On Sun, 08 Nov 2015 22:06:11 -0000, Rod Speed wrote:



"Tough Guy no. 1265" wrote in message
news
On Sun, 08 Nov 2015 21:33:32 -0000, Rod Speed
wrote:



"Tough Guy no. 1265" wrote in message
news On Sun, 08 Nov 2015 20:44:45 -0000, Rod Speed
wrote:



"Tough Guy no. 1265" wrote in message
news On Sun, 08 Nov 2015 20:05:38 -0000, mick
wrote:

On Sat, 07 Nov 2015 16:42:20 +0000, 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?

http://electronics.stackexchange.com...hy-are-we-not-
always-isolating-the-mains-supply


On a "floating" mains supply as described you have to fuse both poles
(I'm calling them A & B rather than L & N) at the transformer
secondary
for safety (there could be a transformer fault).

Fused? They don't fuse them sensibly.

They do here.

My parents' neighbour's roof burnt down because his shorted incoming
didn't blow any fuse. Apparently the only fuse is about 800A on the
transformer,

We have a fuse on each phase at the point of connection.
With a supply from the power line on poles down the
street, that is where the line from the street pole is
attached to the house, with massive great ceramic
blocks with a big fuse cartridge in a ceramic plug that
goes into the fuse holder and pulls out downwards.

Don't you have underground feeds?

Yes we do, and they have a fuse per line too.

We have a fuse for the wire coming from each phase in the substation,

Yes, we have that too and you can see them on the ones half way up the
power
poles.

The ones that sit on the ground don't have them visible, but they have
them
too.

but that branches to several houses, with thinner cables!

We have a big fuse on each of those wires where it enters the individual
houses.

Here, that's a 100A fuse right next to your meter. Hence the wire from
the pavement into your building is unprotected.

and the wire going to his house is 100A.

Now, consider an earth fault after the fuse on pole A. Pole B is now
at
line voltage to earth because neither fuse will blow (there is
insufficient current to earth on
pole B). In this situation it's very easy to get a fatal shock from
pole
B to earth.

But you can already do that now, without it floating.

Nope, if the live is earthed, that will blow the fuse or breaker.

No, I meant you can get a shock from live to earth now.

You can with your hare brained scheme that no one world wide
uses too, when there is an earth in any of the houses that is supplied
by that substation, with no way to detect that at the substation,
and it makes no sense to turn the power off to all those houses
fed from the substation even if you could detect it, and you can't.

That's why no one world wide has ever been stupid enough to do it that
way.

But with my scheme you can only get a shock if there are two faults.

But there is no way to detect the initial earth fault,
so it will go unnoticed until someone gets a shock
and many won't be aware of the significance of that.

They're rare. And probably if a wire comes loose in the appliance and earths it, the wire is no longer where it should be and the appliance doesn't work. That would get noticed.

With the current scheme you only need one fault.

But non faults can kill you because the RCD will protect you.

RCDs didn't used to exist, so isolated mains would have been more sensible then. Even now, not everyone has them. I certainly don't. And I laugh at people who have nuisance trips.

The only problem is with a fault which results in the individual
ending up across the live an neutral with the current flowing
up one arm and down the other etc and that is very very
unlikely indeed and most of the time will trip the RCD anyway.

The RCD wouldn't stop that at all.

--
All the American flags on the moon have been bleached by radiation from the sun (which can only be a good thing).