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Thread: Floating neutral

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    Floating neutral

    Hi guy's.
    We are in serious need of direction.
    We have installed an installation of a MRI, GE is installing a 80kva ups and the magnets .
    They are complaining of floating neutral from 0.2 ohms to 0 6 ohms that is fro N to E bar.
    Below is the earth loop impedance that we got.
    Cable is a 95mm x 4 core ecc
    Main C/B is 250 amp 37ka.
    Main cable is 95mm x 4 core ecc 75 meters
    Red white blue
    0.16 ohms 0.14 ohms 0.08 ohms
    5.72 ka 1.42 ka 1.69 ka
    234 volt 232 volt 230 volt
    N to E bar is 0.215 volt.
    N to E bar is 0.2 to 0.6 ohms.
    E Continuity is 0.01 ohms.
    All readings are good.
    And.
    Mini-sub to MLVP.
    mini-sub has a 450 amp 20 ka C/B.
    Main cable is 120mm x 4 core in parallel 80 meters
    Red white blue
    0.8 ohms 0.06 ohms 0.07 ohms
    3.28 ka 3.08 ka 1.89 ka
    234 volt 232 volt 230 volt
    N to E bar is 0.1 volt
    N to E bar is 0.00 ohms
    E continuity is 1.13 ohms ( not good )

    Do you think our problem is because of the mini-sub earth continuity which should be less than 0.02 ohms that we are having a floating neutral.
    Although all is looking good but there is an issue
    Has anyone out there have some advice

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    Quote Originally Posted by FRENCHIE View Post
    Hi guy's.
    We are in serious need of direction.
    We have installed an installation of a MRI, GE is installing a 80kva ups and the magnets .
    They are complaining of floating neutral from 0.2 ohms to 0 6 ohms that is fro N to E bar.
    Below is the earth loop impedance that we got.
    Cable is a 95mm x 4 core ecc
    Main C/B is 250 amp 37ka.
    Main cable is 95mm x 4 core ecc 75 meters
    Red white blue
    0.16 ohms 0.14 ohms 0.08 ohms
    5.72 ka 1.42 ka 1.69 ka
    234 volt 232 volt 230 volt
    N to E bar is 0.215 volt.
    N to E bar is 0.2 to 0.6 ohms.
    E Continuity is 0.01 ohms.
    All readings are good.
    And.
    Mini-sub to MLVP.
    mini-sub has a 450 amp 20 ka C/B.
    Main cable is 120mm x 4 core in parallel 80 meters
    Red white blue
    0.8 ohms 0.06 ohms 0.07 ohms
    3.28 ka 3.08 ka 1.89 ka
    234 volt 232 volt 230 volt
    N to E bar is 0.1 volt
    N to E bar is 0.00 ohms
    E continuity is 1.13 ohms ( not good )

    Do you think our problem is because of the mini-sub earth continuity which should be less than 0.02 ohms that we are having a floating neutral.
    Although all is looking good but there is an issue
    Has anyone out there have some advice
    I have no idea. I'm new on UPS installation. Here to learn

    Sent from my SM-A305F using Tapatalk

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    Hi

    What is your earth resistance test from the earth bar in the minisub to ground - You should get a reading of below 1 Ohm .

    If your earth and neutral is bridge in the minisub and your earth to ground reading is acceptable you should be OK.

    As a test you could bridge the neutral and earth at the input to the UPS and see if that settles it - If not then they are missing something on the UPS side.

    What is actually happening - Is the UPS not starting at all , or going into standby when they operate the machine.
    Why are they saying that the neutral is floating

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    Hi GCE.
    Spoken to GE technician today and this is what he had to say.
    He has commissioned the UPS but warned us that we have a floating neutral but that it will not affect their equipment.

    Now the issue is that the client is a bit confused who is right or wrong.

    We are concerned that we have a floating neutral and that we must sorted it out.
    We have done all our test and per the sans they are all perfect reading.
    But saying that GE technician insists that we have very earth, we did loop impedance test and according to our calculations the earth is fantastic, the other thing is that he does a resistance test and reading fluctuate from 0.5 Ohms to 1.3 Ohms continuously and he maintains that that the reading what ever it is should be stable and not erratic, as we said to him our sans doesn't require us to do that test as our voltage neutral to earth is 0.125 Volt and we far below 25 Volt dangerous level and 50 Volt we disconnect the installation.

    But he still maintains that we have bad earth
    We have done our earth continuity test from the mini sub through the MLVP to the MRI isolator which feeds the UPS and its 0.01 Omh with a 100 Meters of 16mm PVC wire.

    Now we get from the 315KVA mini sub earth bar to the ground earth test 6 Ohms, GE say that we must have 0.00 Ohms.

    We have request that a GE fully pledged electrical guy comes to retest the system, as we are not really satisfied with the previous guy demands.

    Do you perhaps think that maybe that we are not being compliant with our 6 Ohms at the transformer earth.

    We have always been under the impression that 0 Ohms to 10 Ohms unless the Engineers requests a special earth requirement of 0 or 1 Ohms.
    Thanks
    FRENCHIE

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    Greetings

    From my experience a ups creates this if you do not star the neutral and earth.

    If you could disconnect the ups and test it would be interesting to see and does the floating neutral happen when in ups mode?

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    Hi Frenchie

    SANS 10142-2 , asks for an earth resistant reading of less then 1 ohm on a mini sub.So the GE tech could be right that his earth is not sufficient .Have pasted reg below

    6.2 Installation of a neutral earth and bonding
    6.2.1 The earth electrode shall be installed as close as possible to the transformer and shall have
    an earth resistance not exceeding 1 Ω.
    6.2.2 The transformer tank shall be bonded to the earth stud and to the substation earth bar.
    6.2.3 The LV neutral of a transformer shall be bonded to the earth stud.
    6.2.4 The earth stud shall be connected to the earth electrode.
    6.2.5 All bonding and earthing conductors shall be of cross-sectional area at least 70 mm2 copper
    or equivalent, unless it can be shown by calculation that a smaller cross-sectional area will suffice.
    6.2.6 Earthing terminations shall be installed individually and in such a way that they can easily be
    accessed and disconnected for routine earth testing.
    6.2.7 The neutral earth resistance shall not exceed 1 Ω.

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    Hi guy's.
    Thanks for all the support.
    There is one thing that bothers me is that if you take an Ohms reading between E & N at the primary if the UPS , you are measuring the wires resistance to the bridge from the point of load to the bridge point of the mini sub of the E & N bus bar and not the ground earth that this guy maintains that we don't have.

    The other thing he said that we have fluctuations of Ohms on the UPS primary side, and the voltage shows constant 0.215 Volt, first of all it is a huge building, what think he is picking up static not big enough to register on volt but big enough to affect the Ohms readings, it is most probably lights, plug in equipment and huge A/C units.

    What does the helpful Panel think.

    Thanks.
    FRENCHIE

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    Hi

    I feel that you are correct in your statements above but if the earth was below 1 ohm you may "drag and sink" the static to ground quicker and thereby avoid the funny readings the GE tech is getting.

    Whatever happens somebody is going to pick up the bad earth resistance reading and blame it .I would correct that first before looking further.

    How old is the minisub installation because generally the earthing improves radically within a 3 month period.

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    Hi GCE.
    Mini sub is brand new.
    I can see where you going with that, I do see that you have made the right suggestion, and we are going to have carried it out.
    GCE thanks for all the information and will keep you informed at a later stage once all the issues are over.

    Thanks again
    FRENCHIE

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    Hi GCE it has been awhile since our chat.
    This is an up date sorry that I took so long for that, it's a long story but this what we have found after our chat.
    We have 800KVA transformer on the one side of the building with permission we had installed a new 315KVA transformer at the other end of this building going through a bus bar coupler in the new MLV room.
    We extended our earth mat another 30 Meters of 70mm BCEW with an additional 4 earth spikes, remember we only had 6 Ohms Originally and the new earth we only managed to bring it down to 5 Ohms due to the bad soil.
    And fluctuations was still there, so we went back to our original 800 KVA transformer and we found no fluctuations at all then our brains went into a flat spin.
    NOW we we realised that our issue was only between the 315 KVA TRF via the MLVP and the GE UPS.
    Then we found our fault because we were so close to another stand where they had their own municipal kiosk feeding a very old house and thier meter was mounted on the outside of the house, we complianced that meter panel and found that they had no earth at all coming from the municipality and the voltage between neutral and earth was very erratic but below 25 Volt, the went to the municipality kiosk and there again they no earth, so we twisted the Neutral and earth together on both side of their installation, and we went back to the GE UPS and at last we had found our GREMLIN and our Ohms reading was at last constant which was only reading our copper conductors..
    We got in touch with the municipality electricity dept and a few days later they sorted out their earthing, and they were grateful that we had told them.

    GCE like to thank you for all your encouragement.

    And we would like to thank the person or people for this forum great help and keep up the informative work.

    Wish all the guys on the Forum a MERRY CHRISTMAS and prosperous 2020.
    Thank you
    FRENCHIE.

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