Specifications for outdoor 15kV post insulators - how do you specify creepage distance? 3

[bdn2004]

We're looking over the specifications for some replacement post insulators that a vendor has quoted. Our operationg voltage is about 15kV. He qouted one that is rated 15kV and the creepage distance is about 20". He also quoted a 35kV insulator whose creepage distance is 31". Can someone explain what this is referring to...? And how do we know we are getting the right one?

 

[magoo2]

If you take a string and run it over (press it against the insulator) the insulator surface from the top (where it connects to a conductor) to the bottom (where it connects to a bracket or ground), the lenth of the string will be the creepage distance. This distance is particularly important when you have a source of contamination because it will build up on the surface of the insulator and reduce its ability to operate properly at that voltage. With contamination like salt, it is common to specify a higher rated insulator since this gives a greater leakage distance.

It is not the straight line distance from top to bottom which is referred to as the strike distance.

 

[7anoter4]

See:
epri-Red Book Table 3.6-4
IEC-815

http://my.safaribooksonline.com/boo...lution-control/643_calculation_of_specific_cr

Ch.6 Insulator Table 6.3
ewh.ieee.org/soc/pes/iwg/Publications/Insulators101PanelFinalA.ppt