Why do isolation transformers provide a 0 V between their terminals and grounded instruments and why does a normal grounding not?

Question

Isolation transformers provide a 0 V between their hot wire and ground. After not understanding why that is, I doubted how earth grounding make a potential difference with me in first place.

First I know that there must some sort of a loop (that's basic) for the current to flow and second the must be a potential difference between two points in that loop.

Well in case you touched a grounded instruments and the live wire of the outlet, ground wire is already shorted with the neutral wire in the main panel so that's a loop. My question now is I don't see any kind of a loop when you for example touch a live wire with barefoot. (I mean the grounding rod is connected outside the house it's unlikely that the resistive path between me and it all the way there is allowing any current to flow.)

To sumup my Question ( May be it's to be considered two questions Actually )

• Why Do I get shock When standing on a barefoot while touching a live wire When There's no return path ( at least the way I see it )

• And How does isolation transformers prevent shocks when I touch a grounded instruments or with barefoots on the ground

Answer 1

Why Do I get shock When standing on a barefoot while touching a live wire When There's no return path

If there is no return path you will not be exposed. But there is a return path: the ground wiring in your home is tied to earth through a pad or rod, and to neutral at the panel. This means that the ground under your feet is electrically connected to the neutral wire leading back to the voltage source (e.g. your neighbourhood "step down" transformer)

See the picture below which shows that connection in the panel. (Ignore the subpanel/panel split, the point is that it shows the neutral bonded to the ground at the panel.)

And How does isolation transformers prevent shocks when I touch a grounded instruments or with barefoots on the ground

With an isolation transformer the live and neutral have an electrostatic potential w.r.t. earth, "floating" at possibly very high voltages. They can zap you, but likely won't kill. As long as the neutral or live is not earth grounded, no circuit forms when touched bare feet.

The diagram in your question incorrectly annotates the neutral to ground voltage as 0V. The "stabilization" shown (as 0V) requires a high-ohmic or low-ohmic grounding to neutralize the potential.

A high-ohmic earthing is used to stabilize the potential while avoiding a dangerous ground loop. A low-ohmic earthing is used to protect metal housings or exposed metal parts if a short from live to metal should occur, thus tripping a ground fault interrupter or a over-current breaker before a fire is started or a user is imperilled.

As drawn, there is no earthing, and that voltage to earth is an unknown.

Here is an example of an impedance grounding on the secondary side: (it's for a 3-phase system, but the principle is the same)

The high electrostatic potential on the isolated side would not be caused by the generator/transformer, but rather through other causes of charge accumulation.

Sometimes no path is required to feel a shock, e.g. when taking-in or relieving a charge between two bodies. This would be something like a capacitive in-rush zap. Kids experience this on a playground, when coming off the bottom of a plastic slide.

Whether the transformer or generator (the isolated source) requires earth grounding on the secondary side, depends on whether the consuming device is fully enclosed with the isolated source. If not, electrical codes often require low-ohmic earth grounding, and only a ground-fault interrupter will provide adequate protection, not the isolation.

Livestock electrical fences work on this principle: they provide a high voltage spike to overcome a (high resistive) path through earth.

(Image from https://www.stafix.com/en-us/helpful-information/grounding-your-energizer )

Answer 2

The floor and soil don't have an overly low specific resistance, but it provides a lot of area for the current to flow through (as do your feet compared to the contact area of a wire). Combine this with the fact that wall voltages are high and not much current needs to flow to do some damage to your body.

So it follows that if the resistance between the transformer windings is much higher than the soil and earth, then it should be obvious how the isolation transformer protects you. It disconnects the ground potential on the primary from the 0V on the secondary and lets the 0V float so you must touch both secondary wires to get shocked. If you just touch one secondary wire, the potential at that wire becomes equal to your potential and the potential difference on the other terminal floats higher or lower maintaining the same voltage between secondary terminals.

---

You seem to be questioning how you could possibly be connected to the soil or the ground rod outside when you are several floors up in your building. Let's do some math.

For example, the resistance of rubber is on the order of \$10^{13}\$ Ohms-meters. This should be quite a bit higher than most materials. The resistance of copper is on the order of \$10^{-8}\$ Ohm-meters. So that's a difference of \$10^{11}\$ times. We would need an area of rubber \$10^{11}\$ larger than the area of the copper for it to be equally conductive.

The area for 16AWG wire running to your toaster is \$1.31mm^2\$. The area of your feet is about \$0.09m^2 = 8100mm^2\$ That's on the order of \$10^{3}\$ times more area. So your feet have a lot more area than the wire but but nowhere near \$10^{11}\$, right?

But let's try and guess the area of your entire floor or the cross section of your building. Let's just pick something small, say \$10m \times 10m\$ = \$100,000,000mm^{2}\$ That's \$10^{7}\$ times more area. Getting closer to \$10^{11}\$.

But now consider that you will get hurt by a lot less current than something like a toaster will. Just a few mA is enough. That 16AWG wire was sized to carry 15A to the toaster but you can be hurt by 5mA. That's a difference of current on the order of \$10^{3}\$. The 16AWG wire powering the toaster could be a thousand times higher in resistance and still hurt you.

So if we combine the ratio of \$10^{7}\$ and \$10^{3}\$ together, that gives you \$10^{10}\$ which is getting pretty close to the \$10^{11}\$ ratio. And remember, we used rubber which is a better insulator than most of the things the soil, floor, and building are made of. We also ignored the fact that there are grounded metal beams and pipes that let the charge take shortcuts up your building towards you.

**EDIT: Whoops, \$10^{13}\$ divided by \$10^{-8}\$ is \$10^{21}\$, not \$10^{11}\$. The biggest issue is probably that I should not have used rubber. I had trouble finding good conductivity numbers for dry wood or cured concrete. Wood varied from few gigaOhms when dry to hundreds of megaOhms with just a little bit of moisture content, to few kilOhms when damp. Resistivity of rock is surprisingly low, often much less than 10megaOhms, even when dry.

If you used a few hundred megaOhms, that would cut off about \$10^5\$ compared to using rubber so you would only need \$10^{16}\$ instead of \$10^{21}\$ more area.

https://ieeexplore.ieee.org/document/6441387

http://www.geonics.com/pdfs/technicalnotes/tn5.pdf

Answer 3

One of the misconceptions you are struggling with is earth resistance, you seem to think that earth resistance increases with distance. In reality earth resistance is a constant, if you can measure 1 ohm resistance between two electrodes a metre apart, you will also measure 1ohm between two electrodes 100m apart or 10km apart. What happens is that there are more current paths between points further apart; For electrodes 100m apart, the current paths can go 100m deep and 100m to the side, while resistance does vary with length, the relationship is resistance = length * area/volume , so if length , width and depth all vary together, the resistance is constant. The measured value of earth resistance is determined by the size of your earthing stake :- bigger stake = lower resistance. for lightning protection of large buildings they will often bury a metallic grid in the soil before the start of building.

In a building , there are earthing straps and metal plumbing and a metal building frame that join all the ground potential points together , if all exposed metal parts are connected together, they are the same potential during a fault or a lightning strike.