1，Insulation materials in the electric field will also be destroyed due to its insulation strength and lose due insulation performance, then there will be insulation breakdown phenomenon.

Standards GB4943 and GB8898 stipulate electrical clearance, creepage distance and insulation penetration distance according to existing research results, but these media are affected by environmental conditions，For example, temperature, humidity, air pressure, pollution level, etc., will reduce insulation strength or failure, among which air pressure has the most obvious effect on electrical clearance.

Gas produces charged particles in two ways: one is collision ionization, in which atoms in a gas collide with gas particles to gain energy and jump from low to high energy levels. When this energy exceeds a certain value, atoms are ionized into free electrons and positive ions。The other is surface ionization, in which electrons or ions act on a solid surface to transfer enough energy to the electrons on the solid surface, so that these electrons gain enough energy, so that they exceed the surface potential energy barrier and leave the surface.

Under the action of a certain electric field force, an electron flies from the cathode to the anode and will undergo collision ionization along the way. After the first collision with the gas electron causes ionization, you have an extra free electron. The two electrons are ionized by collisions as they fly towards the anode，So we have four free electrons after the second collision. These four electrons repeat the same collision, which creates more electrons, creating an electron avalanche.

According to the air pressure theory, when the temperature is constant, the air pressure is inversely proportional to the average free stroke of electrons and the volume of gas. When the height increases and the air pressure decreases, the average free stroke of charged particles increases, which will accelerate the ionization of gas, so the breakdown voltage of gas decreases.

The relation between voltage and pressure is：

Thereinto：    P—The air pressure at the point of operation

      P₀—standard atmospheric pressure

      U_p—External insulation discharge voltage at the operating point

      U₀—Discharge voltage of outer insulation at standard atmosphere

      n—Characteristic index of external insulation discharge voltage decreasing with decreasing pressure

As for the size of the characteristic index n value of the external insulation discharge voltage decreasing, there is no clear data at present, and a large number of data and tests are needed for verification, due to the differences in test methods, including the uniformity of the electric field，The consistency of environmental conditions, the control of discharge distance and the machining accuracy of test tooling will affect the accuracy of test and data.

At lower barometric pressure, the breakdown voltage decreases. This is because the density of the air decreases as the pressure decreases, so the breakdown voltage drops until the effect of decreasing electron density as the gas becomes thinner works。After that, the breakdown voltage rises until the vacuum can not be caused by gas conduction breakdown. The relationship between pressure breakdown voltage and gas is generally described by Bashen’s law.

With the help of Baschen’s law and a large number of tests, the correction values of breakdown voltage and electrical gap under different air pressure conditions are obtained after data collection and processing.

See Table 1 and Table 2

Table 1 Correction of breakdown voltage at different barometric pressure 

Table 2 Correction values of electrical clearance under different air pressure conditions 

2， Effect of low pressure on product temperature rise.

Electronic products in normal operation will produce a certain amount of heat, the heat generated and the difference between the ambient temperature is called temperature rise. Excessive temperature rise can cause burns, fire and other risks,Therefore, the corresponding limit value is stipulated in GB4943, GB8898 and other safety standards, aiming at preventing potential dangers caused by excessive temperature rise.

The temperature rise of heating products is affected by the altitude. The temperature rise varies roughly linearly with the altitude, and the slope of the change depends on the structure of the product, the heat dissipation, the ambient temperature and other factors.

The heat dissipation of thermal products can be divided into three forms: heat conduction, convection heat dissipation and thermal radiation. The heat dissipation of a large number of heating products mainly depends on convection heat exchange, that is, the heat of heating products depends on the temperature field generated by the product itself to travel the temperature gradient of the air around the product. At the height of 5000m, the heat transfer coefficient is 21% lower than the value at sea level, and the heat transferred by convective heat dissipation is also 21% lower. It will reach 40% at 10,000 meters. The decrease of heat transfer by convective heat dissipation will lead to the increase of product temperature rise.

When the height increases, the atmospheric pressure decreases, resulting in an increase in the coefficient of air viscosity and a decrease in heat transfer. This is because air convective heat transfer is the transfer of energy through molecular collision；As height increases, atmospheric pressure decreases and air density decreases, resulting in a decrease in the number of air molecules and resulting in a decrease in heat transfer.

In addition, there is another factor affecting the convective heat dissipation of forced flow, that is, the decrease of air density will be accompanied by the decrease of atmospheric pressure.The decrease of air density directly affects the heat dissipation of forced flow convection heat dissipation. Forced flow convection heat dissipation relies on air flow to take away heat. Generally, the cooling fan used by the motor keeps the volume flow of the air flowing through the motor unchanged，As height increases, the mass flow rate of the air stream decreases, even if the volume of the air stream remains the same, because the density of the air decreases. Since the specific heat of air can be considered a constant over the range of temperatures involved in ordinary practical problems,If the air flow increases the same temperature, the heat absorbed by the mass flow is less will be reduced, the heating products are adversely affected by the accumulation, and the temperature rise of the products will rise with the reduction of atmospheric pressure.

The influence of air pressure on the temperature rise of the sample, especially on the heating element, is established by comparing the display and the adapter under different temperature and pressure conditions, according to the theory of the influence of air pressure on temperature described above，Under the condition of low pressure, the temperature of the heating element is not easy to disperse due to the reduction of the number of molecules in the control area, resulting in local temperature rise too high.This situation has little effect on non-self-heating elements, because the heat of non-self-heating elements is transferred from the heating element, so the temperature rise at low pressure is lower than at room temperature.

3. Conclusion 

Through research and experiment, the following conclusions are drawn. Firstly, by virtue of Baschen’s law, the correction values of breakdown voltage and electrical gap under different air pressure conditions are summarized through experiments. The two are mutually based and relatively unified；Secondly, according to the measurement of the temperature rise of the adaptor and the display under different air pressure conditions, the temperature rise and air pressure have a linear relationship, and through statistical calculation, the linear equation of temperature rise and air pressure in different parts can be obtained. Take the adaptor as an example，The correlation coefficient between temperature rise and air pressure is -0.97 according to the statistical method, which is a high negative correlation. The change rate of temperature rise is that the temperature rise increases by 5-8% for every 1000m increase in altitude. Therefore, this test data is for reference only and belongs to qualitative analysis. Actual measurement is needed to check the characteristics of the product during specific detection.