The Material Science Behind Raptor Covers (and What To Look For)

The carbonization of the material is caused by the combination of leakage current, contamination and water vapor. The leakage current steams off the water vapor, creating a dry band that produces a high temperature arc, carbonizing the cover material. In tracking, this conductive track grows out across the material, and the effectiveness of the cover can quickly become compromised. With erosion, the arc remains rooted in one spot and slowly burns a hole through the material. In either case flashovers can occur. At TE, we custom blend our polymers with additives that create a virtually non-tracking material that is used in all our raptor covers. TE uses an Inclined Plane Variable Step Voltage Test (ASTM D-2303), as it gives good correlation to product performance.

Sitting on top of a pole, exposed to the elements, a raptor cover needs to take the abuse for many years, as the expense of putting up a replacement cover is significant. Material degradation due to UV, snow, salt, and chemicals can happen rapidly if the polymer formulation isn’t right. Loss of mechanical strength and electrical performance could happen in just a few short years, causing the covers to fall off or allow such limited protection that flashovers occur through the covers. UV tests such as ASTM G-90 or Xenon lamp (IEC 1109, 5000 hrs.) are excellent tests to use when comparing products. Tests like Tensile Strength and Ultimate Elongation (ASTM D-638), Chemical Resistance (ASTM D-543) and Accelerated Aging (ISO 188) can also give one a good idea if the product will last. TE goes the extra mile by cross-linking the material, providing increased strength and toughness, superior chemical and environmental resistance, and more stable electrical performance.

While we want the product to last mechanically, we must also look at the electrical requirements of the application and how they are tested. Some manufacturers use a dry withstand test when rating the cover’s capability rather than a wet withstand test. Since much of the rating depends on atmospheric conditions, a dry test will show a higher voltage rating for the same air gap than a wet one. But unless the product is restricted to a dry climate like Arizona, the wet test would be the one to give the best assessment of what the product can actually handle. At TE, we typically look at a 1 minute AC wet withstand test for our covers. When you are thinking in terms of protecting against momentary animal contacts, one minute is a very long time. Look for ASTM- D257, D-150 and D-149 for comparing electrical characteristics.

How well the material can take temperature swings over its lifetime is a definite consideration when the manufacturer is making covers that could be used anywhere in the world. Whether you are installing in the cold, dry winters of Canada or in a hot and humid country like El Salvador, the material must be designed from the start to take the beating. There are many thermal stabilizers that can increase the overall robustness of the material and extend its life. Certainly the cross-linking done at TE takes this to a higher level, especially on the high temperature side where the material can take severe hits without melting or igniting. Tests like IEEE 1-1969, IEC 216, and ISO 188 would be good ones to review when comparing materials for thermal endurance.