On Sat, 07 Nov 2015 19:36:23 -0000, wrote:

On Saturday, 7 November 2015 16:42:25 UTC, Tough Guy no. 1265 wrote:
I looked this up, I'm asking the question at the top. The replies don't seem to be able to agree. Any sensible opinions?


Accepted convention is that the body of an article should include the information in the Subject. Note that writing "I'm asking the question at the top" takes more effort that duplicating the subject by copy'n'paste. Also, the question was never at the *top* here and is at present not displayed.

It's in the subject.

"Isolated mains voltage - why not as standard?"
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The supply under the street, or equivalent, is three-phase. Three adjacent houses will normally be on three different live phases, with a common neutral. Less copper is buried that way.

I just drew a simple diagram with a substation at the West end of a street, followed by three houses moving East, on phase 1, 2, and 3.
Both the existing system and the isolated system all use the same amount of copper, as the neutral still goes to all the houses. The only difference is whether the neutral is lashed to the ground in the substation. Eg. my house has two conductors coming to it - live and neutral/earth. The only way that the existing system would use less copper than the isolated system is if the neutral was not in the wire, but relied on the ground to carry it from the house's earth rod to the substation's earth rod. This isn't done, the neutral conductor is always laid.

Providing an isolated voltage would require a power transformer for each property, or a multi-secondary one for each small group of properties, since a supply cannot be guaranteed to be isolated in property A if it is in full parallel with property B.

What do you mean by this? Are you taking about property A and B being on different phases? What is the problem?

If an isolated supply is connected to equipment which generated a higher voltage internally, such as a CRT needs, then an internal fault could raise the floating supply to a high voltage, which would be dangerous throughout the premises.

Impossible for three reasons:
1) How would a circuit be able to pump a higher voltage back into what is supplying it with energy?
2) There is so much current available from a substation, that it would clamp it to 240 volts.
3) If it was possible, then it could also happen with a non-isolated system.

Persons who think they have a need for an isolated supply can, with due care for the Building Regulations, use an isolating transformer where required.

Since builders use them, isolating transformers clearly make things safer, so I see no need to pay attention to any regulations.

--
There once was a time when all people believed in god and the church ruled. This time was called the Dark Ages. -- Richard Lederer