Engineering Pro Guides Question 12 Protection

[eatsleep]

Hey everyone, hope studying is going well. I have a question about the below problem. I must not be understanding what happens during a single phase to ground fault. I thought that one of the phases would be shorted to ground and the other two would be dead in this case IB & IC = 0. Based on the scenario below, i would think that a fault between a1&a2 would mean there is current flowing to ground through phase a but not through phase's b and c.

I am also confused as to what is happening on the primary side. 

Thanks!

 

[ZW_Pub_Power]

Ok,

I thought that one of the phases would be shorted to ground and the other two would be dead in this case IB & IC = 0. Based on the scenario below, i would think that a fault between a1&a2 would mean there is current flowing to ground through phase a but not through phase's b and c.

When a1&a2 is shorted that means the current that was being measured at point a has been sent to ground. You will definitely not read anything at point a. Further more, during a single phase to ground fault all currents go to zero but the ground fault current and the source current feeding the fault. 

On the source side A & B contribute to the winding that sources the fault so you will measure current on both those phases.