Is this a mono-phase distribution or is it two-phase?
NO, Sven, NO! There have been newer-ending threads in some newsgroups on this issue.
My attempt to prevent open warfare:
It should not/can not be referred to as just "two-phase". Two-phase means a system with 90° between the phases. You then need four wires plus earth. The phase-phase voltage is then sqrt(2) of the phase-neutral voltage, not sqrt(3) as with three phase.
Some people will then argue that any phase angle, be it 60°, 90°, 120° or 180°, between two wires and neutral is per definition two-phase. I agree only under condition that you specify the phase angle if this differs from 90°. Thus the 110-0-110V could be called "180° two phase". Two phases plus neutral from a three phase system would then become "120° two phase".
The other group of people will argue that the 110-0-110 is single-phase since it comes from a single transformer. Then the first group will say that single phase means two wires. In my opinion, the term "center-tapped single phase" should be used to describe this system. "Single phase" used alone means two wires. You could call it an Edison system, but then it sounds like a DC system. (Note: You can get three phase from two transformers in open delta configuration. Hence, number of transformers and number of phases need not correspond)
The second group call a service made up of two phases and neutral from a three phase system "network service". I dislike this non-descriptive name, and would prefer to use "120° two phase".
(I wonder if I have added water or gasoline to the fire...)
Yes, I think 230-0-230V is used in a few places, e.g. Australia. (It's on the homepage of some Australian power company)
[This message has been edited by C-H (edited 11-14-2002).]
[This message has been edited by C-H (edited 11-16-2002).]