Skip to content
John Livingston's

The Nuclear Electrical Engineer

An educational resource for electrical engineers in the nuclear power industry

Here you can find design tips on technical skills like cable and breaker sizing, short-circuit evaluations and more, PLUS deep-dive analysis of nuclear-specific topics like design basis, Tech Specs, UFSARs, and safety-related SSCs — built from primary source documents.

The Engineer's Life · Feature

Tan Delta Cable Testing

John Livingston
August 29, 2026 · 2 min read

From Tip of the Week, 7 March 2020. Plants and utilities need a cost-effective, fast way to identify cables that need to be replaced.

That the test should be cost-effective and fast is made obvious by the fact that plants have thousands of cables installed.

Determining which cables need to be replaced is crucial to cable aging management programs, which feed into license renewal. Cables that have been buried underground for a long time can become damaged from water intrusion, especially at connection points.

The water can seep into the insulation and cause “water treeing,” which is a microscopic cracking that results from a combination of water and high-voltage electric field stress. As these “trees” grow, the insulation effectiveness declines.

Wikipedia has a good article on electrical treeing.

How the test works

Tan delta cable testing is a method of detecting insulation compromised by electrical treeing. The name, “Tan Delta,” arises from the parameter being measured: the difference in angles between the resistive current and capacitive current.

Under ideal conditions, zero resistive current would flow. The phase angle would be 90 degrees. But in flawed insulation, a resistive current does flow. The ratio of these currents produces the tan delta — opposite over adjacent. This summary shows a vector diagram and explains the process.

Interpreting the results

There is an IEEE standard, IEEE 400.2, which provides guidance for interpreting tan delta test results.

Another good summary is here.

It’s good to be familiar with this test, what it measures, how it works, and why it’s used. You should also know whether it is destructive — it doesn’t have to be — and at what point it becomes so.