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For example, I am calculating the number of disks required for a 330kV line. Assuming medium pollution (20mm/kV) and each disk having 405mm would my number of disks be 17 (based off line voltage) or 10 (based off phase voltage)

Chris
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    It depends entirely on what the voltage is across the insulation and you haven't said. – Andy aka Oct 01 '20 at 06:40
  • The insulators are connected to a three phase transmission line. So I guess it'd be the phase voltage as there will be an insulator on each line? – Chris Oct 01 '20 at 07:17
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    What does the insulator connect to on both its ends? If it is suspended from a pylon then it will be ground on one end but there's no clue in your question. Only you know this part of the puzzle. – Andy aka Oct 01 '20 at 07:21
  • My apologies, I thought that was obvious when I mentioned that its a transmission line. Yes it is insulating the tower (ground) from the transmission line. – Chris Oct 01 '20 at 07:55

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From comments: -

What does the insulator connect to on both its ends? If it is suspended from a pylon then it will be ground on one end but there's no clue in your question. Only you know this part of the puzzle.

My apologies, I thought that was obvious when I mentioned that its a transmission line. Yes it is insulating the tower (ground) from the transmission line.

In that case you should consider the worst case scenario phase voltage that might be seen. The normal case is just the phase voltage (as per line voltage divided by \$\sqrt3\$) but, if there is a phase lost during distribution and the sourcing transformer is a delta output (no neutral) then there is a possibility that the insulator might see somewhere between phase voltage and line voltage.

You should also consider that lightning strikes are quite common on pylons and this will make an impact so, is there a top earth wire running from pylon-to-pylon that can partially umbrella protect the lines? These are things you might need to consider as per this: -

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Picture from this answer

Andy aka
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    Thank you. I have considered the lightning and switching impulses, it was just the creepage I wanted clarification on. Yes there is also an earth wire. – Chris Oct 01 '20 at 08:09
  • No problems. I just wanted to make sure about things before answering. – Andy aka Oct 01 '20 at 08:11