Contact Resistance Test

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The contact resistance test (commonly known as the Ductor test) measures the resistance of electrical connections such as joints, terminations, connectors, etc. These can be connections between any two conductors, for instance busbar sections or cable connections. The test measures the resistance at the micro- or milli-ohm level and is used primarily to verify that electrical connections are made properly, and can detect the following problems:

  • Loose connections
  • Adequate tension on bolted joints
  • Eroded contact surfaces
  • Contaminated or corroded contacts

This is particularly important for contacts that carry large amounts of current (e.g. switchgear busbars) as higher contact resistance leads to higher losses and lower current carrying capacity.

Test Equipment

Contact resistance tests are normally performed using a micro/milli-ohmmeter or low resistance ohmmeter. "Ductor" was the name originally given to the low resistance ohmmeter manufactured by the Megger company, which became an industry standard.

Test Criteria

The criteria for evaluating the contact resistance of electrical connections largely depends on the type of connection (e.g. bolted, soldered, clamped, welded, etc), the metallic contact surface area, the contact pressure, etc. These will differ by equipment and manufacturer and there is no code or standard that mandates minimum contact resistances. As such, manufacturer recommendations need to be consulted. For example, manufacturers sometimes quote a maximum contact resistance of 10 micro-ohms for large bolted busbar joints.

International Standards

  • ASTM B539-02 (R2008), "Standard Test Method for Measuring Resistance of Electrical Connections (Static Contacts)"