Relationship between insulation strength and insulation resistance

Insulation resistance refers to the ratio of the voltage to the current generated by the polarization of the dielectric under the specified voltage applied between the power line and the ground terminal. Its unit is ohm. The insulation strength refers to the maximum voltage applied between the power line and the ground terminal within a specified time without destroying the insulation of the equipment. The unit of insulation resistance is ohm, and the unit of insulation strength is volt. This is very intuitive. Since the premise of the two is different, it cannot be said that the greater the insulation resistance, the higher its insulation strength.

Insulation resistance refers to the properties of the insulating material itself. It basically conforms to Ohm's theorem. The measurement method is to apply voltage U, measure the current I flowing through it, and then calculate the insulation resistance R=U/I.

The insulation resistance R has nothing to do with the size of the applied voltage. Of course, if the voltage is too large, it will cause the insulation material to be broken down and damaged. So it is said that it is independent of voltage under certain conditions (no breakdown). If the applied voltage is too large, it cannot be said that the insulation resistance has nothing to do with voltage.

The insulation resistance of insulating materials is related to many factors, such as temperature, humidity, whether the material is damp, and also to the duration of the applied voltage, the frequency of the voltage, time and other conditions. The insulation strength of insulating materials refers to its ability to withstand voltage or electric field strength. A voltage is applied to an insulating material sample, and the voltage is gradually increased until the insulation is damaged or broken down. This voltage is called its breakdown voltage. This voltage is divided by the electrode spacing to obtain the electric field strength, which is its breakdown field strength.

Therefore, insulation strength can also be called electrical strength or breakdown strength.

Of course, the breakdown strength and breakdown voltage are related not only to the thickness of the material (electrode spacing), but also to the shape of the electrode, or the uniformity of the electric field and other factors. Therefore, the insulation resistance and insulation strength of insulating materials are two parameters or terms with different concepts. They have no direct relationship. A large insulation resistance does not mean a high insulation strength.

Let's take a common example to illustrate:

A very small air gap, such as 1mm, has a very high insulation resistance, which is infinite, but the breakdown voltage of this small gap is not large.

A piece of insulating paper is very thin, and its insulation resistance is also very large, which can reach several gigaohms, but its breakdown voltage is only a few hundred or several thousand volts.

And a long piece of wood, such as 1 meter long, when it is damp to a certain extent, its insulation resistance is not high, but its impulse breakdown voltage is still very high.