Isolated mains voltage - why not as standard?

[Tough Guy no. 1265]

I would have known about them because a fuse would have blown, then I'd investigate the fault. I've never seen a live chassis.

[Tough Guy no. 1265]

Irrelevant to the fact that they have no voltage WRT ground.

[Tough Guy no. 1265]

Then we could have centre tapped 240V, like the yanks do.

[Tough Guy no. 1265]

But it's very unlikely. Funny how double sockets are rated at 20A. The chances of two 13A devices being plugged in is low.....

[ARW]

Just the one. Ban women drivers.

[Tough Guy no. 1265]

You will get complete agreement from me on that. They have slow reactions and no spatial awareness.

[mick]

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). 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. 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).

[Tough Guy no. 1265]

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.

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

[Dave Plowman (News)]

Of course it's isolating. That refers to the design of transformer. Using an auto transformer to give the 110v would work the tools - but could end up with 240 on one leg of the output to ground if incorrectly wired up.

[Tough Guy no. 1265]

Hmmmm, isolating from the mains perhaps. I assumed isolating was completely isolated, as in floating.

Oh is that what they're called, I call those Variacs. Very useful.

Anything can be dangerous if you wire it up wrong. Don't.

[charles]

Vsriacs have a variable ouput. Autotransfomers can be fixed.

[Tough Guy no. 1265]

Seems pointless. Are they cheaper than two separate coils or something?

[charles]

yes because it's only one coil - with a tap somewhere near the middle (to get 120V)

[Tough Guy no. 1265]

I guess they're more for smaller step downs? Because most are like 240V to 12V, where the output needs more current so a thicker coil.

[philipuk]

Cheaper, smaller, less excitation current (so less leccy wasted when current isn't being drawn from the secondary), better voltage regulation. Yes much less copper is used and a smaller core is needed.

Main disadvantage is that under fault conditions primary voltage can appear on the primary as they are not isolated.

They are used a lot on the high voltage side of the mains distribution networks.

[Tough Guy no. 1265]

This:

Would make you think this:

Is a bad idea.

[Dave Plowman (News)]

That's what it means. The output winding has no reference to the input one.

A Variac is a variable voltage transformer. And entirely different device.

If you could guarantee everything was correctly wired and used safely, there'd be no need for additional safety precautions.

[Tough Guy no. 1265]

It does on one end in the case of an earthed builder's one.

No, it's an auto transformer where you can move the tap. The coil is in the same configuration.

Safety precautions don't tolerate incorrect wiring.

[Dave Plowman (News)]

All girls who attend The High School wear green blazers. Therefore all girls who wear green blazers attend the High School.

[Tough Guy no. 1265]

Stupid analogy. You're talking about something which is a subset of another, then saying you can't reverse the assumption. I didn't reverse the assumption, I never said all auto transformers were Variacs, I said a variac is a special kind of auto transformer.