Lightning arrester selection: how should I decide the nominal discharge current?

[debashish]

I have a 3 phase 75kVA,11kV/415V, 50Hz pole mounted distribution transformer. I want to select a lightning arrester but how should I decide the nominal discharge current? Whether to choose 5kA,10kA?

I have also seen many standards all of them don't have any charts for switching surge Values for 11kV. Why so?

I have read many online articles but couldn't understand properly. Can anyone help me with this please.

Thank you.

[Kalbi_Rob]

I have a 3 phase 75kVA,11kV/415V, 50Hz pole mounted distribution transformer. I want to select a lightning arrester but how should I decide the nominal discharge current? Whether to choose 5kA,10kA?

I have also seen many standards all of them don't have any charts for switching surge Values for 11kV. Why so?

I have read many online articles but couldn't understand properly. Can anyone help me with this please.

Thank you.

There are numerous questions that would need to be answered before we could answer your initial question. There are 2 great papers on this, first being:

http://apps.geindustrial.com/publibrary/checkout/Arresters?TNR=White%20Papers%7CArresters%7Cgeneric

If the actual arrester energy duties are not known and a transient study cannot be performed, then it is suggested that Station class arresters be applied.

It would be helpful to know if you are using this on a grounded or ungrounded system, and what transient fault currents the system will/has seen. I am assuming you are looking into using polymer vs porcelain. It is always better to go more conservative with the current rating to ensure ability of the arrestor to survive an incident, but should not exceed the capability of the grounding system.

As for 11kV (an very unusual Nominal Operating Voltage in US), selecting the MCOV examples are given on this website under MCOV Rating section:

https://testguy.net/content/286-Surge-Arresters-Selection-Application-and-Testing-Overview