> > Are you saying that the DC voltage difference is always 0 VDC? > Measured across the only connection between two otherwise isolated systems, yes.
The systems are isolated, so there is no fixed potential between them, their "DC" potential difference is therefore random (unknown quantity of excess/missing electrons) before they are connected, and you can have charge transfer if you connect both, that will balance the extra electrons on both sides. Electrons will flow in a way that the resulting density is uniform, like repulsing magnets. Therefore, there is no potential difference between any point in these two systems (barring local differences -- like a battery--, or at the very least, no potential difference across the connecting cable).
An analogy that could or could not help is trough gravitational potential energy: take two water buckets, isolated. The potential across them is the height difference between their water lines. We didn't specify where we left the buckets, so it is random. Now if we connect the bottom of the buckets, the system will find its way to the "ground state", or lowest energy state: the water line in both buckets will align (Communicating vessels), and you therefore have no DC potential across those: connect an extra piece of tubing, nothing will flow.
This analogy is quite powerful and can be used for capacitors, diodes, etc. (as most flux/potential analogies). It has its limits, though.
Because as per the linked article, the absolute voltage potential of the earth in a local area can vary significantly over time. This make me think that the difference between a small isolated system and the earth ground must vary significantly. As you point out, everything actually is coupled, there is a time constant involved, but at least immediately after a nearby lightning strike you should have a significant difference. Because of the small capacitance of the isolated system, I'd expect you might see a high initial value that would soon (scale depending on the meter and the capacitance) decay to zero. Do you disagree?
> House wiring is not limited in how much current it can put through you
Absolutely agreed. Per the article, I'm referring to the supposed safety of an isolated transformer used a "razors only" outlet. In addition to having an intentionally small transformer, this system also has an intentionally very small fuse.
> The reason why touching one wire is “safe” is because the return path for the current is usually very high resistance
Yes. I guess the part that I'm failing to make clear is that I'm asking about the articles differentiation between isolated transformers and US standard circuits. In both cases, in normal dry clothed circumstances, there is normally a high resistance to any return circuit through your body. I'm still having trouble seeing the difference between the safety of the isolated system and the standard system. Most of the answers here (including yours) would seem to apply equally to both cases. I feel I understand this, just not the way in which the article explains the safety of isolated transformers.
> Are you saying that the DC voltage difference is always 0 VDC?
Measured across the only connection between two otherwise isolated systems, yes.
> Since much of the article is about the necessity of grounding your electric distribution network to prevent high static voltages, what makes this system different?
The size of the system makes a big difference. The power grid is large enough that you have to worry about things current induced by geomagnetic fields, lightning, solar activity, etc. Lightning has a high enough voltage that it breaks down insulators that would otherwise isolate two systems.
Basically, no system is perfectly isolated from another. It is a question of how much leakage current you have compared to how much current you are using to get work done. With ordinary home wiring, leakage current is negligible, because the resistance of the leakage current paths (many MΩ) is huge compared to the equivalent resistance of the circuit (maybe 100Ω).
Your home wiring system cannot withstand a lightning bolt, it would destroy much of the electronics in your house. The power grid must withstand lightning bolts on a regular bases.
> Instead, I think it's more likely that the difference in DC voltage can vary significantly.
Why do you think this?
There are real electricians and engineers on HN, people with domain knowledge who aren’t guessing but have actually designed power supplies or fixed faulty home wiring systems.
> So rather than there being "zero voltage", I think the reason for safety is related to dfox's answer below: the system is limited in how much current it can put through you, and it's the amperage that harms you rather than the voltage. You might get an initial static shock, but the AC amperage is going to be small enough not to harm you.
Some errors:
- House wiring is not limited in how much current it can put through you. It will kill you dead, given the opportunity. The fuse box or circuit breakers won’t prevent you from being electrocuted, they are there to prevent fires.
- Phrases like “it’s the amperage that harms you rather than the voltage” are a bit simplified. The relationship between voltage and current is fixed. Recall Ohm’s law for resistors, you cannot have current without voltage (except in a superconductor).
The reason why touching one wire is “safe” is because the return path for the current is usually very high resistance—it goes through your shoes to the floor, and through the floor back into the circuit somewhere. This is usually a dubious path for current. But there are a lot of reasons why there might be a better path for current—maybe you are touching something made of metal, maybe the floor is wet and you are not wearing shoes, maybe…
This is why one wire was chosen to be “neutral”, so it would be “safe” to touch even if there was a good path for current. But it turns out that this safety disappears as soon as there is a wiring problem in the house, or whenever there is significant ground current, or one of several other dangerous failure modes, so this is why we have the third ground pin.