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?

We have a fuse for the wire coming from each phase in the substation, but that branches to several houses, with thinner cables!

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.

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.

--
It said, "Insert disk #3," but only two will fit!