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

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