Dielectric strengths for insulators

Dielectric strength in kilovolts per inch (kV/in):

Material*         Dielectric strength 
=========================================    
Vacuum --------------------- 20            
Air ------------------------ 20 to 75      
Porcelain ------------------ 40 to 200     
Paraffin Wax --------------- 200 to 300    
Transformer Oil ------------ 400           
Bakelite ------------------- 300 to 550    
Rubber --------------------- 450 to 700    
Shellac -------------------- 900           
Paper ---------------------- 1250          
Teflon --------------------- 1500          
Glass ---------------------- 2000 to 3000  
Mica ----------------------- 5000          

* = Materials listed are specially prepared for electrical use