Isolated mains voltage - why not as standard?

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?

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Reply to
Tough Guy no. 1265
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Not from me.

Reply to
ARW

You don't have a sensible opinion?!? :-)

Reply to
Tough Guy no. 1265

It is important when electrical equipment/appliances have metal cabinets which can be earthed. Which was everything years ago. Without an earth, ant fault to the cabinet could not be detected and would be dangerous if there were other earth faults. Also get over certain capacitance effects.

Reply to
harry

Two faults is unlikely. One fault would of course not shock you at all. Most shocks are an exposed live to an earth, isolating the mains would remove that possibility altogether, and not change the amount of shocks between live and neutral.

Reply to
Tough Guy no. 1265

Because mains is an AC system, there is no such thing as a totally isolated system. There is always some capacitance between the conductors and earth which all adds up.

Reply to
philipuk

Why do builders have isolating transformers then?

Reply to
Tough Guy no. 1265

seem to be able to agree. Any sensible opinions?

Accepted convention is that the body of an article should include the infor mation in the Subject. Note that writing "I'm asking the question at the t op" takes more effort that duplicating the subject by copy'n'paste. Also, the question was never at the *top* here and is at present not displayed.

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The supply under the street, or equivalent, is three-phase. Three adjacent houses will normally be on three different live phases, with a common neut ral. Less copper is buried that way.

Providing an isolated voltage would require a power transformer for each pr operty, or a multi-secondary one for each small group of properties, since a supply cannot be guaranteed to be isolated in property A if it is in full parallel with property B.

If an isolated supply is connected to equipment which generated a higher vo ltage internally, such as a CRT needs, then an internal fault could raise t he floating supply to a high voltage, which would be dangerous throughout t he premises.

Persons who think they have a need for an isolated supply can, with due car e for the Building Regulations, use an isolating transformer where required .

Reply to
dr.s.lartius

In repair & testing scenarios, we use isolation transformers to remove the Earth reference. It prevents ground loops arising which can destroy sensitive test equipment. But an IT won't save you from a belt if you somehow manage to bridge directly across it.

Reply to
Cursitor Doom

And electric shaver sockets in bathrooms? I would think they're just a small version of whole-house isolation?

Perhaps it's just because it's always been done that way, and that way is cheaper than having a beefy transformer in every garage.

Reply to
Chris Hogg

That is true. My mother once acquired a standard lamp with a hollow metal post. My brother wired it up, and it worked. When I arrived, I could feel the electricity on the post, which was probably at about 120V through of th e order of maybe 100pF. I forget what cable he used; but I ensured that sh e had a three-core cable with all three cores connected as they should be.

Reply to
dr.s.lartius

It's in the subject.

I just drew a simple diagram with a substation at the West end of a street, followed by three houses moving East, on phase 1, 2, and 3. Both the existing system and the isolated system all use the same amount of copper, as the neutral still goes to all the houses. The only difference is whether the neutral is lashed to the ground in the substation. Eg. my house has two conductors coming to it - live and neutral/earth. The only way that the existing system would use less copper than the isolated system is if the neutral was not in the wire, but relied on the ground to carry it from the house's earth rod to the substation's earth rod. This isn't done, the neutral conductor is always laid.

What do you mean by this? Are you taking about property A and B being on different phases? What is the problem?

Impossible for three reasons:

1) How would a circuit be able to pump a higher voltage back into what is supplying it with energy? 2) There is so much current available from a substation, that it would clamp it to 240 volts. 3) If it was possible, then it could also happen with a non-isolated system.

Since builders use them, isolating transformers clearly make things safer, so I see no need to pay attention to any regulations.

Reply to
Tough Guy no. 1265

No it won't, but it won't make it any worse, it'll be identical. But it does remove the shock from live to ground, which is more likely as only one conductor has to be touched.

Reply to
Tough Guy no. 1265

The substation could isolate. Just don't clamp neutral to the ground. 4 wires come out of the secondary side of the substation transformer, leave them away from the ground, and take them to the houses as normal.

Reply to
Tough Guy no. 1265

I would agree with you, but only if you can automatically detect each of the faults and interrupt the power accordingly.

An isolation transformer might well hide some faults as well as introducing some more and so be more dangerous in reality than the conventional single phase supply.

Reply to
Fredxxx

Do you understand about 3-phase distribution?

If you don't clamp neutral to ground, what do you think will happen to each phase voltage wrt ground?

230V is enough for me!
Reply to
Fredxxx

But the two faults would have to be within reach of each other, eg your washing machine and your fridge both having a fault with the opposite conductor, and you touch both. So very unlikely.

Besides, I've never actually observed a fault where the chassis becomes live. The only shocks I've ever had were worn cables on a mower, touching a switch with wet hands when outside, etc. Now if the supply had been isolated, I wouldn't have received a shock.

Reply to
Tough Guy no. 1265

They have transformers at 55 volt per side, so a typical fault would only give you a 55 volt belt. It isnt the isolation that is the main safety factor, it is the lower voltage. I dont think all site transformers are isolating anyway - bathroom shaver sockets are, but 110V plugs have an earth connection. Either way, you are only getting 55volts if touching one live and earth, so should survive getting a belt off one.

Reply to
A.Lee

If isolated, no phase has any voltage with respect to ground, just like a battery sitting on your desk has no voltage with respect to ground.

Reply to
Tough Guy no. 1265

Naw, you want an RCD for that, mate.

Reply to
Cursitor Doom

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