Isolation provided by transformers

Question

I have a few questions about transformers and isolation. I will describe my understanding (maybe wrong) and questions about them.

• Transformers provide safe isolation by disconnecting two circuits electrically. So now instead of touching one hot wire on the isolated circuit, you must touch both to get shocked.
• Why don't my wall outlets work this way? I know there are many transformers used in the power distribution network. Is this because they are referenced to ground (with a center tap or something)?
• If the power lines are referenced to ground, why? Wouldn't it be safer to keep the power lines floating w.r.t. ground, so that for a shock to occur a person would have to touch two wires?
• Could I provide this protection at my home by putting a 1:1 isolation transformer between the power grid and my house?

Answer 1

Could I provide this protection at my home by putting a 1:1 isolation transformer between the power grid and my house?

Consider these scenarios: -

• If one wire (called neutral) is earthy, then touching the live side will give a shock but, that shock isn't going to push hundreds of mA through you and kill or burn you.
• If you touch live and neutral together then that is going to push potentially hundreds of mA through you and give burns or worse.

Transformers provide safe isolation by disconnecting two circuits electrically. So now instead of touching one hot wire on the isolated circuit, you must touch both to get shocked.

Consider the relative benefits of scenario 1 and the advent of the earth leakage circuit breaker. If a current is taken from the live wire through your body to earth that current does not flow down the neutral wire and this can be detected at a low level and trip a circuit breaker.

Modern devices use residual current devices (UK) or ground fault circuit interrupters (in the US). All developed countries use something. So, the RCD passes live and neutral through a magnetic core and the resultant flux in that core is zero because, under normal load conditions live current and neutral current are equal and opposite.

A detection coil (also on the core) generates a signal to trip the breaker if the difference between live and neutral currents is ~30 mA. It trips fairly quickly so if you touch live in a meaningful way, within a few tens of milliseconds, the circuit is rendered safe: -

Without having neutral connected to earth there is no intermediate semi-safe method of preventing full circuit contact and a much higher risk to health.

Answer 2

Your question is one I had, as well. It seemed that a lot of people I'd heard about (back in the day before there were any additional protection circuitry in a home) were hurt or even killed on occasion just touching one conductor and where a transformer would seem to have helped. More, you often read where engineers say you should ADD a transformer for your own protection from shock events. So, when I was learning about these things those questions came up for me, too.

(Keep in mind, too, that these decisions to ground homes and to ground local distribution power transformers were made LONG before there was anything like GFCI or AFCI protection circuitry available. So the primary reasons for using an earthen grounding approach can't include those devices as justification, since they didn't exist when these decisions were being made. And if new devices are instead said to be important, then perhaps the original decisions for earthen grounding systems may no longer be valid and the explanation may fall on that point, as well.)

Consider weather situations such as when storms and lightning are taking place outside. There are huge build-ups of charge going on and these aren't just in the air, but also in the ground from one side of a piece of land to another. A power transformer might be located off your property (elsewhere) with conductors running inside of insulation to your home some distance away, but floating at quite some different voltage from the nearby ground at your home. Or a lightning strike might highly energize something in your home. So you could experience not hundreds... but perhaps thousands... of volts and this could drive a lot more current through your body than if you'd had things "tied down" to the local Earth. (A similar situation might arise with large static charges built up inside of equipment you operate using the power system, too.)

Now, if the decision is taken to use earthen grounding systems and to stake (or use a Ufer grounding system: concrete and rebar in the foundation) the electrical system to ground near the home and to do the same for various nearby homes as well as for the local distribution transformers, then there are very good conductors tying all this into very low impedance net. It will be almost all at very similar voltages, too, because of that. And while there is a large capacity for storing charge in the earth, because of everything being tied together with conductors those things near the conductors will be nearer to similar potentials, too. Charge differentials won't sufficiently accumulate to create large voltages between them, because they are grounded and will bleed that charge out into the larger system. Lightning strikes will move charge into the local earth. But the whole system moves with that, too.

Sure, there's a trade-off of sorts. Grounding everything to earth means that you could be standing in a puddle of water and be half-connected into the system. So yes, touching the hot conductor of the power system could shock you. But the risks of not doing that may be still worse and more likely. It also makes each home's situation more predictable and manageable for those who will then go on to design equipment you may use, as well.

Since then, of course, there are newer ground fault and arc fault protection devices available. So the entire situation is now so much, much better. Plus, the advantages of earthen grounding, as well.

Bear in mind, I'm just a hobbyist in all this and my opinion here is a matter of reading the writing of others. I'm not a trained electrician and don't have an independent, well-informed opinion on this subject. So consider what I've written and then look things up for yourself. I think your question is an important one. Enough so that you should think for yourself and read what you can find about it. But that's my take on it. I hope that helps.

Answer 3

Transformer isolation is only useful if it is done at each appliance. Every house has many steps of transformer isolation between the house and the power station but each time for normal use the consumer side of the transformer has one end (or the mid point in 230V USA) connected to earth to maintain an earth potential rather than an unknown leakage potential. Removing the earth reference from the consumer side of his very own building transformer or the usual street pole transformer feeding 3-12 houses would make things LESS safe. All the consumers would now be at the mercy of all the others when the first leak occurs, the networks would now be at some arbitrary potential (probably between 0 and 230V but perhaps not if a insulation fails in a boiler ignition transformer at 8kV) and the next person to touche the mains would have this fault current grounded through them and have no easy way to detect the fault in advance.

In a transformer isolated appliance you would try to make sure the transformer input is ground referenced and the metal parts were earthed and the insulation was of a high order. All this to try and keep the secondary from connecting to the primary or earth in any way under normal conditions. In this case touching any single point in the secondary should make that the local ground point at your potential and the rest would float as required and no current would flow (no shock) because the insulation would prevent a circuit closing.

This type of application is still seen in places (hotel bathrooms mostly) where you can sometimes find a shaver plug that has a small isolation transformer to make it safer to handle mains rated appliance while standing on a damp floor or holding a metal plumbing fixture.