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480 VAC, 3∅ Distribution Question

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    #16
    have you measured that suspect leg in different locations. toward service, midstream in plant and furthest area from service. 1.75 makes me think it might be quite a ways downstream.(impedance of the wire before it connects to ground)

    if 1.75 is close to the secondary of the 13,5 to 480 transformer, and you measure a lower voltage way downstream it probably is closer to where you're measuring the lower voltage

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      #17
      Originally posted by Wire-Smith View Post
      460, 480 and 1.75, to ground & 460-495 b/w legs sounds like wye to me.
      Sounds like corner-grounded Delta to me.
      Master Electrician
      Electrical Contractor
      Richmond, VA

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        #18
        Originally posted by LarryFine View Post
        Sounds like corner-grounded Delta to me.

        true with the fault it looks like it might be corner grounded delta, but i mean what it was intentionally installed to be. i should've been more clear

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          #19
          Originally posted by Wire-Smith View Post
          true with the fault it looks like it might be corner grounded delta, but i mean what it was intentionally installed to be. i should've been more clear
          If it was a wye, there would be a neutral, likely grounded, leading to a fault.
          Master Electrician
          Electrical Contractor
          Richmond, VA

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            #20
            Originally posted by LarryFine View Post
            If it was a wye, there would be a neutral, likely grounded, leading to a fault.
            doesn't have to be a neutral

            http://apps.geindustrial.com/publibr...388B%7Cgeneric

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              #21
              the main transformer bank is likely 3 single phase transformers(being that old), so it would be easy to see wye. Bretzel, are all of the secondaries(one side of each transformer) at the main transformer bank connected at one point? and the three legs you have are connected to the other bushing's of the transformer's and are all alone(only one wire on that bushing and that wire is not connected to the other wires anywhere else). omit the neutral (N) that runs down the pole in the photo




              or again just omit the neutral wire running away from the bank
              Last edited by Wire-Smith; 08-04-18, 07:22 PM.

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                #22
                when the plant was built there was probably not any concern with imbalance, they just had all 3 phase loads. there was no reason for a neutral.

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                  #23
                  The Whys of the WYEs
                  GE 1967

                  http://apps.geindustrial.com/publibrary/checkout/GET-3388B?TNR=White%20Papers%7CGET-3388B%7Cgeneric

                  page 8 section d

                  Comment


                    #24
                    Two things to keep in mind:

                    With a solidly grounded system, the line to ground fault passes enough current to trip the breaker. The contiguous bonding / grounding system limits voltage differences between any (attached) exposed metal to mitigate any shock hazard. Then the breaker trips, removing the fault from the supply, the sense of urgency to fix it becomes immediate because production is down, and the cost of production downtime is the driving imperative. So the safety hazard is removed by automatic operation of the safety device, the location of the fault is indicated by the circuit out, and fixing it is indicated at the highest priority.

                    A floating or ungrounded system supply has one set of issues. The electrical distribution supply without the contiguous bonding / grounding system, using local peg grounding with ground rods but no EGC conductor or effective fault clearing path back to the source, has a different set of issues. You are describing the coincidence of both, floating and no effective fault clearing path back to the source.

                    Floating or ungrounded systems are still legal with marking or signage and ground fault detection (not ground fault trip). Typically the words I see are 'with qualified onsite supervision'.

                    A floating or ungrounded supply, first line to ground fault simply references the system to ground and it can go undetected for a long time. But with no EGC, there can be and probably is, a local elevated voltage potential above the earth level on the exposed metal frame.

                    This would be especially hazardous outside or around a body of water. The man standing on the ground and grabbing a handhold on the structure / ladder / frame could have the full system voltage across him, from the earth level through the fault to the supply transformer. This would be 1000x if he were standing in water and then grabbed the metal that was faulted to the line.

                    Since the fault is not localized, it can be any piece of metal frame or structure anywhere on the site.

                    Since the breaker does not trip, the hazard is neither indicated or removed. The original design intent of this system is that the onsite qualified personnel will find and fix the fault, with the same or better urgency as if the breaker tripped and production went down.

                    So while possibly a legal and grandfathered system, you are missing several components. No awareness of the very specific hazards, no automatically operating safety devices that are normally expected to be present, no indicating or alarming for voltage hazards on normally grounded and bonded exposed metal.

                    There is a lot else going on, possibly passing enough current to show up on your electric bill but not enough to trip a big breaker.

                    What to keep in mind is that a shock hazard likely exists, and when you are normally expecting your breakers to trip for this type of problem, you have a system that by its original design intent, the breakers do not trip. The original design intent is the power stays on for line faults to ground.
                    Lasciate ogne speranza, voi ch'intrate

                    Comment


                      #25
                      Originally posted by LarryFine View Post
                      If it was a wye, there would be a neutral, likely grounded, leading to a fault.
                      Wye has to have a neutral point to function. If neutral was never utilized for any purpose other then to tie the three coils together and never grounded and all that was connected to the source was the three phase conductors, it wouldn't really act much differently then an ungrounded delta system, as long as the neutral remains intact.

                      With same setup but intentionally grounding a phase conductor - it would act a lot like a corner grounded delta.
                      I live for today, I'm just a day behind.

                      Comment


                        #26
                        Originally posted by __dan View Post
                        Two things to keep in mind:



                        A floating or ungrounded supply, first line to ground fault simply references the system to ground and it can go undetected for a long time. But with no EGC, there can be and probably is, a local elevated voltage potential above the earth level on the exposed metal frame.

                        This would be especially hazardous outside or around a body of water. The man standing on the ground and grabbing a handhold on the structure / ladder / frame could have the full system voltage across him, from the earth level through the fault to the supply transformer. This would be 1000x if he were standing in water and then grabbed the metal that was faulted to the line.

                        Since the fault is not localized, it can be any piece of metal frame or structure anywhere on the site.
                        .
                        so do you think corner grounded deltas are installed initially with that hazard?

                        everything has to be bonded like a grounded system, the secondary winding's are just not tied to ground

                        Comment


                          #27
                          Originally posted by Bretzel View Post
                          The physical plant I manage buys power from the POCO at primary. We have our own 3Mw substation. The beast was built c. 1935. It is, unfortunately, an operating electical museum. The secondary is 480v 3∅ Wye, 3 Wire, local grounds. We have a number of stepdown transformers running our buildings. We have a number of rotating loads up to 100 hp. Recently, we have observed some irregular voltage readings. We're seeing 460 to 495 between the three legs. Two legs fairly consistently measure 460 and 480 to ground. The third leg is 1.75 volts to ground. All rotating loads appear to be operating normally. Some transformers, depending on the legs they are connected across, are 120v on one side and 95 - 102 on the other.
                          Its a bit hard to understand the exact setup. From what I gather, your building is served by customer owned primary. This is stepped down to 480, which is then stepped down to 120. So you have multiple seperately derived systems. Each SDS has its own system grounding configuration that will not effect the others' so you need to start by figuring out which building/system/area has the issue.
                          Ethan Brush - East West Electric. NY, WA. MA

                          "You can't generalize"

                          Comment


                            #28
                            Originally posted by Wire-Smith View Post
                            so do you think corner grounded deltas are installed initially with that hazard?

                            everything has to be bonded like a grounded system, the secondary winding's are just not tied to ground
                            If seconadary windings of a separately derived system are not tied to ground - that secondary has no ground reference and is an ungrounded system. With ungrounded systems you still run EGC's and aGEC and bond them to all non current carrying conductive parts - maybe that is what you were trying to say here?

                            Originally posted by electrofelon View Post
                            Its a bit hard to understand the exact setup. From what I gather, your building is served by customer owned primary. This is stepped down to 480, which is then stepped down to 120. So you have multiple seperately derived systems. Each SDS has its own system grounding configuration that will not effect the others' so you need to start by figuring out which building/system/area has the issue.
                            Correct, every time voltage is transformed you start all over with grounding of each individual system. But stray voltage on a grounded conductor that originates on an upstream system can find it's way across the entire grounding network.
                            I live for today, I'm just a day behind.

                            Comment


                              #29
                              Originally posted by kwired View Post
                              If seconadary windings of a separately derived system are not tied to ground - that secondary has no ground reference and is an ungrounded system. With ungrounded systems you still run EGC's and aGEC and bond them to all non current carrying conductive parts - maybe that is what you were trying to say here?
                              My take is he was responding to some of the posts that were implying this is a dangerous situation - it may be, but there is nothing dangerous about ungrounded systems and even first faults - it just becomes a grounded system. Sure you can come up with scenarios where there is a dangerous situation but you can do that with grounded systems too. I found a first fault in an old 600 volt system once. It had prolly been faulted for decades.
                              Ethan Brush - East West Electric. NY, WA. MA

                              "You can't generalize"

                              Comment


                                #30
                                Originally posted by electrofelon View Post
                                My take is he was responding to some of the posts that were implying this is a dangerous situation - it may be, but there is nothing dangerous about ungrounded systems and even first faults - it just becomes a grounded system. Sure you can come up with scenarios where there is a dangerous situation but you can do that with grounded systems too. I found a first fault in an old 600 volt system once. It had prolly been faulted for decades.
                                I can agree with that, what might be dangerous though is not having any EGC system - which from OP's descriptions so far doesn't appear to be any.

                                If this were originally a grounded phase system, then as old as it is, should have been legal to run just three conductors to separate buildings and then ground at each building like you would have grounded a service. Today you can't do that anymore. But don't sound like anything is grounded period, and no ground fault indication either.
                                I live for today, I'm just a day behind.

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