A multimeter can only measure conductor resistance
A multimeter can only measure the resistance of conductors and cannot accurately measure the resistance of insulators. Only a tramegger can accurately measure the resistance of insulators. Let's talk about why again?
Conductors/Insulators
Conductor: an object with good conductivity
Insulator: An object with poor conductivity (note that it is not a non-conductive object)
Common conductors in our daily lives include copper, iron, aluminum, gold, silver, graphite, etc
Common insulators in our daily lives include plastic, rubber, glass, ceramics, pure water, air, various natural mineral oils, etc.
We should pay special attention here that insulators are objects with poor conductivity, not non-conductive objects. Strictly speaking, objects that are absolutely non conductive do not exist. For example, plastic may be broken down and conduct electricity at high temperatures. So insulators are divided into 5 levels based on their heat resistance temperature: Y, A, E, B, F, H, and C
Similarly, insulators may also be broken down and conduct electricity at high voltages. So, whether an insulator conducts electricity or not is relative to a certain voltage, which is called the rated voltage of the insulator.
In theory, whether the wires are burned or not has little to do with the voltage. Why does he still need to mark the rated voltage? This is because the insulation on the outside of the wire has a voltage bearing 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 spray out. Similarly, when the voltage of the wire exceeds the withstand range of the insulation skin, the insulation skin of the wire will be damaged, and the current will run out, commonly known as "leakage".
Multimeter and megohmmeter
Measuring resistance with a multimeter is actually using Ohm's law. We all know that when measuring resistance with a multimeter, the 1.5V and 9V batteries inside the meter supply power. When two probes are connected to a resistor, the current in the meter starts from the positive pole of the battery, passes through the meter head, resistor, and then returns to the negative pole of the battery. The resistance can be determined based on the current level of the meter head, as the voltage is constant and the current level depends on the resistance level.
For measuring conductor resistance, this is completely no problem; But for measuring insulators, it is not feasible because whether an insulator conducts electricity depends on voltage and temperature. For example, if an insulator is non-conductive at 9V, then when measuring with a multimeter, there will naturally be no current flowing through the meter head, so the displayed resistance value is infinite. However, if a higher voltage is continued to be applied, it may experience breakdown and conductivity. So when measuring whether an insulator is conductive, a voltage must be specified.
There is a manual DC generator inside the megohmmeter, and the output voltage of the generator varies depending on 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 close to 500V, and a 1000V megohmmeter can emit a DC voltage close to 1000V If a 500V megohmmeter is used to measure the insulation resistance of a certain wire, it is simulated to test whether the wire is leaking under a 500V DC voltage.
If a certain line does not experience leakage when measured by a megohmmeter at 500V, then there will be even less leakage at 300V voltage. So when choosing 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 AC, and the peak value of 220V AC can reach 220 * 1.414=311V. So, when measuring the insulation of AC 220V lines, we must choose a 500V megohmmeter.
