> 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?
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.
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?
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?”
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.
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.
Wait, does this apply to AC too? I always believed AC can create current even if the loop isn't closed. Say you have an exposed AC wire and touch it; I'd expect you to get a burn at the place you're touching, as the electrons from your finger at the contact point are pulled into the wire and pushed back to the finger, at 50 or 60 Hz. In general, isn't this how RF burns work?
However, RF is more complicated. The difference between "RF" and "AC" is a matter of what materials and frequencies are involved and how you are using them.
Terrestrial RF systems rely on the Earth as a ground conductor. It becomes more complicated because the impedance (like an AC version of resistance <waves hands>) of a material (such as your body between the antenna and ground) will change depending on frequency. Once a circuit it in place, the impedance mismatch between your finger and the thing it is touching will cause energy to build up at that interface.
This is similar to how light diffracts when it moves from one material to another and you can see the boundary because some energy builds up there and is scattered.
You won't get a burn if there is no circuit.
...but if you are fully enclosed in an RF field, you may get little circuits forming because your body is in contact with that field, and therefore referenced to it, and the potential of the field might vary across your body.
Your body also has a little bit of effective grounding just by existing inside a room, but that's modeled as "a 100pF capacitor in series with a 1.5kΩ resistor". Charging a 100pF capacitor to 120 volts 120 times a second comes out to 0.2 milliamps, which is not going to burn anything. 0.6 milliamps on a 240 volt 50Hz supply. And that's only if your current source is badly isolated. The better it's kept away from the local ground, the less flow you'll see.
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.
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.
> 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.
> > 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.
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.