Ka Rating on breakers

[dewone]

Ever notice that the 7.2 KV breakers and the 15 KV breakers are the same except the nominal voltage is lowered and the Ka rating increased. Been wondering why it gets a higher Ka value?

 

[Senselessticker]

In theory: Imagine if you place a transformer between a 15kV system and a 7.2kV system, the short circuit kAmps on teh secondary (7.2kV side) of the transormer will be greater than the short circuit current on the primary (15kV side).

 

[davidbeach]

The amount of current that can be interrupted is related to the voltage across the gap between the contacts. The more gap, in volts/length, the more current. So, with the same gap length fewer volts is the equivalent of more length for the same voltage.

 

[dewone]

I see so thats where they re-rate the breaker at a high Ka. 15 Kv ussually has around 7 3/8" gap or contact seperation. 5 Kv is around 5 3/4. Yet the 7.2 Kv is at the 7 3/8".
Thank you,
Dave DeWitt