Electrical and electronic equipment as well as components typically are housed in an electrical enclosure designed to provide protection from the external environment. Electronic components are sensitive to temperature changes. At high temperatures, drive performance is de-rated, IC-based devices are adversely affected by erratic output/voltage migration, and the properties of silicone materials change. In wiring insulations, elasticity and strength decrease, ductility temporarily increases and atomic mobility increases.
However, the adverse is no better for electronics. At low temperatures, cooling below the dewpoint leads to condensation that can promote corrosion and battery failure. Condensation also causes IC-based devices to behave strangely.
Therefore, the enclosure space needs to be kept at an optimal temperature point for ideal performance of the electronic components.
Source of Heat Load. Several types of devices are housed in the enclosure of an automation control system: variable-frequency drive (VFD), servo drive, programmable logic controller (PLC), starter kit, power supply, inverter, relays, terminal blocks, indicator lights and, in many cases, a transformer. Electronic components generate heat that has to be removed for maximum component life. As information processing becomes more powerful, the heat generated from electronics continues to increase. All inefficiencies of the devices contribute toward heat generation.
Maintaining Enclosed Equipment. As we understand how temperature extremes can prove dangerous for the equipment, we can begin to find solutions to maintain optimal temperature to run devices and extend service life. The setpoint for electronics can be higher as compared to the average air temperature setpoint inside a house. We should consider choosing a setpoint that is suitable for cooling electronic products instead of what we typically think of as comfortable for the human body.
Moisture in the air by itself is not a problem for electronic equipment or components — until the moisture condenses because of cold surfaces. If the enclosure is being cooled by an air conditioner, most of the moisture is condensed by the evaporator coil and removed through a drain system or active condensate management. However, to avoid the condensation in unwanted locations inside the cabinet, it is necessary to understand the possible sources of moisture and to mitigate those beforehand to reduce the extent of the problem.
Corrosion and short circuiting are two major potentially harmful effects associated with condensed water inside an electronic cabinet. Corrosion causes increased electrical resistance, which in turn generates additional heat and contributes to decreased and inconsistent component performance. In addition, corrosion can lead to rusting of critical electrical components, increasing the risk of circuits shorting out as well as dangerous occurrences of arcing and sparking. Needless to say, any failure will have financial impact as well. To ensure optimal life expectancy of components, users should take several precautions to help prevent these harmful conditions.
Moisture Sources and Control
Moisture can enter an enclosure from numerous sources in many environments and applications. For example, in indoor washdown applications, it is possible for high pressure spray with soap lubricants to penetrate components and gaskets. Plus, in cases where conduit or pipe fittings are not sealed properly, condensation may form in the pipe or conduit and drain directly into the enclosure.
In wet or humid applications and environments, moisture enters an enclosure when the enclosure’s door is opened for service or maintenance purposes. Because internal components generate heat within the enclosure, the warmed air inside can hold even more moisture. When the enclosure surfaces cool to the dewpoint as a result of shutdown, lower evening temperatures or lower outside air temperatures caused by a cool rain and other circumstances, condensation forms.
Large temperature variations between the inside and outside of the enclosure can also result in pressure differences that may create a vacuum and draw water through the fittings or component and gasket seals.
It is evident that all the sources of moisture are external.If moisture is prevented from seeping inside the cabinet, the internal components will be in dry air. A cabinet sealed properly using a gasket can help stop the leakage of outside moist air. Also, making sure the enclosure door shuts fully and properly after every use, and less frequent opening and closing of the enclosure door will prove favorable in preventing the condensation issue. Remember that if there is a continuous drain from the cabinet, it clearly signifies that there is a potential leak of outside, moist air into the enclosure space. The infiltrating moisture is being constantly condensed inside the enclosure.
Preventing Condensation with Optimal Temperature Setpoint
It is necessary to choose the correct setpoint to avoid condensation issues. Condensation occurs when moist air is cooled or comes into contact with a cool surface that is at or below its saturation point, also known as its dewpoint. At this temperature, air can no longer hold all the moisture, and water vapor condenses into droplets, which can contact critical surfaces. The higher the moisture content in the air, the higher the dewpoint, which can result in condensation if the surrounding surface is colder than the dewpoint. So, to help prevent condensation from occurring, it is important to control the moisture amount in the cabinet air and to maintain optimal temperature setting of the cabinet. If the temperature is below the dewpoint of external or internal air temperature, the condensation will occur at the outside or internal side of the enclosure.
Recommendation for Temperature Setpoint. For NEMA 12 cabinets, the maximum permissible temperature before the wiring insulation performance starts de-rating for a Class A wire is 104°F (40°C). Most enclosure test standards are rated at 95°F (35°C) for the inside enclosure temperature, which makes it safe to assume that the electronic components would work fine at or below this maintained temperature. However, a controller might have a hysteresis, which means that if set to a particular setpoint, the unit might actually be operating at a different temperature than the setpoint. The temperature within the cabinet might also vary at different corners.
Therefore, to take care of the variation of the temperature inside the cabinet and the hysteresis of the controller, and still keep the temperature below the permissible limits for optimal performance, the setpoint of 80°F (26°C) is recommended. As an example, if the hysteresis of the controller is 10°F (5.5°C) and the setpoint is 80°F (26°C), the cabinet temperature can elevate to 90°F (32°C), which is still close to optimal operating temperature for electronics. The temperature setpoint should cover any temperature variations across the cabinet to keep it below 104°F (40°C).