* Posts by Spamjunk

3 posts • joined 30 Dec 2010

Diary of a Not-spot: The readers speak


Beware of people that don't know what they don't know

There's some dangerous misapprehensions being propagated on this thread by people that have a GCSE-level theoretical understanding of domestic electricity power supplies. The real world is not so simple and not so predictable ("The difference between theory and practice is that in theory there is no difference...").

It is this level of misunderstanding/incomprehension that has caused to law to be changed to require people working on power supplies (including domestic) to be properly formally qualified. Sad, but probably necessary.

So, if you want to physically interconnect properties, get information from a *properly* qualified electrician.


Three phase and shared earths

Yes of course any single property is on an single phase, but the next property may or may not be on the same phase. Different phases lead to a 415V potential difference.

In my street it is *very* easy to tell which houses are/aren't on the same phase. All I have to do is walk along the street and look at the four wires (three phases plus neutral) running between the "telegraph" poles. Different houses are connected to different cables, and hence to different phases. Yes. delivery people with cranes *hate* unloading here because of the possibility they might touch the cables (they do have a thin plastic insulation, but caution is very prudent!)

Good luck with the assumption that surge protectors will provide protection from lightning strikes. They might (or might not) the first time, but thereafter the protectors may well be invisibly compromised (hint: they fail open-circuit).

Ditto any signal isolation transformers.

As for the concept of "earth", reread Grease Monkey's post "Shared Earth"


Electrical dangers

The dangers associated with connecting wires between different houses is not limited to having "different earths". I use quotations since the concept of a "single earth" is muddleheaded to begin with.

A major issue is that different houses may be on different phases and hence the equipment could experience much more than 230V.

Additionally, if there is a lightning strike nearby, then there can be very large localised potential differences at different points in the ground. Such voltages would appear across the cable and thereby across anything else connected to the cable. Such potential differences are the reason that, if you think you are about to be struck by lightning, you should keep your feet close together and not rest on your backside/hands; a shorter distance means a lower potential difference across your feet, and hence less current flowing through your body.

I strongly recommend consulting someone with *specialist* knowledge before connecting different houses together by cable.

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