First, a side note: when you, as someone unfamiliar with the subject matter, say things like "I have trouble believing" and "this seems unlikely" to someone with subject knowledge (I have an EE degree), you come across as condescending. "I don't understand" is a better way to phrase your confusion than doubting the veracity of the person you're asking for help from, and makes them and others more likely to continue to help you.
> Are you saying that the DC voltage difference is always 0 VDC?
AC vs. DC makes no difference in the statement I made. However note also that DC is not transferred through transformers; any DC offset with respect to earth ground in the distribution system will not be seen by the customer.
> Are you saying that the AC voltage at all points in the isolated system is always 0 with reference to earth ground?
No. But if you measure one at a time, with no other connection between the systems, you will see this. If you measure two simultaneously, what you see will depend on the relative impedance of the two measurement devices. This is easily experimentally verifiable.
> 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.
No. Those statements are not separable. See andyjpb's sibling comment.
Sorry, this wasn't my intent. From your other answers over the years, I recognize your username and actually have a lot of respect for your expertise. I don't doubt your veracity at all, I was just trying to make sure you were focused on the question I was interested in, and encouraging you to go deeper in your explanation of why the (to me) counterintuitive answer might be true. Your side note in the original about the difference between neutral and hot wires made me think you might be answering a different question that the article already covered well, and that I feel I understand.
> However note also that DC is not transferred through transformers; any DC offset with respect to earth ground in the distribution system will not be seen by the customer.
Yes. I was talking solely about the measured DC on the isolated side. This is why I think we might be talking about different question, or at least at aiming at different levels.
Maybe (if you'd be willing to continue) I can substitute a related question that I think addresses my misunderstanding: how is an isolated transformer (powered by an AC current) different in safety considerations than a small electrical generator (powered by spinning magnets)? In my possibly mistaken mental model, a small generator is "safe" in the same way that small transformer would be, but as it grows larger the potential for hazard increases. Is this intuition wrong?
> Yes. I was talking solely about the measured DC on the isolated side.
Ah sorry my mistake. I thought you meant "distribution system" as in "the grid".
Yes, safety-wise, grounding (as opposed to isolation) protects from shocks due to DC static charge, precisely by providing a reference point so that the isolated system cannot "float away". However note that that DC charge is temporary: as soon as something gets "shocked" by it, it is discharged. It is still a safety concern, but of a different nature than getting a continuous shock from AC.
(To further clarify: my initial response to your statement about "60 V" only pertains to the AC voltage intentionally present in a system.)
Additionally (I think the article gets into this, but it is long and I am short on time), isolation is not sufficient for household safety because it only protects against a single accidental connection. As soon as a second, different, point on the AC system makes an accidental connection, you have a circuit and (potentially lethal) current will flow. And, given the number of appliances in modern homes, it's not unlikely that one of them might have an accidental connection to earth ground. If a human then makes a second connection to the other leg of the circuit via some other appliance, game over.
The modern grounding system solves this by forcing all appliances to be designed in such a way that either it's "impossible" for a person to contact either leg of the circuit, or that the entire casing of the appliance is connected to earth ground, so that if the case does get connected to a live wire, the circuit breaker gets tripped. (However I'm approaching the limits of my knowledge here – I have no training as an electrician – so won't make any claims as to whether this is the primary function of the grounding system, vs. preventing static charge.)
BTW if you're interested in this stuff, any material you find online by Mike Holt is very good. He is an unofficial authority on all things US electrical code.
Would refer to NEC 250.21 for discussion of "ground detector" as it relates to the detection accidental first ground connections in ungrounded systems.
When you say things like “you come across as condescending”, the author can feel hurt, because the story in their mind is that a friction in this one interaction is seen as an inherent flaw of the author or of their writing. Hurting your interlocutor’s feelings can actually further sabotage constructive communication, if that is your goal.
I’ve found more success by communicating my own feelings as directly as possible, without theorizing. For example, “When you asked me a question about my area of expertise, and then dismissed my advice, I felt disrespected, because it makes me think that you never sincerely wanted my help in the first place. Do you?”
While I'd strongly prefer that colanderman not be offended, given that he was offended, I'm glad that he voiced his offense as bluntly as he did. Yes, it hurts to think that my writing is that unclear, but I've got to own the audience's reaction even if (especially if) it's not the reaction I was aiming for.
There is no way to have a fully isolated system. At best you get either a static charge or at worst, multiple the voltage and same current flowing to actual Earth due to capacitance. As in literal ground.
Most isolators end at some megaohms in a range of temperatures and non-negligible capacitance. (They're used as capacitor fill for that reason.)
Useful personal isolation and grounding is meant to strike the balance where you still get trickle grounded while not being fully isolated and accumulating dangerous charge.
> Are you saying that the DC voltage difference is always 0 VDC?
AC vs. DC makes no difference in the statement I made. However note also that DC is not transferred through transformers; any DC offset with respect to earth ground in the distribution system will not be seen by the customer.
> Are you saying that the AC voltage at all points in the isolated system is always 0 with reference to earth ground?
No. But if you measure one at a time, with no other connection between the systems, you will see this. If you measure two simultaneously, what you see will depend on the relative impedance of the two measurement devices. This is easily experimentally verifiable.
> 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.
No. Those statements are not separable. See andyjpb's sibling comment.