reg:potential difference between phase nad neutral

[subhu81]

Hello Evrybody,

i am new to this group,i have a doubt which i faced during my commissioning activities in site.
these are the following things which i observed.

Incoming UPS Voltage:

Potential Difference between Phase and Neutral:230V
Potential Difference between Phase and Earth:230V
Potential Difference between Neutal and Earth:0.2v

after UPS we have a isolation transformer:after the isolation:

Potential Difference between Phase and Neutral:230V
Potential Difference between Phase and Earth:0v
Potential Difference between Neutal and Earth:0v

i cannot understand how the p.d between Phase and Earth will become 0v?pls help me.

[Tomi Engdahl]

The results you got makes perfect sense.
The reason why you got the numbers like you have in your measurements lies in the way how the isolation transformer works and how you made the measurements.

The output of isolation transformer is a floating power source. There is no connection to the ground at all through anything. When you make one connection with your multimeter between the "phase" side of isolated source, you will not get any voltage because you are essentially measuring an open circuit consisting of your grounding wire - multimeter - floating 230V output - and open circuit here. No current floating, no measurable voltage withmultimeter. Same thing happens with "neutral" side.
The isolation works. Any single one connection from either side of the isolated output will give you 0V to multimeter.


If you have two identical multimeters, and connect one from "phase" to ground and other from "neutral" to ground, you would get reading of 115V on both of them. The sum of those is that 230V. In this case your measurement circuit would consist of loop consisting of follwoign parts: floating 230V power source - multimeter 1 - ground - multimeter 2 - other end od 230V power source

If your multimeters are not ideantical, then the voltages you get on them can be different, but the sum of measured voltages would be always 230V AC within your measurement accuracy.

[subhu81]

thanks for the reply,but the transformer body ground is grounded with electrical earth whether this will make a difference for the above issue or not ?please clarify?

[Tomi Engdahl]

Grounding transformer body should not make any difference at least with almost all transformers in the world. The transformer body is normally built in such way that it is well electrically isolated from all transformer coils.

There could be some special transformers where some part of transformer scondary could be intentionally wired to the transformer body. I think that the pole transformers used in USA to supply power to houses (convert few kW AC to center tapped 200V AC (110V + 110V) AC) are constructed in such way that the center point of secodnary is connected to the ground/neutral connector onthe transformer, that makes also contact with the transformer body/case.

[Guest]

Hello everybody,

Can anyone explain me about the 0V ground systems.

[Tomi Engdahl]

Hello everybody,

Can anyone explain me about the 0V ground systems.

In the typical plant or house, the 0V ground reference is most often a heavy gauge copper wire embedded around the base of the building and tied into ground rods at the corners as well as into the AC ground feeds at critical junctures.