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. 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, 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.

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).

--
Hickory dickory dock, three mice ran up the clock. The clock struck one, and the others got away with minor injuries.