Neither does well with exposure to gasoline or oil products.
Thermoplastic elastomers behave like beefed-up EPDM but are more costly to produce. Fluorosilicone gaskets are an improvement over silicone with the benefit of better resistance to oxidizing chemicals, as well as gas and oil products.
A more recent improvement is the introduction of a dual seal. Both the door and the enclosure have a gasket installed. The door gasket mates up with the frame of the enclosure, when closed, while the enclosure gasket mates up with the inside surface of the door. This double dam, as it were, greatly improves the ability to keep liquid and dust contaminants from getting inside an enclosure.
It is a good idea to institute a periodic inspection of all enclosures after installation to confirm the integrity of the seal and any penetrations of the surface from normal application like conduits and operator buttons, operator control screens and signal lights.
At time of installation, all of these items should match the NEMA rating of the enclosure itself, but time and use may compromise the initial seal and need to be tightened up or replaced. Most manufacturers of electrical enclosures make replacement gasket kits.
When applied according to the manufacturer’s instructions, these are just as good as the original seal. If in doubt, the manufacturer might also offer the ability to order replacement doors with the gasket material applied as it would on a completely new enclosure.
This is far more cost-effective than replacing the entire enclosure with door and avoids the need to pull back the existing connections and punch new holes in an enclosure.
A final word about enclosures: They do not have to be just for electrical components. In packaging machinery, windowed enclosures are often used to contain and protect pneumatic manifolds. NEMA-rated panel-mount couplings can be used to transition the enclosure and provide a solid connection to the ports on the manifold.
By using a windowed enclosure, the status of the valves can be viewed without the need to open the door and expose the components to the environment outside the enclosure.
