Outdoors, or in wet indoor environments like wash-down areas, Dry Storage Cabinets of electronic systems start out with the design of the enclosures and penetrations, and end with the design and configuration of the components. This article targets a number of these best practices.
Assume your enclosure will leak. Unless the application form calls for a vented enclosure (e.g., for heat dissipation, battery off-gassing), a sealed enclosure represents the initial line of defense against moisture. Unfortunately, even the very best NEMA 4 electrical enclosure works great until poor installation practices or out-year modifications create poorly sealed penetrations (Fig. 1).
It’s better to believe that penetrations into any enclosure are going to leak (as shown by Fig. 2). According to this assumption, top-mounted conduit penetrations where moisture can collect on horizontal surfaces should be avoided. Even if Myers hubs or sealing locknuts are used for code compliance, enclosure penetrations needs to be made below energized parts, whenever possible.
In terms of cable penetrations (versus conduit penetrations), directing water out of the electrical enclosure or housing through the use of drip loops (Fig. 3) is an additional best practice. The next thing is to heat-shrink the connector fittings and alternate wrappings of electrical tape and butyl self-adhesive rubber tape to safeguard against moisture intrusion into the connector.
Maintaining door seals is incredibly important. Door seals needs to be inspected to make certain 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. And finally, seals should be inspected for pinching, tears and proper adhesion to original mating surfaces.
Assume all conduits contain moisture
The following best practice for Moisture Control Cabinets of electronics assumes that even when the conduit penetrations are perfectly sealed, the conduits continue to be likely 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 Dehumidifying Dry Cabinets can transfer water vapor into a sealed enclosure. Typically, when electronics are energized, heat is generated and the air within the enclosure can hold even more moisture than ambient conditions, meaning water vapor is a smaller problem. The situation takes place when the enclosure temperature drops (as a result of equipment being de-energized, cooler nighttime temperatures, cooler weather conditions, etc.) and also the temperature inside xakleh enclosure drops below the dew point, leading to condensation.
Expanding polyurethane foam sealant (Fig. 4) provides an excellent method of sealing around conduit cabling: It’s been found to be better than silicone, primarily because caulking guns used with silicone are hard to insert far enough into the conduit to attain a powerful seal. An expanding foam nozzle attachment can be inserted further in to the conduit to create an effective seal around the cabling.