Insulation resistance test using F1508

Insulation resistance test is a common method to test and inspect the insulation performance of electrical equipment. It is also a preparatory test for high-voltage insulation test. This article introduces how to use Fluke 1508 insulation tester to perform fast, simple and accurate insulation resistance test.

Basic knowledge of insulation resistance measurement

Insulation resistance test is a relatively common method to test and inspect the insulation performance of electrical equipment. Applicable equipment includes motors, transformers, switchgear, control devices and other electrical devices in windings, cables and all insulation materials. It is also a preparatory test for high-voltage insulation test. Before conducting more dangerous and destructive experiments, insulation resistance test can be carried out first. Large insulation defects of insulation materials can be found in advance, and corresponding measures can be taken in advance to avoid complete destruction of the insulation of the test object. The best method will be determined according to the type of equipment under test and the purpose of the test. Among them, in the measurement of test objects or capacitors with windings or dielectric materials, absorption ratio and polarization index are very important indicators for judging their insulation characteristics.

Absorption ratio refers to the result of testing the test object, using the insulation resistance value at 1 minute divided by the insulation resistance value at 15 seconds; polarization index is the result of the insulation resistance value at 10 minutes divided by the insulation resistance value at 1 minute. Compared with insulation resistance, the above two indicators have more advantages. For example, insulation resistance is very sensitive to changes in environmental conditions such as temperature and humidity. In different environments such as temperature and humidity, insulation resistance will also produce very large changes (especially temperature). Therefore, the insulation resistance measurement results in different environments cannot be directly compared and analyzed. The insulation resistance must be converted to temperature and the measurement results must be converted to 20°C before they can be compared and analyzed. The absorption ratio and polarization index do not need to be converted to temperature because their measurement results are measured in the same environment.

Using F1508 to measure insulation performance

The Fluke 1508 instrument is a battery-powered insulation tester that complies with the fourth category (CAT IV) IEC 61010 standard. The IEC 61010 standard defines four measurement categories (CAT I to IV) based on the degree of danger of transient pulses. Category IV testers are designed to protect against transient damage from power busbars (such as overhead or underground utility line facilities). Using the F1508, not only can the insulation resistance be obtained, but the absorption ratio and polarization index can also be automatically obtained. These measurement results can be directly used for design testing, production testing, acceptance testing, verification testing, preventive maintenance testing, and fault location testing. For any of these measurements, the F1508 can quickly, simply, and conveniently obtain very accurate results. Measuring

Insulation Resistance

Insulation testing can only be performed on de-energized circuits. The F1508 has automatic live detection and automatic discharge after detection. The specific operation steps are as follows:

1. Insert the test probe into the V and COM (common) input terminals.

2. Turn the rotary switch to the required test voltage.

3. Connect the probe to the circuit to be tested. The tester automatically detects whether the circuit is energized.

4. Press and hold the T test button to start the test. The test voltage applied to the circuit under test is displayed on the auxiliary display position. The high voltage symbol (Z) is displayed on the main display position and the resistance is displayed in MΩ or GΩ. The t icon appears at the bottom of the display until the T test button is released. When the resistance exceeds the maximum display range, the tester displays the Q symbol and the maximum resistance of the current range.

5. Continue to leave the probe on the test point and then release the T test button. The circuit under test begins to discharge through the tester. The main display position shows the resistance reading until a new test is started or a different function or range is selected, or a voltage of more than 30 V is detected.

Measuring Polarization Index and Dielectric Absorption Ratio

The F1508 can automatically measure and calculate the polarization index and absorption ratio as follows:

1. Insert the test probe into the V and COM (common) input terminals. Considering the time required for polarization index (PI) and dielectric absorption ratio (DAR) testing, it is recommended to use a test clip.
2. Turn the rotary switch to the desired test voltage position.
3. Press the AC button to select polarization index or dielectric absorption ratio.
4. Connect the probe to the circuit to be tested. The tester automatically detects whether the circuit is powered.
5. Press and release the T test button to start the test. During the test, the test voltage applied to the circuit under test is displayed on the auxiliary display. The high voltage symbol (Z) is displayed on the main display and the resistance is displayed in MΩ or GΩ. The t icon appears at the bottom of the display until the test is completed.

When the test is completed, the PI or DAR value is displayed on the main display. The circuit under test will automatically discharge through the tester. If any of the values ​​used to calculate PI or DAR is greater than the maximum display range, Err will be displayed on the main display. [page]

Storage function of F1508

F1508 has the function of storing measurement records. It can be used when the field conditions are bad, it is inconvenient to record data, or there is only one measurement person and it is impossible to record data during the measurement process.

F1508 can save up to 19 insulation resistance or ground coupling resistance measurement values ​​on the tester. The measurement values ​​are saved in a "last in, first out" manner. If more than 19 measurement values ​​are saved, the first saved will be deleted to leave space for the latest measurement value.

Saving measurements is very easy. Simply press the M button to save the most recently read measurement.

Recalling measurements is also very easy. Press the blue button and then the M button to recall the last saved measurement.

The measurement will be displayed in the main display and the number of the saved data will be displayed in the auxiliary display. Press the blue button and the M button again to recall the last saved result. This step can be repeated until the displayed stored data count reaches one. The next displayed measurement will be the most recent measurement. Press the M button to exit the recall display.

Press the blue button and then the L button. The main display will display clr? Press the blue button and then the L button again to clear all memory locations.

Example of measurement and analysis using the F1508
The measurement was conducted at a construction site of a power plant in Inner Mongolia, China. The object of the test was a 400V motor with parameters of 1000W and 400V. The insulation performance tested in the acceptance test was: insulation resistance: 100MΩ; absorption ratio: 13; temperature: 25℃.

During the subsequent trial operation, the upper level 380V switch tripped due to a fault. After inspection, it was found that the water pipe directly above the installation site was leaking, and the area that was wetted was relatively large, but the amount of water leakage was not large. It was difficult to determine whether the tripping was caused by the motor or the distribution box on site or both being soaked, so the motor wiring was disassembled and measured separately.

When testing the stator impedance, make sure that the stator winding and the phase line are disconnected, and measure the insulation resistance between the windings and between the windings and the ground (as shown in Figure 1).


Figure 1: F1508 measurement example

The measurement data is shown in Table 1.


Table 1: Measurement data of the distribution box and the motor

From the results, it can be easily determined that even without temperature reduction, the fault was caused by the distribution box on site being soaked in water, while the more difficult motor rotor and stator windings were not damp. The entire test analysis process only requires one person and can be completed in no more than 6 minutes, and 3 minutes of it is used to remove the connection between the distribution box and the motor.

Compared with the traditional method, using a hand-cranked megohmmeter to check faults, the absorption ratio must be calculated and the test must be repeated (due to external interference causing the hand-cranking to stop), which requires two people at least 10 minutes. Using F1508 makes the entire test process more convenient, safe, fast and accurate.