How to measure insulation resistance with megohmmeter
The megohmmeter is convenient and reliable for measuring insulation resistance, but if it is not used properly, it will bring unnecessary errors to the measurement. In addition, the megohmmeter itself generates high voltage when working, and the measurement object is electrical equipment. Improper operation may cause personal or equipment accidents.
Therefore, the megohmmeter must be used correctly to measure insulation resistance, and the following preparations must be made before use:
1. The surface of the object to be measured should be clean to reduce contact resistance and ensure the accuracy of the measurement results.
2. When using the megohmmeter, it should be placed in a stable and solid place, away from large external current conductors and external magnetic fields.
3. For equipment that may induce high-voltage electricity, this possibility must be eliminated before measurement can be carried out.
4. Before measurement, the power supply of the equipment under test must be cut off and the equipment should be short-circuited and discharged to ground. The equipment must never be powered on for measurement to ensure the safety of people and equipment.
5. Before measuring, check whether the megohmmeter is in normal working condition, mainly check its "0" and "∞" points. That is, shake the handle to make the motor reach the rated speed. The megohmmeter should point at the "0" position when it is short-circuited, and it should point at the "∞" position when it is open-circuited.
After completing the above preparations, you can start measuring. Pay attention to the correct wiring of the megohmmeter when measuring to avoid unnecessary errors or even errors.
There are three binding posts of the megohmmeter: "L" is the line terminal; "E" is the ground terminal; "G" is the shielding terminal, also called the guard ring. Usually the insulation resistance to be measured is connected to "L" and "E" between the terminals, but when the surface leakage of the insulator under test is serious, the shielding ring of the object under test or the part that does not need to be measured must be connected to the "G" terminal. In this way, the leakage current flows directly back to the negative terminal of the generator through the shielding terminal "G" to form a loop, instead of flowing through the measuring mechanism of the megohmmeter, which fundamentally eliminates the influence of surface leakage current.
When using a megohmmeter to measure the insulation resistance of electrical equipment, it must be noted that the "L" and "E" terminals cannot be connected in reverse. The correct connection method is: the "L" wire terminal is connected to the conductor of the equipment under test, and the "E" ground The terminal is connected to the equipment enclosure and the "G" shield is terminated to the insulated portion of the equipment under test. Once "L" and "E" are connected reversely, the leakage current flowing through the inside and surface of the insulator will collect to the ground through the shell, and flow from the ground through "L" into the measuring coil, causing "G" to lose its shielding effect and give the measuring band There is a big error.
In addition, because the insulation degree of the internal lead of the "E" end and the shell is lower than that of the "L" end and the shell, when "L" and "E" are connected reversely, the insulation resistance of "E" to the ground is also measured. The insulation resistance is connected in parallel, which makes the measurement result smaller and brings a larger error to the measurement. When the megohmmeter is placed on the ground and used in the correct wiring method, the insulation resistance between the "E" terminal and the instrument shell and the shell to the ground , which is equivalent to a short circuit and will not cause errors.
When measuring the insulation resistance between the cable core and the surface, be sure to connect the shielding terminal "G", because when the air humidity is high or the cable insulation surface is not clean, the leakage current on its surface will be very large. In order to prevent being To detect the impact of leakage on the internal insulation measurement of the measured object, a metal shielding ring is generally added to the outside of the cable and connected to the "G" terminal of the megohmmeter.
In short, only by using the megohmmeter correctly can the insulation resistance of electrical equipment, etc. be accurately measured. Otherwise, the accuracy and reliability of the measurement will be lost, posing hidden dangers to the safety of electricity.
