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Trace spacing and creepage

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Ok, so I'm working on a new IGBT design with working voltage of 3200V.
I need some clarification on material group as mentioned on a few of the standards.
In determining creepage I need to know the material group, in my case there are no components on the PCB, so does this refer to the PCB as "material"?
If I then pick my boards CTI of 200 I'm in group IIIa, and my pollution degree is 2, this will then be having me using creepage of 32mm, which is insane unless I pass a test to be qualified as pollution degree I? Is this correct?

And I'm using:

UL_60950-1 Table 9 and Table 2N – Minimum creepage distances Continued

Material group classification Material group Comparative tracking index
CTI I 600 ≤
CTI II 400 ≤ CTI < 600
CTI IIIa 175 ≤ CTI < 400
CTI IIIb 100 ≤ CTI < 175

Tony Stewart

Well-Known Member
Most Helpful Member
It is correct and that's why they use air gaps on boards. Air is 3kV/mm the same as a pure clean surface and then it drops to 500V/mm in a dusty moist contaminated surface.


Well-Known Member
Most Helpful Member
Yep, that looks like 32mm creepage. However, you can decerase the physical distances by routing out the board between the potentials and having a longer creepage distance.
And about PD1, unless your board is in a hermetically sealed box I wouldn´t even think about it. Don´t forget that high voltage naturally attracts dust particles and deposits them around high voltage parts. Ever seen the inside of a CRT?


Active Member
I agree with the above comments. Also remember that if you put an air gap in it needs to be sized to the PEAK voltage as flashover (which drive clearance distance) is a short term effect. Tracking and material breakdown (drives creep) is a longer term effect so only need to consider working voltage.
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