power - Isolation provided by transformers

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

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.