An electrical system is like a pipe system. Voltage is like liquid pressure, current is like liquid flow rate, and electrical insulation is like pipe wall. Insulation prevents the leakage of electrons from the conductor and its effect is expressed by insulation resistance. An effective insulation resistance system has a high resistance value, usually greater than a few megaohms (M). A poor insulation system has a lower insulation resistance. In order to find a leak in the piping system, you need to pressurize it. Since leaks are easily found under water pressure, you cannot turn off the tap water to check for leaks. However, you will limit the available tap water so that you will not spray too much water around if you find a big leak. The ideal test is to provide a limited amount of water under high (but not particularly high) pressure. This is exactly what the electrical insulation tester does. An insulation tester (megohmmeter) will apply a DC voltage to the insulation system and measure the resulting current. In this way, it is possible to calculate and display the resistance value of the insulation (the degree to which the insulation confines current in the wire, or the degree to which it prevents current leakage). Portable insulation tester (megohmmeter) generally output test voltage. As in piping systems, the purpose is to provide a pressure (voltage) that is not particularly high. We want to find existing leaks, but we don't want to overstress the system and create new leaks. Lower voltages are used in low-voltage systems, such as networks or control lines; higher pressures are used to test the insulation of power systems.

The basic principle of the insulation resistance tester can be compared to a typical Ohm's law diagram: the meter has a known internal resistance, will output a constant test voltage, and check the current at the same time. In order to ensure personal safety and equipment safety, the insulation meter is equipped with current limiting protection. If the measured insulator resistance is low, or local breakdown occurs, or the distributed capacitance is large, the current limiting protection function will work, and voltage fluctuations will be observed at this time. The insulation meter will calculate the insulation resistance based on the actual voltage and current, and the measurement results can still be accurate.

Terminology of insulation resistance tester:

1. Insulation resistance meter (megohm meter)-an electrical measuring instrument used to measure insulation resistance with direct action analog indication, and its unit is expressed in MΩ.

2. The measuring terminal button-insulation resistance meter is used to connect the terminal to be measured. The measuring terminal buttons of the insulation resistance meter should be marked with the line terminal button L, ground terminal button E, and shield terminal button G respectively.

3. Terminal button voltage-the voltage between the line terminal button and the ground terminal button of the insulation resistance meter.

4. Rated voltage-the nominal value of the output voltage when the measuring terminal of the insulation resistance meter is in an open circuit state.

5. Open circuit voltage-the measured value of the output voltage measured when the insulation resistance meter measuring terminal button is in an open circuit state.

6. Median voltage-the output voltage value measured by connecting the L and E of the insulation resistance meter to the median resistance.