Earthing

[snowplough]

Hi guys,

Wonder if any of you out there could try and explain to me why the earth we stand on that is soil,rock etc is refered to at zero potential. Is it because it is connected to the source back at the star transformer via the earth electrode which is at zero potential, or is it because the earth itself is one massive conductor that cannot be charged up. I find this very confusing and seem to get different answers ever where i look but everthing says the earth is at zero potential, but nothing actually tells you why exactly.

One of the reasons i am asking is because im also trying to get my head around the secondary transformer connected in star. The bottom side of the star point is coonected to the earth and so called is at zero potential?.

I think you can see why i need the earthing question ansering before i can understand the other question regarding the star point.


Many Thanks

Snowplough

 

[Phil Mumford]

Cor ! One for the electrical theory area really. Here is my best shot:

The mass of earth is simply a reference. It doesn't matter if its actually 0V or 100V as long as the star point, which is neutral and earth for our purposes, is connected to it. Then earths/general mass of earth and the neutral are all at the SAME potential. As they are the same (whatever it actually is) you have not potential difference between them and therefore cannot get a belt !

Generators less than 10kVA do not require the frame (or star point) to be earthed. Bigger ones do (must be less than 20 Ohms). The former are considered to 'float' as the star point or neutral may be at a different potential to that of the general mass of earth.

Hope I made some kind of sense !

 

[snowplough]

Thanks Phil,

Im still trying to get my head around it, it seems to throw more questions to me then answers still, my own train of thought was because the neutral was connected to the source (zero potential) that you wont get a belt because the circuit is already complete whever or not its connected to the general mass of the earth. if you just had the star transformer but not connect it to earth, would you have two line wires each having the voltage going back and forth throught the load , (A.C) like an isolated transformer, but the if you connected the bottom line to earth as in the normal transformer then that line then becomes neutral and is said to be at earth potential (zero) , so thats why im thinking because it is connected to earth what difference doe it make.


Many Thanks

Snow plough

 

[Guest123]

Well I'll have me 10pence worth.


Look at it the other way....take the line volatge of 230V....this is only 230V with reference to the zero potential of earth/ground.

If the earth were at 230V then there would be no line voltage as there would be no potential difference between the two.

As we know both neutral and earth connections are derived from the star point in the winding if the supply is to be TN-C-S then the PEN conductor id spiked along it's route to earth to keep it as close to zero potential as possible, again as we know.

Earth must be at zero potential in order for us to have our U and Uo of 400 and 230V

As Phil has touched on with the generator scenario it is what we call a 'floating earth'.
If it's single phase there is no star point nor is neutral strapped to the gen frame so will be at a different potential to earth.

230V is only 230V when referenced to 0V, because the neutral is not earthed and is not at 0V, then the line is not 230V. Between line and neutral you get 230V, but between line and earth and between neutral and earth you get roughly half of 230.

So earth is referred to as 0V in relation to the line conductor in order to give us our volatge.