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How circuit breaker electrical system clears ground faults.

  1. Dec 27, 2012 #1
    Hi I am taking generals now but I will eventually take EE degree. I am studying electrical systems now. Can somebody help me on system configuration regarding circuit breakers clearing ground faults?

    In this picture (the bottom figure), if X2 faults at the transformer, would the fault current ever trip the breaker? I know that it should but I just don't see how. The fault by X2 in the picture will travel through a bonding jumper (the short green horizontal piece between the transformer and the disconnect panel), then to the disconnect, then eventually to the X0 of the transformer. The current will cycle like that but will not travel through the breaker. From reading the web page where this picture came from, I know the bonding jumper will make the breaker trip when the X2 faults at the transformer. But I just don't see how. Any help is appreciated.
    250-30A102web.jpg
     
  2. jcsd
  3. Dec 27, 2012 #2
    The fault current DOES go through the CB. The current will go from the X2 terminal to the CB, to the load, to the ground wire and then back to the center tap of the transfromer. This completes the circuit.
     
  4. Dec 27, 2012 #3
    Why does the fault current take the path that has the load resistance when another path with virtually no resistance now became available?
     
  5. Dec 27, 2012 #4
    I'm not sure what you mean. Under normal condition, current flows through the load (represented by the resistor symbol). Under a ground fault condition, current flows through the ground wire. In the picture below, I drew the path of the ground fault current. Very very little current will flow through the load during a ground fault because the ground wire has almost no resistance.

    250-30A102web_zps43453f54.jpg
     
  6. Dec 27, 2012 #5
    My apologies. I used the picture for describing the involved equipments but not for the situation illustrated in the picture. In my situation, the fault happens not at the load, but at the transformer enclosure.
     
  7. Dec 27, 2012 #6

    jim hardy

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    So the pictures don't go with the question in first post?
    Circuit breakers protect downstream equipment................

    Dave90 did a good job with the pictures given.
     
  8. Dec 27, 2012 #7
    I see. If a ground fault occurs at the transformer, then no the CB will not trip. However, the ground fault will trip the CB or fuse ( Not shown in your picture) that protects the primary winding of the transformer.
     
  9. Dec 27, 2012 #8
    So the primary winding's fuses open. Okay I see. Now, what if I got rid of the green wire that electrically bonds the transformer enclosure to the disconnect panel. And then I bond the transformer enclosure to the X0. Would that not produce the same protection, same result?
     
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