Many thanks for all the info. I'm always interested in the origins and historical development of different systems.

I ran across your 4-w unbalanced delta here a few months ago in relation to the use of orange to identify the high-leg. I can see how it originated to provide 120 from a simple 240 delta, but it seems quite an odd system from a British perspective. Do you know when these systems were last installed as standard? I think somebody else said that even a regular 240V delta would be rare now and that all new commercial LV supplies are a simple 120/208 or 277/480 Y.

We have some isolated houses on a 1-ph xfmr fed from two legs of an 11kV delta, but the 2-wire residential service here is usually tapped off a 240/415 Y distribution network, the same xfmr often feeding 3-ph power for commercial buildings, as you mentioned for Europe. The 240 nominal became our national standard by the early 1970s; prior to that it varied 200 to 250 in different areas.

Most of Europe standardized at 220/380, although they had some older 127/220 systems in some areas until quite recently. There is now a move toward a common European standard of 230/400.

The main difference in this respect between the U.K. and mainland Europe is that only a very large, power-hungry house would be fed 3-phase here, but on the Continent 3-ph is very common, although often of low power rating (e.g. a lot of older homes in France are fused 20A per phase). Many electric ranges in Europe are 3-ph 5-wire.

I'm not aware of any 1-ph 3-wire AC systems ever being used over here, although 3-wire DC (200/400 to 250/500V) survived in the older parts of some towns until the 1960s.

The "cut-off" point for safety in homes here seems to be regarded as 250V. That was the highest residential utilization voltage under the old 250/440 Y or 250/500 DC systems, and still figures quite prominently in many parts of our code (e.g. 250V max. on regular lampholders).

On the very rare occasions where 3-ph is introduced to a domestic system, the regs. specify quite a number of rules to minimize the risk, such as the requirement that all receptacles in one room be on the same phase.

The use of a greater number of smaller xfmrs in a typical American neighborhood is one of the things I noticed the first time I was over there. Interesting that the primaries on some of these are fed phase-to-neutral; I've not been involved with the HV distribution side of things here, but I'm not aware of any such HV feeds in England.