How to use a megohmmeter to measure insulation resistance and what preparations to do before measurement
The megohmmeter is convenient and reliable for measuring insulation resistance, but if it is not used, it will bring unnecessary errors to the measurement. In addition, the megohmmeter itself generates high voltage when it is working, and the measurement object is electrical equipment. Improper operation will cause personal or equipment accidents.
Therefore, the megohmmeter must be used correctly for insulation resistance measurement, and the following preparations must be made before use:
1. The surface of the measured object should be clean to reduce the contact resistance and ensure the correctness of the measurement results.
2. When using the megohmmeter, it should be placed in a stable and firm place, and away from large external current conductors and external magnetic fields.
3. For equipment that may induce high voltage, this possibility must be eliminated before measurement can be performed.
4. Before the measurement, the power supply of the device under test must be cut off, and the ground should be short-circuited and discharged, and the device is never allowed to be charged for measurement, so as to ensure the safety of the person and the device.
5. Check whether the megohmmeter is in normal working condition before measurement, mainly check its "0" and "∞" points. That is, shake the handle to make the motor reach the rated speed. The megohmmeter should be at the "0" position when it is short-circuited, and it should be at the "∞" position when it is open-circuited. the
After doing the above preparations, you can start measuring. Pay attention to the correct wiring of the megohmmeter during measurement to avoid unnecessary errors or even mistakes. the
There are three terminals of the megohmmeter: "L" is the line terminal; "E" is the ground terminal; "G" is the shielding terminal, also called the protection ring. Usually the measured insulation resistance is connected to "L" and "E". However, when the surface leakage of the measured insulator is serious, the shielding ring of the measured object or the part not 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 the 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 reversed. The device case with the terminal button grounded and the "G" shield terminated to the insulated part of the device under test. Once "L" and "E" are reversed, the leakage current flowing through the insulator and the surface will be collected to the ground through the shell, and will flow into the measuring coil from the ground through "L", so that "G" will lose its shielding effect and give the measurement band There is a big error.
In addition, because the insulation degree of the inner lead of the "E" terminal and the shell is lower than that of the "L" end and the shell, when the "L" and "E" are reversed, the insulation resistance of "E" to the ground is the same as the measured The insulation resistance is connected in parallel, so that the measurement result is too small, which brings a large error to the measurement. When the megohmmeter is used on the ground and the correct wiring method is used, the insulation resistance of the "E" terminal to 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 outer surface, be sure to connect the shielding terminal button "G", because when the air humidity is high or the surface of the cable insulation is not clean, the leakage current on the surface will be large. The influence of the measured object on its internal insulation measurement due to leakage, generally a metal shielding ring is added to the outer surface of the cable and connected to the "G" end 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, and hidden dangers will be laid for the safety of electricity use.
