Acceptance test reports can tell you the condition of your cables when they were installed, but they tell you nothing about the present condition. Startup alone can radically change the data.

To ensure reliability, you need to:

  1. Perform insulation resistance testing at the correct intervals.
  2. Trend the results over time.

Several factors determine what the correct intervals are. Refer to the relevant standards, such as IEEE Standard 141: Recommended Practice for Electric Power Distribution for Industrial Plants (the Red Book). Another good reference is the ANSI/NETA Standard for Maintenance Testing Specifications.

You probably already know that conducting these tests requires a specialized test set. However, you may not know that such a test set isn't directly measuring resistance; it's measuring absorption current, capacitive current, and leakage current. Using all three values, the test set determines the total current flowing through and on the insulation. We'll discuss these three quantities further in our next issue.