1. Requirements and Challenges of Insulation Resistance Testing

In sectors such as power generation, telecommunications, meteorology, data centers, oil fields, mechanical and electrical installation and maintenance, and power supply for industrial and mining enterprises, insulation resistance testing of large-scale equipment—including electrical equipment and power transmission lines—is a critical step in ensuring the safe operation of such equipment. As equipment capacity increases, voltage levels rise, and operating environments become more complex, insulation resistance testing faces numerous challenges. In environments with strong induced electric fields, such as substations, traditional test instruments are susceptible to electromagnetic interference, leading to inaccurate test data. In scenarios with high distributed capacitance, such as testing long cables, standard instruments lack sufficient output power to perform tests reliably. Furthermore, traditional test instruments have limited functionality, capable only of basic insulation resistance testing. They cannot meet the testing requirements for multiple parameters such as the polarization index, absorption ratio, and dielectric discharge index. Additionally, they are complex to operate and require extensive training for specialized personnel to use them proficiently.

Furthermore, the storage and analysis of test data present significant challenges. Most traditional instruments lack data storage capabilities, requiring manual recording of test results—a process prone to errors. Moreover, they cannot effectively analyze historical data, making it difficult to predict trends in equipment insulation conditions in advance.

2. Solution: EM3520 Series High-Voltage Insulation Resistance Tester

The EM3520 Series High-Voltage Insulation Resistance Tester is a high-performance instrument meticulously developed by Iridium Measurement & Control. It addresses the pain points of current insulation resistance testing and provides a comprehensive solution. This series of instruments features comprehensive testing capabilities for various insulation resistance parameters, a user-configurable test voltage function, and excellent anti-interference performance, offering significant advantages in environments with strong induced electromagnetic fields and high distributed capacitance.

The EM3520 series instruments feature a true-color touchscreen, displaying all test data and battery level clearly on a single screen. The combination of rotary switches and buttons makes operation extremely simple. Users do not need to memorize operating procedures; simply tapping the “HELP” button on the screen brings up a guide to effectively assist with instrument operation. The instrument’s test timer automatically records test duration and stores results with date and time stamps. Historical data can be conveniently reviewed via the touchscreen, while fully isolated USB ports and Bluetooth connectivity allow test data to be securely uploaded to a PC or mobile device for user analysis.

Figure 1: ETCR3520 Insulation Resistance Tester

3. Product Features

3.1 Key Performance Advantages

• Wide-range, high-precision measurement: The insulation resistance measurement range covers 0.005 MΩ to 30 TΩ, meeting the testing requirements for equipment at various voltage levels. Measurement accuracy within the standard range reaches ±5% of reading ±3 digits.

• Multiple Output Voltage Settings: The unit offers up to 8 rated output voltage settings (50 V, 250 V, 500 V, 1 kV, 2.5 kV, 5 kV, 10 kV, 15 kV) and supports a custom output voltage mode, allowing users to freely set test voltage and duration to accommodate specific application requirements.

• Strong anti-interference capability: With high output power and a maximum short-circuit current of 7mA, it delivers stable testing performance in scenarios with high distributed capacitance (such as long cable runs) and in environments with strong electromagnetic interference (such as substations).

• Comprehensive Test Parameters: Supports testing of multiple parameters including insulation resistance (IR), polarization index (PI), damping ratio (DAR), step (STEP), ramp (RAMP), dielectric discharge index (DD), distributed capacitance (CAP), and AC/DC voltage (V), meeting diverse testing requirements.

3.2 Product Feature Overview

Figure 2: Introduction to the Product Testing Interface

Figure 3: Insulation Resistance (IR)

The principle behind insulation resistance measurement is as follows: a high-voltage generator produces a voltage V, which is applied across the resistor under test. By measuring the current I flowing through the resistor, the resistance value R is calculated using Ohm's law.

Figure 4: Damping Ratio (DAR)

The damping ratio (DAR) is defined as the ratio of the insulation resistance measured over a 1-minute period to the insulation resistance measured over a 15-second period. The damping ratio test must be completed within 1 minute. Therefore, for all insulation tests lasting less than 1 minute, the damping ratio data is considered invalid. When the insulation test lasts 1 minute or longer, the damping ratio data is included in the results.

Figure 5: Polarization Index (PI)

The polarization index (PI) is defined as the ratio of the insulation resistance measured over a 10-minute period to that measured over a 1-minute period. The polarization index test takes 10 minutes to complete. When the insulation test duration is 10 minutes or longer, the polarization test is completed and the results are saved.

Figure 6: Standard Mode and Ramp Mode (RAMP)

Under normal circumstances, a fixed voltage is used to measure the insulation resistance of the circuit under test; this is referred to as the standard test mode. Ramp test mode is an automatic test mode used to detect insulation faults. During a ramp test, the output voltage starts at 10% of the preset voltage and increases in 10% increments until it reaches the preset voltage or a sudden drop in the measured resistance is detected, at which point the ramp test stops. If the preset voltage is not reached, only the data from the completed test points will be recorded, and the test results will be invalid.

The ramp mode is a type of overvoltage test. The principle behind this mode is that, at all voltages, an ideal insulator should produce the same reading; however, when the insulator is subjected to excessive voltage, it will display a lower insulation resistance value at higher voltages. The ramp mode can be applied to the stator and rotor windings of synchronous and asynchronous AC motors, as well as the armature and field windings of DC motors.

Figure 7: Filter Test Mode

In certain environments with strong interference, insulation resistance values may fluctuate due to external factors. In such cases, you can select the filter mode and use the buttons to set the filter duration (e.g., 10 s, 20 s, 30 s, or 40 s). The instrument calculates the average value over the filter period to reflect the actual insulation resistance value. The test results will be displayed on the FR.

Figure 8: Capacitance Test

As part of the insulation test, the 3520 series is capable of measuring and storing the capacitance of the circuit under test. Press the corresponding button to select whether to measure capacitance. When capacitance measurement is selected, press the test button to begin the test. Once the test has stabilized, press the test button again to stop the test. The instrument calculates the current value based on the discharge time, and the fast discharge function is disabled at this point. The discharge time is longer than when capacitance measurement is not selected.

Figure 9: Voltage Test

This function measures AC and DC voltages. When measuring insulation resistance, this function is used solely to verify whether the circuit under test is live.
Do not measure AC or DC voltages exceeding 1000 V, as this may damage the meter.

Figure 10: Record Query Function

Figure 11: Custom Output Voltage Test Mode

Custom output voltage ranges and corresponding models: ETCR3520: 40 V to 5 kV, ETCR3520B: 40 V to 10 kV, ETCR3520C: 40 V to 15 kV

Figure 12: Custom Time and Calculation Formulas

The instrument allows you to set the date and time and select calculation formulas.

To change the date and time, place your finger on the screen and swipe left. The screen will switch to the test record view page. Swipe left again to enter the clock settings interface and adjust the time. In this interface, swipe to select the time, then tap “OK.”

To modify a formula, check the box next to the formula you wish to use. Once all desired changes have been made, press the Accept button to apply the changes and return to the previous page.

3.3 Product Wiring Instructions

Figure 13: Cable Insulation Resistance Test, Conventional Two-Wire Measurement Configuration

1) Conventional two-wire measurement configuration: Leakage current is present on the surface of the inner insulation layer near the cable end. This leakage current is also included in the current measured at the “–” terminal, which will cause the measured resistance reading to be lower than the actual insulation resistance. This method may be used for measurements that do not require extremely high resistance values. 

Figure 14: Cable insulation resistance test, three-wire measurement configuration for ultra-high resistance

2) Three-wire measurement configuration for ultra-high resistance: A bare metal wire with good conductivity is wrapped around the outer surface of the inner insulation layer. By connecting the safety terminal to the conductor surrounding the inner insulation layer, surface leakage current from the test object is prevented. Surface leakage current is directed to the safety terminal, thereby eliminating surface leakage current in the measurement path between the positive and negative terminals and improving the accuracy of the measurement reading.

Figure 15: Cable Insulation Resistance Test, Three-Wire Measurement Configuration for Ultra-High Resistance

3) Three-Wire Measurement Configuration for Ultra-High Resistance: Wrap a bare metal wire with good conductivity around the outer surface of the inner insulation layer, and connect the safety terminal to the conductor surrounding the inner insulation layer and to an unused cable. Surface leakage current will be directed to the safety terminal, thereby eliminating surface leakage current in the measurement path between the positive and negative terminals. This ensures that the measured insulation resistance represents the resistance between the selected cable and the outer insulation, while eliminating leakage paths between cables.

Figure 16: Transformer insulation resistance test; insulation resistance test between the primary winding and the secondary winding (grounded)

Figure 17: Transformer insulation resistance test; insulation resistance test between the primary winding (grounded) and the secondary winding

Figure 18: Transformer Insulation Resistance Test; Insulation Resistance Test Between Secondary Windings

3.4 Operational and Design Advantages

1) User-Friendly Operation: A 5-inch color touchscreen displays test data on the same screen, making test operations and history queries very convenient. The combination of rotary switches and buttons accommodates both traditional operating habits and the need for intelligent operation.

2) Safety Protection Design: Built-in voltage monitor automatically detects the voltage of the test object; testing is automatically disabled if the voltage exceeds 36V, effectively protecting both the instrument and the operator. The device features an automatic discharge function that rapidly releases the stored charge from the test object after testing, eliminating the need for an external discharge circuit.

3) Durability Design: Features a robust double-layer housing structure with an IP65 protection rating, preventing the ingress of moisture and dust during transport and storage, as well as protecting the instrument from impacts.

Long Battery Life: Equipped with a high-capacity rechargeable lithium-ion battery pack (14.8V, 6400mAh) to ensure extended testing sessions and meet the demands of prolonged outdoor operations.

Figure 19: Safety and Precautions

Figure 20: Safety and Precautions

3.5 Data Management Advantages

1) Large-Capacity Data Storage: Automatically stores up to 1,000 sets of real-time test data, including test dates and durations, allowing users to easily review historical test records.

2) Multiple Data Transfer Methods: Supports USB and Bluetooth communication, enabling test data to be uploaded to a PC or mobile phone for secure storage and professional analysis.

3.6 Model Specifications

3.7 Test Range and Accuracy

3.8 Other Technical Specifications

4. Application Scenarios and Test Cases

4.1 Cable Insulation Resistance Testing

In a cable line maintenance project for a power company, the EM3520C high-voltage insulation resistance tester was used to perform insulation resistance tests on 10 kV cables. During the testing process, the instrument maintained stable operation even in the high electromagnetic interference environment of a substation and successfully completed measurements of insulation resistance, polarization index, and absorption ratio.

The test employed a three-wire measurement configuration with ultra-high resistance. A bare metal wire with good conductivity was wrapped around the outer surface of the inner insulation layer. By connecting the safety terminal to the conductor surrounding the inner insulation layer, surface leakage current from the test object was prevented. This effectively eliminated the impact of surface leakage current on the test results and improved measurement accuracy. The test results showed that the cable had an insulation resistance of 8.5 TΩ, a polarization index of 2.8, and an absorption ratio of 1.6. The insulation condition was found to be good and in compliance with operational standards.
 

Figure 21: Diagram of Cable Insulation Resistance Testing

4.2 Transformer Insulation Resistance Testing

During maintenance work on a transformer at a certain factory, an ETCR3520B high-voltage insulation resistance tester was used to perform insulation resistance tests on a 10 kV transformer. The tests included measuring the insulation resistance between the primary winding and the secondary winding ground, between the primary winding ground and the secondary winding, and between the secondary windings.

The tests accurately determined the insulation resistance values between the transformer’s windings. The insulation resistance between the primary winding and the secondary winding ground was 5.2 TΩ, between the primary winding ground and the secondary winding was 4.8 TΩ, and between the secondary windings was 6.1 TΩ. All values met the insulation requirements for transformer operation. Additionally, the dielectric discharge index test confirmed that the transformer’s multi-layer insulation is in good condition, and no maintenance is required.

Figure 22: Diagram of Transformer Insulation Resistance Testing

4.3 Preventive Testing of Electrical Equipment

During the annual preventive maintenance of a certain data center, the ETCR3520 high-voltage insulation resistance tester was used to perform insulation resistance tests on electrical equipment such as UPS units and power distribution cabinets within the facility. During the testing process, the instrument’s customizable test voltage function was utilized to set appropriate test voltages based on the withstand voltage ratings of different devices, ensuring safe and accurate testing.

After the tests were completed, the test data was uploaded to a mobile app via Bluetooth, and a test report was generated to facilitate data analysis by management personnel. The test results showed that the insulation condition of most equipment was good; only one power distribution cabinet had a low insulation resistance value. Maintenance was performed promptly to prevent equipment failure.

5. Summary

5.1 Summary of Application Results

The ETCR3520 series high-voltage insulation resistance testers have demonstrated outstanding performance in practical applications. They effectively address testing challenges in environments with strong induced electric fields and high distributed capacitance, improving the accuracy and efficiency of test data while enabling digital management of test results. The application of this series of instruments provides a strong guarantee for the safe operation of equipment in fields such as power, telecommunications, meteorology, data centers, oil fields, mechanical and electrical installation and maintenance, and power supply for industrial and mining enterprises.

By using this series of instruments, enterprises can promptly detect changes in equipment insulation status and perform maintenance in advance, thereby reducing the incidence of equipment failures and minimizing economic losses caused by such failures. At the same time, it lowers labor and training costs associated with testing and improves work efficiency.

5.2 Summary of Product Advantages

The EM3520 series high-voltage insulation resistance testers offer a wide range of high-precision measurements, multiple output voltage settings, strong anti-interference capabilities, comprehensive test parameters, a user-friendly interface, safety protection features, durable construction, long battery life, and robust data management functions. Capable of meeting insulation resistance testing requirements across various industries and scenarios, this series represents a high-performance, highly practical solution for high-voltage insulation resistance testing.

Guangdong Yidian Measurement & Control Technology Co., Ltd. specializes in the R&D and production of power testing instruments and power quality management equipment. With nearly two decades of deep expertise in the power industry, the company is committed to technological innovation and independent R&D, meticulously crafting every product and providing attentive service to every customer. The company actively supports OEM and ODM customization services, tailoring solutions to meet specific client needs to create greater value for customers. Yidian Measurement & Control is the manufacturer of the ETCR3520 series high-voltage insulation resistance testers. Yidian Measurement & Control Service Hotline: (86-757)66860936

For more information, please visit the Yidian Measurement & Control official website: www.etcrmeter.com