The Dielectric Strength of Insulating Materials
By Jon Rennie
Electrical insulators are used throughout transmission and distribution circuits to separate voltage from ground. The materials used in the design and production of electrical insulators have very unique characteristics. These materials prevent internal electric charges from flowing freely in the material which make it nearly impossible to conduct electrical current.
Not all insulating materials are the same though. Some perform the task of isolating electrical current better than others. In fact, the way to understand the capability of material to prevent electrical conductivity is to look at its Dielectric Strength.
Dielectric Strength is simply the maximum electric field that a material can withstand without experiencing failure of its insulating properties. It’s measured in megavolts per meter (MV/m). The higher the Dielectric Strength, the better a material is to prevent electrical conductivity.
So what material has the highest Dielectric Strength? It might surprise you to know that a perfect vacuum is actually the best electrical insulator. A perfect vacuum has the highest dielectric strength, rated at 1×1012 MV/m. A perfect vacuum contains no material to breakdown and is, therefore, the perfect electrical insulator. In reality, a perfect vacuum is nearly impossible to achieve but a high vacuum is also a great insulator, rated at 30 MV/m. High vacuums are used as an insulation method in equipment like vacuum circuit breakers.
So, how do other insulating materials rank? The following is a partial list of the Dielectric Strengths of various materials, including those used in transmission and distribution applications:
| Material Dielectric Strength | |
| Perfect Vacuum | 1×1012 MV/m |
| Mica | 118 MV/m |
| Teflon | 60 MV/m |
| High Vacuum | 30 MV/m |
| Transformer Oil | 24 MV/m |
| HDPE Insulator | 20 MV/m |
| Silicon Insulator | 20 MV/m |
| Glass Insulator | 14 MV/m |
| Nylon | 14 MV/m |
| Rubber | 12 MV/m |
| Porcelain Insulator | 12 MV/m |
| Air | 3 MV/m |
HDPE, Silicon, Glass, and Porcelain are all very close in terms of dielectric strength. Each of these materials are used extensively in transmission and distribution insulator applications.
Dielectric Strength a critical characteristic for materials used to design insulators. Preventing electrical conductivity and maintaining a gap between voltage and ground is an important function of all insulators. Choosing the right material will ensure your insulator performs at the highest level.
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