Arc resistance of polymers

Upon subjected to electric arc, many inorganic materials become incandescent thus, resulting in electric current conductivity. When cooled, they become insulators again. Another factor includes carbonization of the surface once there is enough carbon to conduct the electric current across the material. Additionaly, factors such as degree of ionization, length and cross-section of the arc define arc resistance of the given material. 

Different plastics have different arc resistance performance. Thermoset phenolics tend to carbonize easily and therefore have relatively poor arc resistance. While, on the other hand, alkyds, melamine and fluorocarbons are excellent arc resistance materials 

The arc resistance of thermoplastics can be improved substantially by the addition of reinforcements such as fiber glass, minerals and other inorganic fillers. 

 

The most generally used standard tests to calculate arc resistance is ASTM D495 (of course there exists several other methods as well, but they are not discussed here). ASTM D495 test method evaluates dry, uncontaminated samples, Test Methods ASTM D 2132, D 2303, and D 3638 involve wet, contaminated specimens.
 

Schematic High Voltage, Low Current Dry Arc Resistance Test 

ASTM D495-14 - Standard Test Method for High-Voltage, Low-Current, Dry Arc Resistance of Solid Electrical Insulation

This test method covers, in a preliminary fashion, the differentiation among similar materials w.r.t their resistance to the action of a high-voltage, low current arc close to the surface of insulation. This arc tends to form a conductive path in that location or may cause the material to become conducting due to the localized thermal and chemical decomposition and erosion. 

The arc resistance of a material is determined by this method by measuring the total elapsed time of operation of the test until failure occurs. 

There are four general types of failure which have been observed:
 

• Many inorganic dielectrics become incandescent, at which point they are capable of conducting the current. However, when cooled, they return to their earlier insulating condition
• Some organic compounds burst into flames without formation of a visible conducting path in the substance
• Some organic compounds fail by tracking (i.e. a thin wiry line is formed between the electrodes)
• Some compounds experience carbonization of the surface until sufficient carbon is present to carry the current

Generally, this method is not used in product design or material specifications. However, the results obtained in the test are values only used to distinguish materials of nearly identical composition such as identification, quality control and development.

 

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