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Practical Engineering: Material Group Selection and How it Affects Spacings

Introduction

Often overlooked during the development of appropriate spacings (creepage distances) for safety-certified products is the failure to account accurately for the material group of the components involved. This oversight can have significant implications. Let us briefly explore this issue to raise awareness among readers.

Material Group Review

One of the items from which creepage distance is determined is the Comparative Tracking Index (CTI) rating of the insulating material of the component. Below is a list of the material group number and its associated CTI value obtained in accordance with IEC 60112.

Material Group CTI Rating
I 600 ≤ CTI
II 400 ≤ CTI < 600
IIIa 175 ≤ CTI < 400
IIIb 100 ≤ CTI < 175

 

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As the table above indicates, Material Group I has the highest CTI rating, while Material Group IIIb ranks as the least favorable.

Pro Tip: Components with Material Group I CTI ratings are the preferred choice for highly reliable applications.

Pro Tip: Before finalizing the use of a component, consult the manufacturer’s datasheet to ascertain the specified material group. If this information is unavailable, consider contacting the supplier for clarification or exploring alternative parts from different suppliers.

Bonus: For glass, ceramics, or other inorganic insulating materials that do not track, maintaining creepage distances is not required.

Base Test Case

To best see the effects that material group selection has on creepage distances, let us use a best test case using a working voltage (root-mean-square or direct current) of 300 V, a pollution degree 2 environment, an insulation location of “other insulating materials” and a location where reinforced or double insulation is required. Spacings are derived from UL 61010-1 Third Edition.

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Results for Material Group III

Starting from worst to best, here are the results for the case where the component selected has a CTI failing under the Material Group III category:

Results for Material Group III

In the above, that light green denotes input to the spacings calculator, and pinkish reflects the required creepage distance in millimeters (mm).

Results for Material Group II

Results for Material Group II

Results for Material Group I

Results for Material Group I

Summary of Results

Material Group Required Creepage Distance (mm)
I 3.0
II 4.17
III 6.00

 

Conclusion

As demonstrated above, the required creepage distance for a component with a Material Group III CTI rating is twice that of a component with a Material Group I rating! This result may be acceptable or problematic depending on the specific application and whether your design accommodates these doubled spacings. Carefully considering the material group of components well in advance of your design layout is crucial. Imagine being locked into using a particular part, only to discover later that it has a Material Group II or III CTI rating, while your available space only allows for a Material Group I part.

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