The relationship between creepage distance, insulation gap and pollution level

When conducting high-voltage design of battery systems, including the design of high-voltage components and the PCB design of high-voltage circuits in the BMS, creepage distances or how to improve insulation are often mentioned. Most of the time, this is done based on experience. Design, I have learned some knowledge about this aspect intermittently before, but I have never been able to completely understand this point, so I took the time to find the standards I had seen before and carefully sorted out the content in this aspect.

First, find the annotation part in the standard and introduce these contents. Then I will share my understanding with you in the order of my understanding of this issue. Of course, there may be some deviations in understanding, and you are welcome to correct me.

The microscopic environment determines the impact of pollution on the insulation. Some condensation or solid particles and dust will definitely affect the insulation effect if they are bridged at the microscopic level. For example, pollution is conductive when it is wet, and pollution caused by some dirty water, soot, and carbon ash containing metal ions is also conductive. Therefore, the pollution levels of the microenvironment are defined in IEC60664-1 . A total of 4 pollution levels are established here:

——Level 4 pollution: pollution caused by conductive dust, rain or snow that produces persistent conductivity.

For the above classifications, the fourth one is basically not suitable for the scope we commonly use.

It is relatively simple to understand the electrical clearance literally. It means the shortest distance between two conductive parts, or between a conductive part and the accessible surface of the appliance.

The shortest path measured along the surface of an insulating material between two conductive parts, or between a conductive part and an accessible surface of an appliance.

Therefore, from the above definition of terms, we can see that the electrical clearance is based on the spatial dimension, while the creepage distance can be understood as based on the plane. In the GB8898 standard, the calculation of electrical clearances and creepage distances is also introduced in detail based on the previously mentioned concepts. One of the more special ones is for the case with narrow trenches. The creepage distance and electrical clearance are the same. .

For this kind of narrow trench, the condition is that the measurement path of the creepage distance includes a trench with any depth, width less than Xmm, and groove walls that are parallel or contracted. The measurement rule is to measure creepage distances and clearances directly across the trench.

The value of X is strongly related to the pollution level. The corresponding relationship is as shown in the following table:

For some insulation grilles and insulation trenches that we often use in the design of electrical components, how do they increase the creepage distance through some means? Two commonly used methods are digging trenches and vertical grilles.

For digging trenches, first of all, narrow trenches are not enough, and wide trenches need to be dug. For wide trenches, the creepage distance is obviously longer than the electrical clearance. (The dotted dot is the creepage distance, the solid line is the electrical clearance)

For the operation of adding ribs, the electrical clearance is the straight-line distance connected after crossing the top of the ribs, while for the creepage distance, it is the road extending along the edge contour of the ribs. Therefore, it can be seen that whether it is digging trenches or adding ribs, the creepage distance can be greatly increased.

So what impact do creepage distances and clearances have on our design? In the standards mentioned above, they are all defined. Both clearance distances and creepage distances are set according to our voltage level. Specifically The corresponding relationship is as shown in the following table:

Basic insulation, supplementary insulation, and reinforced insulation are mentioned in the electrical clearance. What do these mean? Directly copy the parts defined in the standard as follows:

For the design of our PCB or electrical components, creepage distance is actually more commonly used. The relationship between the creepage distance under basic insulation defined in the creepage distance and the working voltage is relatively clear. It can also be found in the standard, and can also be found in the standard. See this content more clearly.

In the table of creepage distance, a new concept is mentioned, that is, material group, and then the definition of material group is obtained through standard search as follows:

For the FR4 boards we generally use for PCBs, they generally belong to the IIIa material group.

However, for the creepage distance of the PCB board, due to the addition of the solder resist layer and the fact that it is in the plane layer, special instructions are given for the insulation distance in GB8898, and a very detailed formula is used to define the working voltage and creepage distance. Require.