6 ms·

> and hence exhibits zero voltage across it This is the part that I'm doubting. Are you saying that the DC voltage difference is always 0 VDC? Since much of

by nkurz 7y ago

> and hence exhibits zero voltage across it This is the part that I'm doubting. Are you saying that the DC voltage difference is always 0 VDC? 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? Instead, I think it's more likely that the difference in DC voltage can vary significantly. Are you saying that the AC voltage at all points in the isolated system is always 0 with reference to earth ground? Again, this seems unlikely. If you take 3 simultaneous measurements (Isolated A to earth ground, Isolated B to earth ground, and Isolated A to Isolated B), I have trouble believing that you will find 120V between A and B and 0 between both and ground. 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. But I'm still confused why this is the case. I think it may have more to do with fusing and generation power than with isolation per se. Yes on the side note. The article does a great job of explaining (at least for the US) why neutral and ground are distinct, and why hot and neutral are not symmetric.

colanderman 7y ago
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.
- 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.
andyjpb 7y ago
It's not very intuitive, but it is true! When you have a power source it needs two wires to make a current flow: a wire "out" and a wire "back". You're only part of the circuit if you're connected to both sides. When the two wires are not connected to anything, there is infinite resistance between them. If you grab one side then there's no path through you to the other side so no current flows: the resistance between the two wires is still infinite. Due to Ohm's law, (V=IR), if the current through you is zero then the voltage across you is also zero. The hand touching the wire is at the same voltage as the rest of your body. So you "float up" to whatever voltage the wire you are touching is at. If you complete the circuit then current starts to flow. The shock you receive is related to the current that flows through you. However, even tiny currents cause a big shock because humans are quite sensitive to currents in the micro or milliamps range as that's more than our nervous system uses to move our muscles.
- nkurz 7y ago
  Thank you, and I think I agree with everything you say. Yes, at steady state, as long as you don't complete a circuit, no current flows. Helicopters can be used to do live repairs on high voltage power lines, and if the pilot keeps the copter from running into anything, no one dies: https://www.youtube.com/watch?v=IgrN0fyDRsQ https://www.youtube.com/watch?v=IgrN0fyDRsQ Maybe what I've failed to make clear is that my real question is about the relative difference in safety of the isolated transformer as used in a "razors only" plug and the normal breaker protected outlets used in US houses. For both of them, you could stand on a dry wooden ladder and momentarily grab either wire. But the focuses on the increased safety of the isolated transformer. I'm trying to understand what exactly about this system provides for the increased safety. From what I can tell, everything in your answer applies to both systems. It's the difference in safety between the systems that I don't feel I understand.
  - andyjpb 7y ago
    "Normal breaker connected outlets used in US houses" are already connected to ground on one side. If you grab the neutral side, you will (ideally) get no shock. If you grab the phase side then you'll complete the path to the other side, through ground. An isolating transformer is a 1:1 transformer where the input can be referenced one way and the output another. The output is not connected to ground on either side so when you grab one of the sides, that's the first time it gets referenced to ground. ...and then the explanation continues as per my previous post.
chrisBob 7y ago
It might be more intuitive if you think of the wires as the two sides of a battery. You can safely touch either the + or - side of a battery, even a high voltage one.
klodolph 7y ago
There are a large number of serious errors in this comment which I will try to address. > 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.

by nkurz 7y ago

colanderman 7y ago

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.

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.

andyjpb 7y ago

It's not very intuitive, but it is true! When you have a power source it needs two wires to make a current flow: a wire "out" and a wire "back". You're only part of the circuit if you're connected to both sides. When the two wires are not connected to anything, there is infinite resistance between them. If you grab one side then there's no path through you to the other side so no current flows: the resistance between the two wires is still infinite. Due to Ohm's law, (V=IR), if the current through you is zero then the voltage across you is also zero. The hand touching the wire is at the same voltage as the rest of your body. So you "float up" to whatever voltage the wire you are touching is at. If you complete the circuit then current starts to flow. The shock you receive is related to the current that flows through you. However, even tiny currents cause a big shock because humans are quite sensitive to currents in the micro or milliamps range as that's more than our nervous system uses to move our muscles.

nkurz 7y ago
Thank you, and I think I agree with everything you say. Yes, at steady state, as long as you don't complete a circuit, no current flows. Helicopters can be used to do live repairs on high voltage power lines, and if the pilot keeps the copter from running into anything, no one dies: https://www.youtube.com/watch?v=IgrN0fyDRsQ https://www.youtube.com/watch?v=IgrN0fyDRsQ Maybe what I've failed to make clear is that my real question is about the relative difference in safety of the isolated transformer as used in a "razors only" plug and the normal breaker protected outlets used in US houses. For both of them, you could stand on a dry wooden ladder and momentarily grab either wire. But the focuses on the increased safety of the isolated transformer. I'm trying to understand what exactly about this system provides for the increased safety. From what I can tell, everything in your answer applies to both systems. It's the difference in safety between the systems that I don't feel I understand.
- andyjpb 7y ago
  "Normal breaker connected outlets used in US houses" are already connected to ground on one side. If you grab the neutral side, you will (ideally) get no shock. If you grab the phase side then you'll complete the path to the other side, through ground. An isolating transformer is a 1:1 transformer where the input can be referenced one way and the output another. The output is not connected to ground on either side so when you grab one of the sides, that's the first time it gets referenced to ground. ...and then the explanation continues as per my previous post.

chrisBob 7y ago

It might be more intuitive if you think of the wires as the two sides of a battery. You can safely touch either the + or - side of a battery, even a high voltage one.

klodolph 7y ago

There are a large number of serious errors in this comment which I will try to address. > 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.