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    CT Shorting Block connection

    Hi,

    I need to install a ct shorting block on an existing GE EPM 9900 energy meter. Most of the commercially available shorting blocks direct me to tie together and ground the ct negative leads. The GE manual recommends shorting blocks as an option, but the installation block diagram of the energy meter show 3 completely independent ct's. (3 phase 4 wire wye system.)

    Is there any advantage to shorting the negative leads? Any way this could affect the meter?
    I'm installing the shorting block on the next shutdown, I don't want to cause an additional shutdown to correct the wiring. The goal is to be able to replace the meter in the future without a power outage.

    Thanks,
    Andy

    #2
    Shorting blocks prevent individual CT's from becoming PT's when the secondary circuit is opened, which is a result of removing the meter.

    3ph CT secondary circuits normally have the "negatives" tied together and grounded through the supply neutral, like any 3ph wye circuit.

    One has nothing to do with the other.
    Master Electrician
    Electrical Contractor
    Richmond, VA

    Comment


      #3
      It typically does not really depend on the number of CTs or system configuration, but rather on how your instruments are wired.

      Some people prefer to make the ground/common connection as close to the CT as possible, thus all shorting bars just need to be 4-pole style.
      For other CT installations, especially where multiple instruments will be connected in series, it is often better to use 6-pole shorting blocks up to the final device.
      Just because you can, doesn't mean you should.

      Comment


        #4
        We use these at the CT end, all other connections are near the metering equipment.

        Click image for larger version

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        Rob

        Moderator

        All responses based on the 2017 NEC unless otherwise noted

        Comment


          #5
          Originally posted by andy32821 View Post
          Hi,

          I need to install a ct shorting block on an existing GE EPM 9900 energy meter. Most of the commercially available shorting blocks direct me to tie together and ground the ct negative leads. The GE manual recommends shorting blocks as an option, but the installation block diagram of the energy meter show 3 completely independent ct's. (3 phase 4 wire wye system.)

          Is there any advantage to shorting the negative leads? Any way this could affect the meter?
          I'm installing the shorting block on the next shutdown, I don't want to cause an additional shutdown to correct the wiring. The goal is to be able to replace the meter in the future without a power outage.

          Thanks,
          Andy
          Shorting the “negative” together in a multiple array of CT's is a recognized practice. This is the traditional modality in terms of simplicity and even labor-saving that gives an overall economic advantage. ..a darling of contractors.


          Anything to save money.


          I am not keen in calling one leg of the CT terminal “negative” because it gives the impression of a DC source.. . .but since you initiated the post… I will use it for the sake of decorum.


          Generally, when you have a current transformer you should maintain a closed circuit. It could be either a shorting block or the meter must be always connected.
          Leaving the CT secondary leads open--could pose a hazard.. . . a shocking hazard. It acts like a power transformer.
          In a situation where there is a need to replace the meter, it is important to short the two terminals to avoid this shocking hazard.


          The GE recommendation of providing shorting block is a good option. You did not mention the “tying together of negative terminals” in the GE manual.
          For the GE engineers, having three independent energy meters provide flexibility in the field. It insures that each set of leads to the meters can be handled independently without having the other meters accidentally disturbed or disconnected during maintenance.


          Shorting the negative leads or even the two leads should not affect the meters.

          You don't have to shut the power off to replace the meter/s as long as the CT leads are properly shorted before disconnecting the meters.


          All of the above of course is contingent upon the proficiency of the electricians working on the system. . . and your knowledge of the system.

          Comment


            #6
            Originally posted by infinity View Post
            We use these at the CT end, all other connections are near the metering equipment.

            [ATTACH=CONFIG]22096[/ATTACH]
            What is the purpose of the shorting block at the CT’s?
            I have never seen it before, only at the meter.
            Tim
            Master Electrician
            New England
            Yesterday's Technology at Tomorrow's Prices

            Comment


              #7
              Originally posted by tkb View Post
              What is the purpose of the shorting block at the CT’s?
              I have never seen it before, only at the meter.
              In the case of the ones in the photo they serve two purposes, one to provide a splice point for the CT conductors (which were too short to reach the metering equipment) and secondly to provide points within the switchboard section where the CT's could be shorted so that they may be worked on in the future.
              Rob

              Moderator

              All responses based on the 2017 NEC unless otherwise noted

              Comment


                #8
                Originally posted by infinity View Post
                In the case of the ones in the photo they serve two purposes, one to provide a splice point for the CT conductors (which were too short to reach the metering equipment) and secondly to provide points within the switchboard section where the CT's could be shorted so that they may be worked on in the future.
                What kind of work do you anticipate with the CTs that you would be able to do with the CTs connected to the shorting block?

                It seems like an extra unnecessary expense when butt splices would do.
                Out of all of the CTs that I have installed over the years, we never had to go back to do any work on them, just the meters.
                Tim
                Master Electrician
                New England
                Yesterday's Technology at Tomorrow's Prices

                Comment


                  #9
                  STATES test switches work quite well, also.
                  "Just because you're paranoid, doesn't mean they're not out to get you"

                  Comment


                    #10
                    Originally posted by tkb View Post
                    What kind of work do you anticipate with the CTs that you would be able to do with the CTs connected to the shorting block?
                    There have been several project I have had to connect relaying and power quality metering where I was not involved in the installation and the CTs were installed backwards to what I needed. Having access to all 6 CT leads, at a shorting block allowed us to effectively change the CT polarity as needed.
                    Just because you can, doesn't mean you should.

                    Comment


                      #11
                      Originally posted by tkb View Post
                      What kind of work do you anticipate with the CTs that you would be able to do with the CTs connected to the shorting block?

                      It seems like an extra unnecessary expense when butt splices would do.
                      Out of all of the CTs that I have installed over the years, we never had to go back to do any work on them, just the meters.
                      A lot of CT manufacturers don't follow marking conventions when it comes to designating the “polarity” of the secondary.

                      Sometimes, the schematic symbols differ from the actual marking on the CT itself.
                      In the schematic. . . one end of the coil has a square dot. . . on others, it has a solid round dot.
                      This designates where the “positive” lead goes together with the source lead.

                      This is where the face of the CT is marked “source”.

                      The secondary wires (the ones connected to the meter) are color-coded. It is usually black and white. White being “negative”.



                      There is a way to check the polarity in case all these markings are missing. . . by using a 9 volt battery.

                      But you have to know the procedure on how to do this. . .don't just connect your battery to a 480 volt AC.
                      Last edited by roger; 01-22-19, 02:28 PM.

                      Comment


                        #12
                        Just to throw this in, every CT shorting device I've seen installs directly onto the terminals on the body of the CT itself.
                        Master Electrician
                        Electrical Contractor
                        Richmond, VA

                        Comment


                          #13
                          Originally posted by LarryFine View Post
                          Just to throw this in, every CT shorting device I've seen installs directly onto the terminals on the body of the CT itself.
                          And yet probably none of the window style 600V CTs I have ever specified has had integral shorting devices. Which is good because most of the CT placements have not been easily accessible, especially in energized gear.
                          Just because you can, doesn't mean you should.

                          Comment


                            #14
                            It's been years, but I remember a bent metal bar that probably came with the CT, and was removed after connecting meter wiring.
                            Master Electrician
                            Electrical Contractor
                            Richmond, VA

                            Comment


                              #15
                              In line shorting devices ant CT theory

                              In response to the OP I just want to add that there was a case we ran into where the shutdown of load would take
                              a pile of paperwork in order to change the power meter where no shorting block existed. We talked the customer into installing a shorting block to facilitate future repairs.
                              We used an in-line shorting device that used a needle and shorting wires that would pierce the wire when clamped on it to short out the 3 signal wires to neutral coming from the CTs. It really worked slick and allowed us to install a shorting block and new meter without a power shutdown. The devices were then removed without any damage to the control wires. I forget the name of the device but you can google it.

                              The other subject has to do with the open circuit voltage at the CT secondary terminals. I recall Westinghouse stating that depending on the load current thru the CT, the terminals can cause the CT to act as a PT (as others have mentioned) and the voltage can rise to infinity. I can attest to this when I was thrown against a chain link fence of a MV substation when I was installing a test plug on a 50/51 protective relay. The Port Engineer who I was working with replied " What's the matter Tony, did you trip?"
                              He had no Idea what shocked me!
                              Ifyoucan'texplain itsimply youdon'tunderrstanditwellenough- Albert Einstein

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