Cable Tan Delta Test

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The person typing “cable tan delta test” wants to know what the test measures, how it is run on medium and high voltage cable, and how to interpret a number that comes back. A tan delta test measures the dielectric loss in a cable’s insulation, expressed as the tangent of the loss angle, which is also called the dissipation factor. Insulation that behaves close to a clean capacitor produces a small loss value. Rising loss usually points to contamination, moisture, or degradation somewhere in the cable system, though the number alone does not name the cause.

Quick answer

• What it measures: the ratio of the resistive current to the capacitive current in the insulation, which is the tangent of the loss angle (tan delta).
• Where it is used: on medium voltage and high voltage power cable, usually while the cable is disconnected from the grid.
• Common frequencies: 60 Hz, or very low frequency (VLF) in the 0.01 to 1 Hz range as NEETRAC describes it.
• How to read it: compare the result with the same cable’s earlier readings, with adjacent phases, and with reference values for the same insulation type.

What “tan delta” means

The name comes from the phasor diagram of a cable insulation system. A perfect capacitor would draw a current that leads the voltage by exactly 90 degrees. Real insulation draws a little current in phase with the voltage, and that part is the resistive loss. The Wikipedia entry on dissipation factor describes the parameter this way: a capacitor’s dissipation factor “is equal to the tangent of the angle between the capacitor’s impedance vector and the negative reactive axis,” and this gives rise to the loss tangent tan δ.^[1] The same article notes that the dissipation factor is the reciprocal of the quality factor, so a large loss angle means a less efficient, more lossy dielectric.^[1]

The NEETRAC Cable Diagnostic Focused Initiative chapter on dissipation factor puts the cable version of the idea in practical terms. Tan delta measurements “determine the amount of real power dissipation in a dielectric material,” and the measured value is compared with a known reference value for that dielectric to establish how the tested system is doing.^[2] The same chapter defines the measured value as the ratio of the resistive and capacitive currents at a given frequency and voltage.^[2]

How the test is run on a cable

A cable under test is isolated from the power system and energized from a separate supply. NEETRAC describes the cable as “disconnected from the grid and energized from a separate power supply with a fixed ac frequency (e.g. 60 Hz or Very Low Frequency (VLF) ac),” with the test voltage typically between 0.5 and 2 times the system phase-to-ground voltage.^[2] The same source adds that each tan delta system connects differently, so the equipment literature and the vendor’s instructions govern the exact hookup.^[2]

Frequency matters for how the result looks. NEETRAC’s comparison table states that tan delta is less sensitive at 60 Hz than at lower frequencies, because the capacitive current is larger at 60 Hz. At VLF in the 0.01 to 1 Hz range the capacitive current is smaller, so the resistive part of the loss stands out more.^[2] That is why many field programs use VLF equipment. The Wikipedia article on VLF cable testing explains that VLF systems can be built small and light enough for field work, and that the test is typically run over a range of voltages from 0.5 U0 to 2 U0 depending on the standard being followed.^[3]

What the numbers can tell you

A single tan delta value is hard to use alone. The NEETRAC chapter lists several reference points that make the result meaningful: values measured on adjacent phases, values measured on cables of the same design and vintage in the same location, values from when the cable was new, and how the value changes over time.^[2] The chapter also says the value is more useful when the specific cable and its accessories are known, because splices and terminations contribute to the reading.^[2]

Three patterns come up in the source material:

• Trend over time. A value that climbs from one test to the next is more concerning than one high reading taken once.
• Water treeing. NEETRAC states that the tan delta value “can be an indication of the overall degree of water treeing in XLPE cable.”^[2]
• Change with cable length. NEETRAC notes that measured values that change as a function of test system length can point to problems such as corroded neutrals.^[2]

Where the test has limits

The main limit is that a fixed pass or fail line is hard to set. NEETRAC explains that the values “depend not only on the cable system quality and degradation/aging, but also on the cable and accessory technologies employed on the tested cable system.”^[2] The Wikipedia VLF article notes that for tan delta, IEEE 400.2 establishes the criteria for assessment.^[3] Treat any published threshold as a starting point, and check the standard that applies to your system.

How this relates to a handheld multimeter

A tan delta test is a high voltage cable diagnostic. It is not something a handheld digital multimeter can do. A bench or field insulation tester on a cable is a different job, and the readings it gives are insulation resistance rather than a loss angle. If you are choosing equipment for insulation work at lower voltages, the insulation resistance tester guide covers the test categories that a handheld tool can reach. For the broader picture of insulation test equipment, see the insulation resistance test equipment overview.

Safety and qualified work

Cable diagnostic testing involves high voltage stored in long cable runs. The NEETRAC source describes the cable being disconnected from the grid and energized from a separate supply during the test, and that separation is the basic safety condition the procedure depends on.^[2] The sources we opened do not provide step-by-step safety procedures for a specific site, and they do not replace your employer’s lockout and grounding rules or the test equipment manual. Work on energized or live cable systems should be left to qualified personnel.

How we chose

This article is an explainer, so it does not pick products. We chose the sources based on what they explain directly: the NEETRAC CDFI chapter is a utility-focused reference on the cable test, and the two Wikipedia entries give an accessible definition of dissipation factor and an overview of VLF testing. We avoided using a single vendor brochure as the basis for any claim, and we did not use a source for safety claims that it does not make.

What we could not verify

• We could not confirm a single universal tan delta threshold. The sources we opened tie the criteria to the cable and accessory technology and to the standard in use.
• The sources we opened do not give a site procedure for testing a specific cable, so this article does not describe one.
• We did not confirm the current edition of each IEEE or IEC standard that the sources mention. Check the standard itself before applying a number.

FAQ

Is a tan delta test the same as an insulation resistance test?

No. An insulation resistance test applies a DC voltage and reports a resistance. A tan delta test measures dielectric loss as a ratio of resistive to capacitive current, and it is usually run with AC or VLF. The two results answer related but different questions about the insulation.

Why is VLF used for tan delta?

The Wikipedia VLF article says VLF systems can be manufactured small and light, which helps in the field where transport and space are limited. NEETRAC adds that lower frequencies increase the sensitivity of tan delta because the capacitive current is smaller.^[2][3]

What does a rising tan delta value mean?

A rising value suggests more resistive loss in the insulation, which can come from moisture, contamination, or degradation. The source material treats that as a reason to investigate, not as a final diagnosis.^[2][3]

Do I compare one phase against another?

Yes. NEETRAC lists comparison with adjacent phases as one of the reference points that make a tan delta reading useful.^[2]

Can I run this test with a handheld multimeter?

No. The test needs a separate high voltage source and dedicated measurement equipment. A handheld multimeter is for lower-voltage electrical checks.

Where can I learn more before a test?

Read the equipment manual and the standard that governs your system. The NEETRAC chapter is a useful background reference, and the Wikipedia VLF article gives a short overview of the test types.^[2][3]

Bottom line

A cable tan delta test measures how lossy the cable insulation is, and the result is most useful as a trend compared with the same cable’s history, its adjacent phases, and its reference values. Judge the number against the standard for your system rather than a single universal limit.

Sources

1. [1] Dissipation factor, Wikipedia. https://en.wikipedia.org/wiki/Dissipation_factor (opened 2026-10-29)
2. [2] NEETRAC, Cable Diagnostic Focused Initiative (CDFI) Phase II, Chapter 6, Dissipation Factor (Tan Delta), 2016. https://neetrac.gatech.edu/files/2023/09/6-Tan-Delta-21_with-Copyright.pdf (opened 2026-10-29)
3. [3] VLF cable testing, Wikipedia. https://en.wikipedia.org/wiki/VLF_cable_testing (opened 2026-10-29)