ESD Diodes Selection: How do I understand what Ipeak Current to look for?

Hi,

I was going through the datasheets for ESD diodes and couldn't make out what Ipk is to select for my application. I see that most to the ESD diodes such as ESD321 comply to the IEC 61000-4-5 8/20 surge current. But they still have varying Ipkk values that each can tolerate . Why is this so?

I understand that the IEC 61000-4-5 8/20 specifications exceed much more than what an actual ESD strike can cause (am i correct?) , How much voltage can I practically expect to see? and What is the approximate peak current that I could see at a voltage strike in a real-life ESD strike?

I have also come across that fact that we have to derate ESD diodes based on temperature. How is this done?

Thanks ,
Tony