The multimeter can only measure the resistance of conductors and the shaker can only measure the resistance of insulators
The multimeter can only measure the resistance of conductors, but not the resistance of insulators. Only the shaker can accurately measure the resistance of insulators. Let me talk about why?
Conductor/Insulator
Conductor: an object that conducts electricity well
Insulator: an object with poor electrical conductivity (note, not an object that does not conduct electricity)
The common conductors in our life are: copper, iron, aluminum, gold, silver, graphite, etc.
Common insulators in our lives include: plastic, rubber, glass, ceramics, pure water, air, various natural mineral oils, etc.
What we should pay special attention to here is that an insulator is an object with poor conductivity, not an object that does not conduct electricity. Strictly speaking, there is no such thing as an absolutely non-conductive object. For example, plastics may be broken down at higher temperatures and thus conduct electricity. Therefore, insulators are divided into five grades according to the heat resistance temperature: Y, A, E, B, F, H, and C.
Also, insulators can break down at higher voltages and become conductive. Therefore, whether an insulator conducts electricity is relative to a certain voltage, and this voltage is called the rated voltage of the insulator.
Logically speaking, whether the wire is burned or not has little to do with the voltage. Then why does he still have to mark the rated voltage? This is because the insulation on the outside of the wire has a voltage withstand range. We can simply understand that when the water pressure exceeds the bearing range of the water pipe, the water pipe will be damaged and the water inside will be sprayed out. Similarly, when the voltage of the wire exceeds the tolerance range of the insulation, the insulation of the wire will be destroyed, and the current will come out, commonly known as "leakage".
Multimeters and Megohmmeters
Measuring resistance with a multimeter is actually using Ohm's law. We all know that when the multimeter measures resistance, the 1.5V and 9V batteries in the meter supply power. When the two test leads are connected to the resistor, the current in the meter starts from the positive pole of the battery, then passes through the meter head, the resistor, and then returns to the negative pole of the battery. The size of the resistance can be judged according to the current size of the meter head, because the voltage is constant, and the size of the current depends on the size of the resistance.
This is perfectly fine for measuring the resistance of conductors, but not for measuring insulators, because whether an insulator conducts electricity depends on voltage and temperature. For example, an insulator is non-conductive at 9V, then when measured with a multimeter, there will naturally be no current passing through the meter head, so the displayed resistance value is infinite. But if you continue to apply a higher voltage, it may break down and conduct electricity. Therefore, when measuring whether an insulator is conductive, a voltage must be specified.
There is a hand-operated DC generator inside the megohmmeter, and the output voltage of the generator is also different according to the voltage level of the megohmmeter. A 250V megohmmeter can emit a DC voltage close to 250V, a 500V megohmmeter can emit a DC voltage of nearly 500V, and a 1000V megohmmeter can emit a DC voltage of nearly 1000V... If you use a 500V megohmmeter to measure a certain The insulation resistance of the wire is simulated under 500V DC voltage to measure whether the wire is leaking.
If a line does not leak under the measurement of 500V by the megohmmeter, then there will be no leakage under the voltage of 300V. Therefore, when we choose a megohmmeter for measurement, we must ensure that the voltage level of the megohmmeter is higher than the actual voltage of the line. In addition, the megohmmeter emits direct current, while the commonly used 220V is alternating current, and the peak value of 220V alternating current can reach 220*1.414=311V. Therefore, we must choose a 500V megohmmeter when measuring the insulation of AC 220V lines.
Megohmmeter voltage level selection
For electrical equipment and lines with a rated voltage below 220V (such as 110V, 48V, 36V, 24V, etc.), generally use a 250V megohmmeter;
For electrical equipment and lines with a rated voltage of 220V, a 500V megger is generally used;
For electrical equipment and lines with a rated voltage of 380V, a 1000V megger is generally used;
For porcelain bottles, insulators, etc., a 2500V megger is generally used;
