Sizing surge arrestors for lightning

[kaspor]

Hello

I am trying to specify BIL ratings for some equipment based on lightning strikes. In order to mitigate risk of equipment damage, I am installing surge arrestors around critical pieces of equipment. To understand the BIL rating, I need to understand the maximum 8/20uS current wave, which will determine the maximum surge arrestor discharge voltage. I can then ensure equipment BIL ratings are greater than this. There is a lightning protection system (i.e. LPS), but there is always a risk that the strike will bypass the system (i.e. LPS shield failure scenario) and this equipment is very critical.

The issue I have is I cannot find what the max value of the 8/20uS current wave is. Plenty of standards talk about what the anticipated level based is on strike distance then specify a probability that it will be exceeded (i.e. IEE998 standard).

Wanted to understand what others do. If the minimum based on a strike distance of X is say 5.4kA, do you just double it and say there's a low risk of it ever being this high? And you just use a conservative strike distance to ensure you're well above the tallest asset?

 

[bacon4life]

Direct strikes to equipment may cause equipment failures regardless of surge arrestors.

I live in an area with very little lighting, so we have very little shielding. A direct strike at one of our substations is likely to cause equipment failure. We have spare equipment that can be used to replace critical equipment regardless of whether the loss was due to lighting or some other cause.

 

[jghrist]

There is no maximum stroke current. The probability that the stroke current will be higher than I (in kA) is:

P(I) = 1/[1+(I/31)^2.6]

IEEE Std 998-2012 Equation 9