For the arc resistance, two different situations can occur: high Voltage, low current arcing and Low voltage, high current arcing. UL 746 has defined a test for each situation: High Voltage Tracking Resistance (HVTR), High Amperage Arc Ignition (HAI) and the high Voltage, dry arc resistance.


High Voltage Tracking Resistance (HVTR)
This test determines how well a material resists forming a conductive path on the surface when it is subjected to repeated high voltage arcing. To this effect, two electrodes charged with 5200 V are brought on the surface of the part. As soon as an arc forms, the electrodes are separated. This process is repeated for two minutes. At the end of the test, the length of the conductive path is measured and the tracking rate (in mm/min) is determined. UL recognizes the following classes:



High Amperage Arc Ignition (HAI)
For this test, two electrodes are placed on the surface and charged with 240 V (60 Hz): they are moved closer together until an arc forms on the surface. When this happens, the electrodes are separated and the process is repeated. This happens at a rate of 40 arcs per second. This test is carried out at a far lower voltage, but far higher amps than the HVTR test. The HAI is defined as the number of arc needed to ignite a sample. Polyamide 66 has excellent resistance against this type of arcing. With a resistance of over 120 arcs, it falls into the highest performance class defined by UL.



High Voltage Dry Arc Resistance
This test determines how well a material resists forming a conductive path on the surface when it is subjected to a continuous arc. This test is described in ASTM D495. Two electrodes are applied to the surface at a specified distance. According to a defined cycle, the electrodes are charged with a Voltage of 15000 V. The time to failure (either through ignition of the part or by a current that starts to run between the electrodes) is noted. Polyamide 66 does not have a very good resistance to this type of arcing; failure times are between 60 and 120 seconds.