A tale of woe, including carelessness, stupidity and laziness....

[Jon Elson]

On 08/27/2015 04:27 AM, Peter Coghlan wrote:
> The neutral is defined by being connected to the star 
> point on the transformer secondary as well as being 
> grounded. If it is not (at least intended to be) connected 
> to the star point, it's a ground, not a neutral. It is 
> possible to get a three phase power feed with no netural 
> provided but this is only suitable for use with balanced 
> three phase loads, not a typical building load which 
> includes single phase lighting and power outlets.
There have been a number of crazy systems used in the US, 
generally referred to as "open delta".
One of them uses two single-phase transformers, and the 
common connection between them is grounded.
This, you have two unbalanced 220 V circuits (on different 
phases) with a common point.  Any 3-phase motor or device is 
perfectly happy with this, although one "corner" of the 
three-phase triangle is grounded.  A benefit is you can use 
common split single-phase panels and double-pole breakers 
for it.  This system is called "corner grounded open delta".

Another system uses one center-tapped residential 120/240 V 
transformer and one 240 transformer (or can use two split 
transformers as long as the center tap of the 2nd 
transformer is not used).  The center tap of the split 
transformer is grounded.  This allows you to derive standard 
120/240 V single-phase service AND 240 V 3-phase service 
from only 2 transformers.  But, it gets really confusing, as 
the 3-phase service is grounded at the center between two of 
the 3-phase lines.  You have to use 3-pole breakers on this, 
as none of the 3-phase lines is at neutral/ground 
potential.  This system is called "center-grounded open delta".

Then, the typical 3-phase service can be 120/208 V Wye 
service, with the center tap of the Wye grounded.  This 
gives both 120 V for office equipment and 208 for industrial 
loads.

Also, there is 240 V Delta service, where loads cannot be 
placed between line and neutral.  These transformers 
generally have a small balance transformer that is just 3 
windings with one end all tied together and grounded.  
Often, they have a ground fault interrupter connected to the 
the balance transformer.  If any load draws more than a 
small current to ground, the transformer is shut down.  But, 
you can run unbalanced loads from one line to another.  
Welders and big computer gear were often set up this way.

Jon