1. Principle of digital insulation resistance test
The insulation resistance tester calculates the insulation resistance value based on Ohm's law. By outputting a DC voltage that is much lower than the dielectric strength test, and then measuring the current, the insulation resistance value is calculated.
In fact, the resistance value of the insulation resistance is very large, but it is not infinite, so it can be calculated by measuring the leakage current. The megohmmeter can display the resistance value of the insulation resistance, and the results are generally kΩ, MΩ, GΩ , In individual cases, it even reaches TΩ. The size of the insulation resistance value directly represents the insulation quality between the two conductors.
Insulation Resistance Tester
2. Factors affecting the results of the insulation resistance tester
The reason why the results of the insulation resistance tester will be affected is because the current value is obtained by applying a constant voltage to the test object. These external factors, such as temperature and humidity, may seriously affect the test results.
First, let's analyze the properties of the current generated during the insulation test, and assume that none of these factors will affect the measurement.
There are three components of the total current of the insulating material when a voltage is applied: the capacitive current, also known as the capacitive charging current
Capacitive current: When testing insulation resistance, the charging current must first fill the capacitance of the insulating material to be tested. This current is relatively large at the beginning of the test moment, and once the circuit under test is charged, it will drop at a rapid rate to close to Zero. It will become stable after a few seconds or ten seconds, and this current is so small that it can be ignored relative to the total current
Absorbing current, the absorbing current is different from the capacitor current. The speed of the absorbing current dropping is much slower than the charging current of the capacitor. It generally takes several minutes to reach a value close to zero and stable.
Leakage current, also known as polarization current, is a current generated during the polarization process of the insulating medium under the action of voltage. The magnitude of the current indicates the quality of the insulation. Motors with large capacity or cables with long distances and large cross-sections It may take about 30 minutes for the capacitor charging current and sinking current to decrease and stabilize, so that the insulation test can obtain a correct result
Summary: When a steady current is applied to the circuit, the total current in the insulating material being tested changes over time. This means that the insulation resistance also changes significantly over time
Influence of temperature: The value of insulation resistance will also change when the temperature rises or falls. In the preventive test, the insulation resistance test must be carried out at a similar temperature. If this condition is not met, a reference temperature should be set As a standard, it is converted and corrected. According to a general guideline, if the temperature rises by 10°C, the resistance value will drop to half of the resistance value of the reference temperature point; if the temperature drops by 10°C, the resistance value will be twice the reference value.
The influence of humidity on the test results of the insulation resistance tester mainly depends on the degree of contamination on the surface of the insulator. However, special attention should be paid, and the insulation resistance measurement must not be carried out when the temperature drops below the dew point.