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EIS—preventive solution for premature dielectric breakdown

An electrical insulation system (EIS) is composed of a unique combination of materials that have been verified for chemical compatibility when used at certain maximum temperatures. These combinations are arranged to form insulation systems for applications such as motors, transformers, relays and solenoids. Electrical Insulation Systems play an important role in product safety and are now emerging as a global standard.

Typically, engineers select electrical insulation materials (EIMs) based on the relative thermal index (RTI) value required to meet the product target or maximum hot-spot temperature limit. However, when the operating temperature reaches 105˚ C or above, the chemicals of the individual insulation materials will react with each other.

For example, when coil winding hot-spot temperature rises to 105°C or above, the chemicals will react with each other and deteriorate insulation properties—such as dielectric strength, ignition resistance and flame resistance—especially those on the insulation coating of conductor wires. The deterioration of insulation performance will result in premature product failure or even electric/fire hazards.

As the rate of chemical reaction increases with higher temperature, the unexpected decay will present a higher risk of product liability. Such chemical compatibility issues cannot be predicted by models. That’s why Underwriters Laboratories (UL) and the International Electrotechnical Committee (IEC) established testing standards (UL-1446, IEC-60085 and IEEE 117) to verify the compatibility of EIMs at elevated temperatures.

If you do not use an EIS that is pre-approved by UL and/or recognized by IEC, there are a multitude of tests that must be done to gain recognition, such as:

  • Polymeric insulation materials must go through component plastics testing for properties such as flame rating (UL94, UL746), arcing resistance (HAI, CTI), hot wire ignition resistance (HWI) and relative thermal index (RTI).
  • Chemical compatibility test (CCT), also known as a sealed tube test, which lasts for 1 to 3 months.
  • Full thermal aging test, also known as a motorette test, uses a general-purpose model that undergoes temperature, electrical, vibration and humidity cycles for 9 to 12 months.