Creep Distance and Contamination Levels

By Rodolfo Elizondo


Different contamination levels require special consideration when designing or specifying electrical distribution equipment.  This is critical to ensure products will perform safely over time.  Specifying engineers need to define not only the type of product they will be using in the application, but also the environment where the equipment will be installed.


In the world of insulators, it is important to understand the relationship between creep distance & contamination levels. Creep distance is the shortest distance along the surface of the insulator between the conductive ends of the insulator, basically it is the shortest path between the energized end and ground through the external shape of the body.  Contamination is any substance which does not belong to the original component design for example dust, fungus, paint, pollutants etc.


So, let’s imagine that we have a clean insulator body with no contamination.  After performing the appropriate dielectric testing, the insulator should operate as designed.  However, if we take that same insulator and cover it in contaminants which have different dielectric properties than the insulator, the dielectric properties of the insulator will be overridden by the dielectric properties of the contamination.  This will create an unplanned electrical path between the high voltage and the ground which will cause a premature failure of the insulator.


This type of situation happens in the field on a daily basis.  Every installed insulator, over time, will acquire a small layer of dust and other contaminants which eventually could create a conductive path between high voltage and ground.   This is why it is recommendable to use insulators with a higher creep distance for applications where the contamination levels are higher.  The longer the creep distance, the lower the opportunities are to have a premature insulator failure.


IEEE recognizes four contaminations levels:


Class I (Light): Areas without industry and low population density.

Class II (Medium): Areas where the industry does not produce pollution or with high populated areas but combined with frequent winds or rain.

Class III (Heavy): Areas with a high industry a population Density where pollution is present every day, it could also be the areas close to the ocean or with winds from the ocean.

Class IV (Very Heavy): Areas subject to conductive dust or pollution, close to the coast or where there is no rain for long periods of time.


For every contamination class, a minimum creepage distance (in mm) is recommended per kV:


Pollution Level mm/kV of line to line Voltage mm/kV of line to ground Voltage
Class I 16 28
Class II 20 35
Class III 25 44
Class IV 31 54


Keep in mind, these are recommendations only.  Conditions in your area may be different.  Always air on the conservative side for creepage distance.


For more information about this subject, please review following Standards IEEE C37.40 and IEEE 100.


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