Arcing Horns

This is incredibly, dangerously wrong!

I don't know where you got 28kV/mm as the breakdown voltage of air (try around 3kV/mm as a better general rule of thumb, and this clearance distance is still much too low when talking about power transmission and distribution systems). Also based on the original poster's information (and previously asked questions on CR4) I believe he/she is after clearance distances used for transmission line arching horns for a 132kV overhead transmission line.

http://en.wikipedia.org/wiki/Arcing_horns

I have not come across these here on our local transmission lines (I think we use surge divertors due to the protection schemes we employ on the network), but given the application (and the fact the distance needs to be correct to properly protect insulator strings from overvoltage transients) you could either look in the electrical standard specifically covering these devices (which will depend on what country the arc horns are being sold in, and may likely be a part of either the insulator standard or the protective device standard) or try an internet search.

Remember, these devices are more than just a simple spark gap and must be designed, tested and certified correctly.