Air gap clearance between busbars

Hello everyone,
I am new to this form and hope to learn more about arc flash safety practices. My questions are :
1- What is the allowable air gap clearance between live busbars (line-to-line) in 600V system (indoor).
2- Is there any possibility for air ionization between live busbars which may cause arc flash in 600 V, what is the air gap clearance which can cause this.

Thanks