Tan Delta Test is one of the most advanced and reliable diagnostic tests for detecting hidden insulation issues in electrical systems. The operation and maintenance of high-voltage equipment is highly dependent on the reliability of its insulation system. By tracking subtle changes in insulation’s electrical properties, tan delta tests provide a clear picture of its condition, enabling the maintenance team to take timely action and avoid unplanned downtime. 

In transformers, power cables, motors, and switchgears, insulation acts as an invisible shield. However, over time, this insulation deteriorates, leading to unexpected failures and dangerous outages. Detecting this deterioration early is crucial to preventing costly breakdowns and extending the equipment's service life. 

At Technomax, our team of skilled professionals carefully understands each system and performs TD testing to assess the condition of high-voltage insulation accurately.

In this article, we’ll explore the core concept of Tan Delta testing, walk through its step-by-step process, and highlight its key features—giving you a clear understanding of the service before you choose it.

What is Tan Delta Testing?

Tan Delta (TD), also called Loss Angle or Dissipation Factor, is a non-destructive diagnostic test used to evaluate the condition and quality of cable insulation in electrical systems.

The power cables used in electric systems are not just a conductor wrapped in insulation. But electrically, they behave like a capacitor, with a central conductor and an outer metallic sheath or screen separated by an insulation, which is dielectric. This behavior enables capacitance measurement, a crucial component of Tan Delta testing that determines the dielectric properties of the insulation.

In the case of a perfect cable, i.e, a cable with no defects, the current that passes through the conductor is the capacitive current( IC ). As per the phase relationship, this current is 90 degrees ahead of the applied voltage.  (Figure 2(a))

In reality, no insulation is perfect and is subjected to stresses, contamination, and damage over time. This gives rise to a resistive current ( IR) to flow with the voltage, along with the capacitive current. 

Fig.1. Current (I) v/s Voltage (U) Phase Diagram.

On the phase diagram, this resistive current shifts the total current vector slightly away from the ideal 90° position (Figure 2(b). The angle between the ideal capacitive current ( IC) and the actual current is called the loss angle (δ ). 

The total leakage current IL can be given by,

IL = IR + IC

In the Tan-Delta testing method, we calculate the dielectric loss factor,  tan δ, at different test voltages. So,  from the vector diagram, we get the Dissipation factor,

tan  δ = IR / IC

Where IR is the resistive element of the current, and IC  is the capacitive element of the current.  The value of IC is determined through capacitance measurement, letting us know the insulation’s ability to store electrical charge.

By tracking how tan δ changes, we can identify whether the insulation is still in good condition, starting to deteriorate, or approaching failure.

Why Tan Delta Testing?

Tan Delta Testing is vital for identifying early-stage insulation deterioration in high-voltage assets like cables, transformers, and bushings. The loss angle (δ), measured in a tan delta test, represents the phase difference between the ideal capacitive current (IC) and the total leakage current (IL).

A larger δ means more resistive losses, indicating the insulation is no longer performing like a pure capacitor. The key benefits of conducting tan delta testing include:

• Early fault detection: Identifies moisture ingress, contamination, and insulation aging before failure occurs.
• Non-destructive testing: Performed at low frequency and controlled voltages to protect equipment.
• Trend analysis: Comparing results at normal and elevated voltages reveals deterioration patterns.
• Cost savings: Helps plan maintenance and avoid unplanned outages.
• Extended asset life: Maintains safe operating conditions and delays expensive replacements.
• High accuracy: Sensitive to even small changes in insulation properties.

Process of Tan Delta Testing

Tan delta testing involves the following processes

1. Preparation of Equipment: Disconnect the cable, potential transformer, bushings, current transformer, or winding to be tested from the rest of the system, so the insulation can be assessed independently.
2. Apply Test Voltage: Begin the examination with the minimal frequency level of test voltage on the equipment whose insulation is under analysis.
3. Initial Measurement at Normal Voltage: Apply the equipment’s rated operating voltage. Record the tan delta values at this level using the tan delta controller. This serves as the baseline reading.
4. Increase to High Voltage: Raise the applied voltage to 1.5 to 2 times the normal operating voltage to stress the insulation under test. 
5. Measurement at High Voltage: Record the tan delta values again at this elevated voltage for comparison.
6. Data Comparison and Analysis: Use a loss angle analyzer to compare the tan delta values from the normal voltage test (Step 3) with those from the high voltage test (Step 5).If values remain stable, insulation is likely in good condition. If values rise significantly at higher voltage, insulation deterioration is suspected.

How to Predict the Result of Tan Delta Testing

The health of an insulation system using the tan delta test can be predicted using two main methods:

1. By Comparing Trends Over Time: Compare the latest test results with previous measurements. A noticeable increase in tan delta values over the years often points to insulation deterioration caused by aging, moisture ingress, or contamination.
2. Direct Value Assessment: The value of tan δ reading is used to determine the condition of insulation. A healthy insulation system will show almost the same tan delta value across all test voltages. But if the insulation is weak or damaged, the tan delta value will rise noticeably at higher test voltages—a clear sign that the insulation is under stress and losing its effectiveness.

What are the Different Modes of the Tan Delta Test?

Tan delta testing can be performed in three main modes. Each mode focuses on different aspects of insulation behaviour.

1. GST Guard Mode: In Ground Specimen Test (GST)  Guard mode, the tan delta test measures only the leakage current flowing to the ground. This mode is useful when you want to focus only on ground leakage and ignore other leakage paths.
2. UST Mode: Ungrounded Specimen (US) Test mode measures the insulation between two ungrounded points in the equipment. This mode is ideal for checking phase-to-phase insulation without interference from ground connections. 
3. GST Mode: In GST mode, all possible leakage paths are included in the measurement. The results from GST Guard mode and UST mode, when added together, should ideally match the GST results.

If the sum of GST Guard and UST does not equal the GST reading, it could indicate:

• A fault or calibration issue in the test set, or 
• Incorrect test connections or terminal setup

Tan Delta Testing in UAE and Middle East with Technomax

At Technomax, we combine advanced diagnostic equipment with expert analysis to deliver precise and reliable Tan Delta testing for electrical systems.  Our team conducts the test under controlled conditions to measure the insulation loss angle and detect even the earliest signs of deterioration.

By comparing results across multiple voltage levels, we give you a clear, data-backed picture of your equipment’s health—helping you predict failures, optimize maintenance schedules, and extend asset lifespan. Whether it’s Tan Delta for transformers, cables, or rotating machines, our testing ensures your systems remain safe, efficient, and fully compliant with industry standards.

Our expert team also perform condition monitoring services like vibration analysis, laser alignment services, infrared thermography testing and more.

Don’t wait for a fault to disrupt your operations. 

Book your Tan Delta Testing in UAE or across Middle East with Technomax now for a fast, accurate, and compliance-ready assessment of your insulation’s condition.

FAQs

1. What does tan delta indicate?

Tan delta indicates the condition of electrical insulation by showing how much leakage current is present compared to the ideal capacitive current.

2. What is the difference between hipot and tan delta test?

Hipot testing is a pass/fail check to see if insulation can handle high voltage, while tan delta testing analyzes insulation quality, aging, and moisture content.

3. How to check tan delta? 

The tan delta value of a cable insulation is checked by applying a controlled voltage through specialized test equipment and measuring the loss angle or dissipation factor.

4. How do you interpret tan delta results?

If the tan delta test values are consistent across test voltages, it means healthy insulation. But rising values at higher voltages indicate degradation or contamination of the insulation.