As mentioned previously, the goal was to develop a film capacitor with increased capacitance stability under high humidity, elevated temperatures, and AC loading, with small dimensions and suitable to withstand the requirements of X2 capacitors. In order to avoid degradation of the electrode due to high AC voltage, EPCOS designed the new series with an internal series connection that reduces the AC voltage per connection by one half. This effect is of high importance because the speed of degradation is significantly reduced. On the other hand, this design, frequently used for higher AC voltages, is normally larger than with a standard design. In order to compensate for this and achieve the goal of optimizing the volume, the heavy duty is based on polyester (PET). Polyester is a material with higher dielectric constant than polypropylene (PP) which is used in the majority of X2 class EMI capacitors used today. Polyester also has the advantage of having a better resistance to high temperatures so the film capacitors are able to withstand higher operating temperature up to 125°C. The other key requirement is the résistance to humidity, which is achieved with use of specially designed materials for box, resin and metal spray. A critical improvement is using plastic film that is not metallized with zinc. The zinc, commonly used in X2 and AC capacitors, is very sensitive to humidity and corrodes very easily when exposed to harsh conditions.