Why don't we use isolated supplies in our homes?

[Surefire]

I'm sure most people here know that shaver sockets have an isolating transformer inside. This means that should a person come into contact with the live conductor and earth, the risk of electrical shock is greatly reduced because the secondary side has no reference to earth.

My question is a simple one: why don't we use this kind of transformer to supply a complete premises.

There must be good reasons why not, I can think of a couple such as floating voltages and problems which may be encountered when connecting two premises together on different phases. But I'm sure the possibility of eradicating all risk of electric shock must out weigh the disadvantages.

Would be interested to hear others views.

 

[Pete999]

A logistical nightmare, along with tremendous costings I would imagin.

 

[Darkwood]

Having isolated supplies like you mention have their own problems especially regarding safety, consider a fault in such a set up where L went to some exposed metal work now a PD may not exist for you to receive a shock by touching the metal but you may be the bridge to the return path which is dangerous.

Another issue is how we isolate the natural ground earth which will would need isolating from any exposed conductive parts in the house, a task that is very difficult to achieve especially as you could bridge the 2 yourself.

We have grounded systems strapped to N at source usually to ensure faults are successfully seen and removed by protective equipment, there is also added issues with fire risk if a fault was allowed to exist uninterrupted.

Also to remember is been able to detect problems by testing which would be a big issue if you had no reference earth to test to expose the fault, its hard enough to learn the new school the existing methods but throw a curved ball into the issue and your asking for trouble.

I use isolated systems in my control panels and they have to be designed correctly to be safe with correct fusing in the right places.

 

[Surefire]

consider a fault in such a set up where L went to some exposed metal work now a PD may not exist for you to receive a shock by touching the metal but you may be the bridge to the return path which is dangerous.

Thanks for the reply and I agree with your reasoning but I'm a bit confused about your bridge comment. If there is no PD then no current should flow should it?

 

[Darkwood]

Thanks for the reply and I agree with your reasoning but I'm a bit confused about your bridge comment. If there is no PD then no current should flow should it?

I meant of the circuit was down to isolated earth you wouldn't receive a shock unless you touched this conductive part and another part of the circuit where a PD would exist thus bridging the gap... pretty much like having a broken N on a circuit normally the N would have no PD to earth but if the circuit isn't complete then you may become the bridge to the return N or to E.

 

[Surefire]

Thanks for clearing that up Darkwood. I also believe that it's more common to get over voltage issues when the neutral is not earthed especially on three phase systems.

 

[Lucien Nunes]

Load overvoltage arises when the neutral is broken between the star point of the transformer and an unbalanced load, rather than when it's disconnected from earth. The line voltage then divides unequally according to the impedances of the load phases. Earthing the neutral point maintains the L-E voltage at the lower of the two system voltages - if one phase became casually earthed then the full line voltage to earth exists on both the others.

Isolated supplies (IT) are good for single localised loads only like the shaver outlet, because they almost eliminate hazards from a single shock fault path to earth such as a wet shaver connector. Such a shock hazard might be cleared by an RCD if the neutral were earthed, but the shock has to happen first and the RCD has to react. With the isolated supply, the shock simply doesn't occur.

However, as soon as multiple loads are connected to one isolated source of energy, the safety of one depends on the sound insulation of the others and the wiring in between, which may be out of sight or out of your control, and overall not as trustworthy as the transformer itself. A single fault that breaches the isolation causes no symptoms so it will remain until the second fault arises possibly in a different location, creating a hazard as per Darkwood's explanation.

 

[Darkwood]

Explained a lot better than me there Lucien