Short Circuit Calc Trouble 1

[kingtutley]

I have beat this horse before, but this time it comes from a different source. I hav attached a page from a manual produced by the Army Corp of Engineers for their training. I have a real problem understanding where they are getting the 18.271A for the fault at LC BUS. Nothing I do gets that number or even close.

As I understand it (and this is ALL the information given about this simplistic system), the fault just from the 1MVA transformer would be (1M/(480*sqrt(3)))/.0475 = 25.3kA.

So what am I doing wrong, ,or what did they do wrong, or what am I missing?

Understand, there is NO other information given about the system. This is NOT a homework problem, and I KNOW this is probably very simple.

I ask here because I would rather look like an idiot in front of a bunch of strangers than ask some one in my office and have someone I KNOW think I'm an idiot.

I hope some one here will please answer this, as I really don't understand this.

 

[ijl]

burnt2x, with "see" I mean the following: If there is a three-phase fault at the 480V bus, the fault current from the motor does not reach any of the current transformers.