Water Absorption on Electrical Insulation

The Gund Company fabricates custom electrical insulating components for power systems equipment, including generators, transformers, switchgear, power supplies, electric motors, induction furnaces, electric arc furnaces (EAF), and related equipment.

Insulation material selection can be quite a challenge due to the factors influencing insulation system engineering and insulation component design.

Moisture, especially when combined with aggressive gases and dust, causes atmospheric corrosion. This corrosion can fail circuit breakers, bus bars, relays, integrated circuit boards, and transformers. The greatest danger lurks in conditions where electronic equipment is exposed to relatively high air humidity or extreme variations in temperature, such as day-and-night operation or outdoor installation. Failure of components in such cases is usually caused by changing contact resistances, flashovers, creepage currents, corona, or reduced insulation properties.

The moisture absorption of a material is a critical property that is often overlooked. According to ASTM D570, the American
Standard Test Method for Determining the Water Absorption of Plastics, “the moisture content of plastic is very intimately related to such properties as electrical insulation resistance, dielectric losses, mechanical strength, appearance, and dimensions.” Of course, with insulation materials, the absorption of water from high humidity conditions can result in the original design insulator becoming a conductor.

The chart below shows the water absorption properties for several common rigid insulation materials used in electrical and electronic equipment applications. Note that the values below are the maximum allowable water absorption percentages per the NEMA Standards for laminate insulation materials.

The ASTM D570 test for water absorption most commonly uses a 24-hour immersion test to determine the material’s water absorption property. “The conditioned specimens shall be placed in a container of distilled water maintained at a temperature of 23C +/- 1C and shall rest on the edge and be entirely immersed. At the end of 24 hours, the specimens shall be removed from the water one at a time, all surface water wiped off with a dry cloth, and weighed to the nearest 0.001 gram immediately.” The percentage change in weight is then determined.

According to W. Tillar Shugg, the author of the Handbook of Electrical and Electronic Insulating Materials: “In selecting a material for a specific use, these steps are important:

1. All possible environmental conditions for which the material could be exposed should be determined, including (but not necessarily limited to)
temperature, humidity, chemicals, and radiation.”

The effect of environmental humidity on an insulation material’s performance can be critical. A specific example may help illustrate the point.

The tremendous growth in the electronics industry-driven partly by the proliferation of computers, servers, and network devices, has resulted in great demand for power quality equipment such as power supplies. In large server farms, keeping this equipment cool can be quite a challenge. The higher the air temperature, the more invisible water vapor it can absorb. Even if the amount of water vapor remains the same, the air will become saturated as the temperature drops.

When the air can no longer hold the water vapor it contains, it has reached the “dew point.” Observers will note fine drops of water (condensation) forming on the surfaces. In everyday life, droplets form on window panes, eyeglass lenses, and drinking glasses. Unfortunately, the simple physics of condensation can be very destructive inside electronic enclosures.

Up to a relative humidity of 65 percent, the probability of condensation and corrosion remains low. Above 65 percent, the likelihood of condensation occurring increases significantly. These problems can be easily reduced by keeping the environment inside an enclosure temperature as little as 5C higher than the ambient air temperature.

In electronic equipment applications such as power supplies, the industry moves away from high water absorption materials such as Nomex and Vulcanized Fiber due to their high water absorption rates. Both of these materials and fiber-based insulating papers, in general, tend to have water absorption rates higher than 15%.

Today’s solution is using PolyPro FR™ material with water absorption in the range of 0.01% to eliminate the potential adverse effects of water absorption on insulation materials in this environment.


The Gund Company is a vertically integrated manufacturer and fabricator of engineered material solutions. Since 1951, we have listened to our customers and learned about the demanding operating environments of their industries. We are AS9100D Certified and ITAR Compliant. Our custom fabricated parts are manufactured according to ISO 9001:2015 certified quality systems.

We understand the challenges of material selection and the demanding operating environment of your application. Our Application Engineering Team takes a consultative approach to understand your requirements. By relying on our material specialists, our customers gain valuable insight into improving component designs to increase efficiency and functionality while reducing cost. In addition to helping with material selection, we challenge ourselves to optimize production for material yield or fabrication efficiency. As a lean enterprise, we focus on continuous improvement and finding the most cost-effective, efficient solutions for our customers.

Please contact us today if we can help answer material property questions or provide an application-specific quote. Thank you for the opportunity to earn your business.