ASTM A976 Insulating Coatings for Electrical Steels by Composition, Relative Insulating Ability

Scope:

ASTM A976 categorization divides insulating coatings into categories based on their chemical composition, insulating performance, and typical applications. These categories explain the coatings’ general physical and chemical properties, which are important in estimating their insulating ability and suitability for diverse applications.

Procedure:

In the ASTM A976 procedure, ten fixed-area metallic contacts are put to one of the test specimen’s surfaces, and two drills make electrical contact with the base metal. A measurement of the average electrical current flowing between the contacts and the base metal under a particular applied voltage indicated the effectiveness of the coating (surface insulation). This current value is known as the “Franklin Current,” and it is used as a direct indicator of the insulation’s quality. A Franklin Current of zero, for example, corresponds to a perfect insulator.

Video 01: Electrical Insulator Materials

Specimen size:

This ASTM A976 test method is especially useful for electrical steels with insulating coatings that have surface insulation resistivities greater than 300 Ω·cm2.

Data:

The insulating layer’s apparent surface insulation resistance value is determined as follows:

Ri = 6.45 {(1 / I) – 1} in Ω·cm²/lamination

Where:

Ri = surface insulation resistivity of test sample in Ω·cm² /lamination

I = ammeter reading in A (also known as Franklin Current)

Conclusion:

Insulating coatings are divided into three categories based on their overall composition, relative insulating capabilities, and utility. The goal of this classification is to establish  terminologies for different types of coatings. The purpose of this classification is not to specify coating requirements.