Multimeter can only measure the resistance of conductors and shaking table can measure the resistance of insulators
Conductor/Insulator
Conductor: an object that conducts electricity well
Insulators: objects that conduct electricity poorly (note, not objects that do not conduct electricity)
Common conductors in our life are: copper, iron, aluminium, gold, silver, graphite, etc.
Common insulators in our life are: plastic, rubber, glass, ceramics, pure water, air, various natural mineral oils, etc.
Here we should pay special attention to is that the insulator is poor conductivity of the object, not non-conductive objects. Strictly speaking, absolutely non-conductive objects do not exist. Plastics, for example, can be punctured at higher temperatures and thus conduct electricity. Therefore, insulators are classified into five grades according to their heat-resistant temperature: Y, A, E, B, F, H and C.
Similarly, insulators may be punctured at higher voltages and thus conduct electricity. Therefore, whether the insulator conducts electricity is relative to a certain voltage, the voltage is called the insulator's rated voltage.
By definition, whether a wire burns or not has little to do with the voltage. Then why does he have to mark the rated voltage? This is because the wire outside the insulating skin has a voltage tolerance range. We can simply understand that when the water pressure exceeds the range of the water pipe, then the pipe will be destroyed, the water inside will spray out. Similarly, when the voltage of the wire more than the range of insulation skin, wire insulation will be destroyed, the current will run out, commonly known as "leakage".
Multimeter and Megohmmeter
A multimeter actually uses Ohm's law to measure resistance. We all know that when measuring resistance, the multimeter is powered by 1.5V and 9V batteries. When the two pens are connected to the resistor, the current in the meter starts from the positive terminal of the battery, then passes through the meter head, the resistor, and then returns to the negative terminal of the battery. Based on the current size of the meter head, you can judge the resistance size, because the voltage is certain, the current size depends on the resistance size.
For measuring the resistance of a conductor, this is perfectly fine; but for measuring an insulator, it won't work, because whether the insulator conducts or not depends on the voltage and temperature. For example, an insulator at 9V is not conductive, so when measured with a multimeter, the meter naturally has no current through the head, so the display resistance is infinite. But if you continue to apply a higher voltage, it may be breakdown conductive. So when measuring whether an insulator is conductive or not, a voltage is specified.
The megohmmeter has an internal hand-cranked DC generator, and depending on the voltage level of the megohmmeter, the output voltage of the generator varies. 250V megohmmeters can emit DC voltages close to 250V, 500V megohmmeters can emit DC voltages close to 500V, and 1000V megohmmeters can emit DC voltages close to 1000V... If you use a 500V megohmmeter to measure the insulation resistance of a particular wire, you are simulating the measurement of whether the wire is leaking under a 500V DC voltage.
If a certain line in megohmmeter 500V measurements do not occur under the leakage, then in the 300V voltage will be even more will not occur under the leakage. So when we choose megohmmeter for measurement, we must ensure that the megohmmeter voltage level is higher than the actual voltage of the line. In addition, megohmmeter is issued by DC, and we commonly used 220V is AC, 220V AC peak can reach 220 * 1.414 = 311 V. So, we must choose 500V megohmmeter in the measurement of AC 220V line insulation.
Multimeter can only be used to measure the size of the resistance of the conductor, measuring the size of the resistance of the insulator or pass or fail must be megohmmeter. Because only megohmmeter can truly react in a certain voltage, the insulator conductive or not! When an object insulator damage is particularly serious, such as a motor coil insulation is seriously damaged, copper wires directly connected together, then the multimeter can also be measured. Because the insulation has been completely destroyed, the connection point becomes a conductor.
