"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.

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.

A second earth fault or overcurrent, now on pole B, will blow
one or both of the fuses - but it's anyone's guess which one as they
will
have to have the same rating. Now, is the supply earthed or not? Which
side is live to earth (if either)?

This sort of system is possible (it is actually used in some specialist
situations).

But not with normal house supplys.