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I have following reading of voltages:

  1. RY 397V

  2. RB 398V

  3. YB 393V

  4. RN 231V

  5. YN 227V

  6. BN 228V

  7. RG 386V *

  8. YG 385V *

  9. BG 10V *

  10. NG 216V *

If you clearly observe data above phase-to-phase voltages are good and phase-to-neutral voltages are also good, but when I measure phase voltages with respect to ground unusual value is observed which should be actually somewhat equal to phase-to-neutral voltage. Also the neutral-to-ground voltage is too high, but should be minimum. Now the problem is it is not affecting the normal electrical equipment, but highly sensitive equipment like UPS doesn't take it as a healthy voltage. So what might be the cause behind it?

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  • \$\begingroup\$ Neutral to ground isn't connected but blue to ground is at some distance away maybe. \$\endgroup\$ – Andy aka Mar 26 '16 at 12:01
  • \$\begingroup\$ Did something (like a load switching on) add a huge phase imbalance after you made the phase-phase measurements? \$\endgroup\$ – user16324 Mar 26 '16 at 12:59
  • \$\begingroup\$ I´d start by putting a load, eg 100W incandecent lamp between Neutral and Earth and see what happens. \$\endgroup\$ – F. Bloggs Mar 26 '16 at 13:03
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    \$\begingroup\$ It looks to me as though the blue phase has been grounded at the transformer instead of neutral. \$\endgroup\$ – brhans Mar 26 '16 at 13:22
  • \$\begingroup\$ @brhans In that case, would one not observe a higher voltage (400 V) between the other phases and "neutral"? \$\endgroup\$ – Jonas Schäfer Mar 26 '16 at 13:26
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. The likely cause of the low voltage reason on the blue phase.

It sounds as though your supply is from a star (European term) or wye (American term) connected transformer. Normally the star point, marked 'N' for neutral, is connected to earth to "neutralise" it.

In your case it seems as though either

  • the blue phase has been earthed for some reason or
  • that the transformer secondary star / wye point was not grounded but has now developed an earth fault on one phase.

The second arrangement can be used in the right circumstances to give single-fault tolerance of a ground fault. The circuit will continue to operate (as you have noticed) until a second ground fault occurs. When this is done it would be normal to add ground fault detection to warn of the event. Three bulbs in place of the voltmeters would suffice: when a ground fault occurs that lamp would go out.

The problem with this arrangement is that devices rated for 230 V to ground now have 400 V to ground. This may stress the insulation, causing early failure, and also provides a higher shock hazard to maintenance personnel.

schematic

simulate this circuit

Figure 2. Earthing of star / wye point on transformer. This is standard European practice.

I'd recommend understanding the design intent, examining the system and repair, if required.

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  • \$\begingroup\$ Thanks for your attention. with ur circuit diagram i'm clear that the fault may be, because of earth fault in blue phase. But as you draw two cases, my question to 1st case is :Any where in the circuits node (in distribution wire) earth fault to blue phase can lead this problem? and question to 2nd case:"that the transformer secondary was not grounded" ,which part of transformer secondary you are taking about, is it Neutral Grounding Or Body Grounding? \$\endgroup\$ – Sahan Mar 26 '16 at 16:02
  • \$\begingroup\$ (1) The earth connection on the blue phase could be anywhere. That's the problem: you have to find it! It could be damaged insulation, etc. (2) I've clarified my answer. The star / wye / neutral point is the normal earthing point - otherwise the neutral won't be neutral. \$\endgroup\$ – Transistor Mar 26 '16 at 18:37
  • \$\begingroup\$ the problem was solved , it was because of loose connection in blue phase. thanks every one for participation \$\endgroup\$ – Sahan Mar 28 '16 at 6:45

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