Outdoors, or in wet indoor environments like wash-down areas, Low Humidity Control Cabinets of electronic systems begin with the design of the enclosures and penetrations, and end with the design and configuration of the components. This article focuses on a number of these best practices.
Assume your enclosure will leak. Unless the application form requires a vented enclosure (e.g., for heat dissipation, battery off-gassing), a sealed enclosure represents the first line of defense against moisture. Unfortunately, even the very best NEMA 4 electrical enclosure is effective until poor installation practices or out-year modifications create poorly sealed penetrations (Fig. 1).
It’s better to assume that penetrations into any enclosure are likely to leak (as shown by Fig. 2). Based upon this assumption, top-mounted conduit penetrations where moisture can collect on horizontal surfaces needs to be avoided. Even if Myers hubs or sealing locknuts are used for code compliance, enclosure penetrations should be made below energized parts, if at all possible.
When it comes to cable penetrations (versus conduit penetrations), directing water out of the electrical enclosure or housing by using drip loops (Fig. 3) is an additional best practice. The next step is to heat-shrink the connector fittings and alternate wrappings of electrical tape and butyl self-adhesive rubber tape to protect against moisture intrusion to the connector.
Maintaining door seals is equally important. Door seals ought to be inspected to make sure panel doors are sealing properly by observing surface wear on the seals. Larger doors with few latches are particularly problematic as flexing in the door may prevent a uniform seal. Lastly, seals should be inspected for pinching, tears and proper adhesion to original mating surfaces.
Assume all conduits contain moisture
The next best practice for Moisture Control Cabinets of electronics assumes that even if the conduit penetrations are perfectly sealed, the conduits are still going to contain moisture. Underground conduit often is left unsealed during construction (allowing moisture accumulation), and conduit runs can potentially have multiple points where moisture can enter. Conduit with Moisture Control Cabinets can transfer water vapor into a sealed enclosure. Typically, when electronics are energized, heat is generated and also the air within the enclosure can hold even more moisture than ambient conditions, meaning water vapor is less of a problem. The issue occurs when the enclosure temperature drops (as a result of equipment being de-energized, cooler nighttime temperatures, cooler climate conditions, etc.) and also the temperature inside xakleh enclosure drops underneath the dew point, leading to condensation.
Expanding polyurethane foam sealant (Fig. 4) provides an excellent approach to sealing around conduit cabling: It’s been found to get preferable over silicone, primarily because caulking guns used in combination with silicone are difficult to insert far enough in to the conduit to attain an effective seal. A growing foam nozzle attachment can be inserted further to the conduit to generate a powerful seal across the cabling.