3 ms·

> you come across as condescending Sorry, this wasn't my intent. From your other answers over the years, I recognize your username and actually have a lot of

by nkurz 7y ago

> you come across as condescending 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?

colanderman 7y ago
> 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.
- phasetransition 7y ago
  Second Mike Holt as excellent learning source. Would refer to NEC 250.21 for discussion of "ground detector" as it relates to the detection accidental first ground connections in ungrounded systems.
phasetransition 7y ago
nkurz I wrote out all the steps for operation of the typical american house electrical system lower in the thread, perhaps that will answer your questions? https://news.ycombinator.com/item?id=20383117 https://news.ycombinator.com/item?id=20383117

by nkurz 7y ago

colanderman 7y ago

> 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.

phasetransition 7y ago
Second Mike Holt as excellent learning source. Would refer to NEC 250.21 for discussion of "ground detector" as it relates to the detection accidental first ground connections in ungrounded systems.

phasetransition 7y ago

nkurz I wrote out all the steps for operation of the typical american house electrical system lower in the thread, perhaps that will answer your questions? https://news.ycombinator.com/item?id=20383117 https://news.ycombinator.com/item?id=20383117