Dielectric Strength Test: How is it Determined for Insulating Materials?

The EN 60243-1 and ASTM D149 standards outline the procedures for testing the dielectric strength of solid insulating materials under alternating voltage at industrial frequencies. Dielectric strength represents the ratio between the voltage capable of causing an electrical fault and the thickness of the material at the point where the fault occurs. It is expressed in kilovolts per millimetre (kV/mm).

Electrical fault: what it is and what the breakdown voltage indicates
Electrical failure indicates a significant loss of the insulating properties of the material following electrical stress, which manifests as perforations, carbonisation, or decomposition of the material due to the passage of electrical current. The voltage at which the material breaks down under the prescribed test conditions is defined as the breakdown voltage. The higher the dielectric strength, the better the insulating properties of the material.


Dielectric strength test: what factors influence the outcome?
The test results may be influenced by several factors, including:

• Thickness and homogeneity of samples
• Temperature, humidity and presence of gaseous inclusions
  
   

Dielectric strength tests: how do they take place?
The test involves placing the sample between two metal electrodes, the geometry of which depends on the specimen. An increasing voltage is then applied to the electrodes until the sample breaks. The application of tension can be done continuously or in steps, as specified by the standard. Next, the thickness of the sample at the breaking point is measured to calculate the dielectric strength value.


Testable materials according to EN 60243-1 and ASTM D149
The Istituto Giordano Low Voltage Laboratory can perform the dielectric strength test according to EN 60243-1 and ASTM D149 on the following types of insulating materials:

• Cast materials
• Printed materials
• Thermosets
• Thermoplastics
• Elastomers
• Moulded solid pieces
• Paints (EN 60464-2)
• Filling compounds

The dielectric strength test is performed as follows:

Maximum applicable voltage	50 kV @50 Hz
Maximum sample size	125 x 125 mm plates (or discs Ø 125 mm) and preferred thickness 1 mm
Standard environmental conditions	23°C and 50 % RH
Special environmental conditions on request	- 40 °C to 300 °C (when tested in a silicone oil bath)