Moisture is a key contributor to a variety of electronic-system failure modes. One particularly challenging aspect of such failures is their intermittent nature. As a result, troubleshooting can result in the removal of parts that subsequently retest as acceptable. This can generate great numbers of suspect parts and significantly increase the consumption rate of your spares.
Outdoors, or in wet indoor environments like wash-down areas, effective moisture protection 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 several of these best practices.
Assume your enclosure will leak
Unless the application calls for 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 best NEMA 4 electrical enclosure works great until poor installation practices or out-year modifications create poorly sealed penetrations (Fig. 1).
Fig. 1. Water accumulation in a poorly sealed electrical enclosure
It’s best to assume that penetrations into any enclosure are going to leak (as shown by Fig. 2). Based on this assumption, top-mounted conduit penetrations where moisture can collect on horizontal surfaces should be avoided. Even if Myers hubs or sealing locknuts are being used for code compliance, enclosure penetrations should be made below energized parts, if at all possible.
Fig. 2. Poorly sealed conduit penetrations
In terms of cable penetrations (versus conduit penetrations), directing water away from the electrical enclosure or housing through the use of drip loops (Fig. 3) is another 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 into the connector.
Fig. 3. Cable drip loop
Maintaining door seals is equally important. Door seals should be inspected to ensure panel doors are sealing properly by observing surface wear on the seals. Larger doors with few latches are particularly problematic as flexing of 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 next best practice for moisture protection 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 can transfer water vapor into a sealed enclosure. Typically, when electronics are energized, heat is generated and the air inside the enclosure can hold even more moisture than ambient conditions, meaning water vapor is less of a problem. The problem occurs when the enclosure temperature drops (due to the equipment being de-energized, cooler nighttime temperatures, cooler weather conditions, etc.) and the temperature inside the enclosure drops below the dew point, resulting in condensation.
Expanding polyurethane foam sealant (Fig. 4) provides an excellent method of sealing around conduit cabling: It’s been found to be superior to silicone, primarily because caulking guns used with silicone are difficult to insert far enough into the conduit to achieve an effective seal. An expanding foam nozzle attachment can be inserted further into the conduit to produce an effective seal around the cabling.
Fig. 4. Expanding foam sealing of conduit
Assume moisture will infiltrate an enclosure
Despite the best efforts to seal doors, conduits and cabling, moist air will still infiltrate the electrical enclosure. At this point, one of several decisions must be made.
Option 1: Do nothing…
One option is to assume that the electronics can handle potential moisture, and do nothing. This decision is not as short-sighted as it initially sounds. If correct, no additional expenditures are required. If incorrect, data on the nature of moisture-induced problems will inform better decisions going forward. If the electronics cannot handle the actual moisture that infiltrates the enclosure, the specific problems will self-declare, which will assist with the decision to remove the moisture or to moisture-harden the electronics.
Fig. 5. Temperature/humidity data logger
Option 2: Remove moisture. . .
A useful first step to remove enclosure moisture is to characterize the enclosure environment using a temperature/humidity data logger (Fig. 5). These inexpensive, battery-powered devices (~$200) record relative humidity and temperature. They also indicate the dew-point conditions inside the enclosure (Fig. 6). Maintaining enclosure temperatures above dew-point temperatures is a requirement for condensation prevention.
Fig. 6. Enclosure temperature/humidity/dew point
Pursuing this option can be accomplished in a number of ways, ranging from desiccant to thermoelectric dehumidifiers—the challenge is to select an option that is inexpensive to both implement and maintain. The water-absorption capability of desiccant is dependent on a variety of factors (e.g., desiccant type, humidity, temperature). For example, silica gel can absorb up to 40% of its weight in water. A 4’ x 6’ x 2’ electrical enclosure in a hot/humid environment would saturate 125 g of desiccant in about two air exchanges. Therefore, the resulting frequency of required desiccant change-outs (which affects maintenance costs) is largely driven by how well the enclosures are sealed. Unfortunately, when it comes to desiccant regimes, each act of opening an enclosure to inspect the desiccant serves as an air exchange.
Dehumidifiers are relatively inexpensive, although finding convenient available power inside an enclosure may be problematic. The positive feature is that dehumidifiers eliminate the manual intervention associated with a desiccant regime. The negative feature of dehumidifiers is that they introduce one more piece of equipment that can ultimately fail.
Another strategy is to minimize the potential for condensation through internal heaters (or light bulbs) to keep the internal enclosure temperature well above dew-point temperatures. The downside is that higher temperatures may be detrimental to some heat-sensitive electronic components, and the higher temperature actually allows the air to hold more moisture. Venting and fans can help avoid condensation in some situations—although the humidity still exists. One interesting product the makers of GORE-TEX® have produced involves screw-in vents that enable enclosures to breathe, while providing a barrier to moisture and contaminants. The theory behind this type of venting is that it reduces the stress on door seals when there are pressure differentials between the enclosure and the environment. By equalizing pressure, the possibility of moist air at higher pressure defeating your door seals is lessened.
Fig. 7. Use of potted electrical components for improved moisture protection
Option 3: Moisture-harden the electronics . . .
Moisture-hardening of electronics includes a variety of techniques. In terms of connectors, using waterproof connectors or hardening existing connectors and splices with heat-shrink tubing can be useful to minimize water intrusion and corrosion. Avoiding horizontal orientation of components like printed circuit boards inside the enclosure can minimize surfaces where condensation may collect for extended periods of time. Conformal coatings for lower-voltage printed circuit boards and the use of potting (see Fig. 7) of higher-voltage components greatly increase the moisture resistance of components. Potting costs vary according to the size of order, material selection and part geometry, but representative costs for very small orders (less than 10) typically fall in the range of $18 to $45 per part. An additional advantage of potting is the added protection from shock and vibration.
Moisture protection of electronics is best approached by pursuing practices that maximize moisture barriers during equipment installation, coupled with being prepared to mitigate failure through any one moisture-protection measure during operations. This strategy, along with tracking equipment-maintenance performance to understand how well moisture-protection measures are working, can lead to long-term minimization of electronics moisture-induced problems. MT
*Pacific Northwest National Laboratory is operated for the United States Department of Energy by Battelle, under contract DE-AC05-76RLO 1830. PNNL-SA94933.