Announcement

Collapse
No announcement yet.

Ground fault- Why doesn't anyone get shocked?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    #76
    Originally posted by mbrooke View Post
    Hypothetical scenario- What if the substation neutral was earthed via a correctly tuned Peterson coil and high speed clearing? I'd think the current that would pass through the person would be rather small.
    Those are for ungrounded systems. We were discussing MGN systems.

    What person currents did you calculate and for how long?
    BB+/BB=?

    Comment


      #77
      Originally posted by mivey View Post
      It must have been determined that the probability of that happening in conjunctio. with the failure of high-speed clearing is low. I have not researched it.
      But if you can't design a system to clear fast enough to save a person from touching a 4.16kv busbar, then how can a distribution system clear fast enough to protect a person?
      Our comedian shamelessly joked about a blackout. Talk about dark humor.

      Comment


        #78
        Originally posted by mivey View Post
        Those are for ungrounded systems. We were discussing MGN systems.

        What person currents did you calculate and for how long?
        Yes, because the massive amount of current in an MGN produces tremendous neutral to remote earth voltage, where as in an ungrounded system the voltage to remote earth is small during a fault.
        Our comedian shamelessly joked about a blackout. Talk about dark humor.

        Comment


          #79
          Originally posted by electrofelon View Post
          What is that thing? Please elaborate....
          A Petersen coil tries to cancel the capacitive current on an ungrounded system by paralleling an inductive coil.

          The idea is to keep the current low during transient line to ground faults because the coil current is opposite in phase to the capacitive current and thus, ideally, no net ground current due to line capacitance would flow.
          BB+/BB=?

          Comment


            #80
            Originally posted by mbrooke View Post
            But if you can't design a system to clear fast enough to save a person from touching a 4.16kv busbar, then how can a distribution system clear fast enough to protect a person?
            There is a difference between touching a 4 kV line while standing on the ground vs. touching a grounded line in contact with a 4 kV line.
            BB+/BB=?

            Comment


              #81
              Originally posted by mivey View Post
              There is a difference between touching a 4 kV line while standing on the ground vs. touching a grounded line in contact with a 4 kV line.
              Or rather there is no difference touching a 4kv line vs touching the MGN of a 34.5kv line during a phase to MGN fault.
              Our comedian shamelessly joked about a blackout. Talk about dark humor.

              Comment


                #82
                Originally posted by mbrooke View Post
                Yes, because the massive amount of current in an MGN produces tremendous neutral to remote earth voltage, where as in an ungrounded system the voltage to remote earth is small during a fault.
                If you want to change over to discussion of faults on ungrounded systems, that is a whole different discussion. Designs for the two are not the same of course.
                BB+/BB=?

                Comment


                  #83
                  Originally posted by mivey View Post
                  If you want to change over to discussion of faults on ungrounded systems, that is a whole different discussion. Designs for the two are not the same of course.


                  Of course, but electrical theory tells us that the more current you have the more voltage drop you have.
                  Our comedian shamelessly joked about a blackout. Talk about dark humor.

                  Comment


                    #84
                    Originally posted by mbrooke View Post
                    Or rather there is no difference touching a 4kv line vs touching the MGN of a 34.5kv line during a phase to MGN fault.
                    Of course there is. One occurs during a fault while protective devices are reacting and the other occurs with no protective devices reacting.

                    The kV present during a L-N fault on a MGN should normally clear on instantaneous in about 5 cycles or less.

                    The kV present for just touching a line will be there all day and all night.
                    BB+/BB=?

                    Comment


                      #85
                      Originally posted by mbrooke View Post
                      Of course, but electrical theory tells us that the more current you have the more voltage drop you have.
                      Why thank you Capt.
                      BB+/BB=?

                      Comment


                        #86
                        Originally posted by mivey View Post
                        Of course there is. One occurs during a fault while protective devices are reacting and the other occurs with no protective devices reacting.

                        The kV present during a L-N fault on a MGN should normally clear on instantaneous in about 5 cycles or less.


                        5 cycles? Real world often takes longer then that.
                        Our comedian shamelessly joked about a blackout. Talk about dark humor.

                        Comment


                          #87
                          Originally posted by mivey View Post
                          Why thank you Capt.
                          So it is not out of bounds of the discussion we are having and the Peterson coil RJ brought up.
                          Our comedian shamelessly joked about a blackout. Talk about dark humor.

                          Comment


                            #88
                            Originally posted by mivey View Post
                            The kV present for just touching a line will be there all day and all night.
                            Not if all loads are connected in delta and your zero sequence is set very low.
                            Our comedian shamelessly joked about a blackout. Talk about dark humor.

                            Comment


                              #89
                              Originally posted by mivey View Post
                              If you want to change over to discussion of faults on ungrounded systems, that is a whole different discussion. Designs for the two are not the same of course.
                              you mean other earthing systems ?


                              https://en.wikipedia.org/wiki/Earthing_system


                              I always felt it somewhat a challenge getting my head outta the 'nec' box.....

                              ~RJ~

                              Comment


                                #90
                                Originally posted by mbrooke View Post
                                5 cycles? Real world often takes longer then that.
                                Perhaps you have some setting or breaker selection issues. 2-8 cycles is normal total clearing with 5 pretty common.
                                BB+/BB=?

                                Comment

                                Working...
                                X