(Also side note: in the US system with hot and neutral, neither is 60 V from ground. Hot is 120 V from ground, neutral is (ideally) 0 V. Only in a 240 V system are the voltages symmetric.)
In some apartment buildings, commercial properties, and densely-populated neighborhoods, two phases of a 208V three-phase supply are used instead, which still yields a 120V phase-to-neutral voltage. Large appliances often have a wiring option to operate on 208V.
[ETA: The second hot wire is usually red, and 208V three-phase systems use black, red, and blue for the hot wires.]
Why is this the case historically? It's been a huge annoyance for me personally, as I want to use my arc welder in my garage that has only 120v circuits, as opposed to my cluttered shop that has a 240v circuit available.
I hear you on this; I'm in the situation where neither my small shop nor my garage has 220-240V available. Also, I don't know if my current service will allow for another such circuit - in any case, I'd have to likely add an auxiliary panel to accommodate the extra breaker(s) needed, as my main panel is full.
But I do have one possibility - and maybe you do, too?
From the door to my garage that leads into the house, the laundry room is just around the corner. In that room, I have both the dryer and water heater, on separate circuits. So in theory, I have the voltage needed.
Now, the water heater is hard-wired, so nothing I can really do there (well, I could - but I won't). The dryer, on the other hand, is plugged into a 220V outlet (as they usually are).
So - what I have considered doing is making an extension cord; one with a plug for the dryer, and on the other end a socket for the welder. In case you aren't aware, the plug on a welder for 220VAC is different than the plug on a dryer. And technically, you're not supposed to use a welder on such a socket (there's something in the code that forbids it).
But people have been making and using such extension cords for almost the entire time such welders have been available, and I have not heard of any reasons against them. You can even purchase such cords thru some places, so they're even legal to sell, purchase, and own. It may be something having to do with the strain a welder puts on the plug and extension; that the extra connection can heat up too much?
Anyhow, maybe you could do something like this; get an IR thermometer and monitor the temperature of parts as you weld and use the circuit; see if there is any issues in that regard.
Another thing I've thought about doing, since the wall where the socket for the dryer is shared with the garage, is mounting a junction box, bringing in the 220, then tapping off it inside the junction box to a new socket and the old dryer socket. Then I could plug right into the socket in the garage without needing a custom "extension cord", so long as I unplug the dryer (maybe add a cover to both as well that can be locked to prevent both being used at the same time).
Something tells me, though, that such a setup would not be up to code - but I don't know for certain.
If you’re using an adapter to plug a high-current device into a low-current socket, there is a risk of fire from putting too much current through the in-wall wiring. Tripping a circuit breaker is more likely, though.
See https://en.m.wikipedia.org/wiki/NEMA_connector for lots of detail.
You can get in trouble if the welder expects a safety ground or neutral intended to carry current that you don't have (neither is safe substitute for the other). You can get in trouble if you connect the welder to a circuit that can deliver more current than it's designed for and something fails (it was designed to fail safely assuming that the circuit is limited to X amps by a fuse or breaker).
If the dryer outlet has all the wires the welder wants (and some extra) and the same current rating, an adapter that connects the desired wires and ignore the rest should be safe as long as it's made from big enough wire.
This is not possible for AC due to capacitive coupling between everything. In a large-ish ungrounded AC distribution system, a fault from one phase to ground will carry a large amount of current due to capacitive coupling from the other phases to ground. To reduce this current, an inductor can be installed from neutral to ground [0].
Even for use within a single room, a safe isolated AC system is complicated [1]. If you’re doing benchtop AC experiments, you should not use an isolation transformer — you should use a GFCI.
[0] https://circuitglobe.com/peterson-coil-grounding.html [1] https://www.pglifelink.com/line-isolated-monitors
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.
> 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.
Maybe I can make it clearer with a parallel more specific question. Assume I'm running an gasoline powered generator which produces 120VAC for a single outlet. As far as I can tell, this is an ungrounded isolated system analogous to the isolated transformer. If I'm barefoot and standing on wet concrete, and stick my wet finger into only one side of the plug, will I receive a dangerous shock?
The article seems to claim that as long as I touch only one of the wires of an isolated system, I am "safe". My counter intuition is that that the capacitance of the earth is such that an AC current will flow through my body and cause me harm. Am I wrong? Is the article wrong? Or is there some essential difference between my proposal and the example in the article? If I'm wrong, I'd love to have a more technical explanation of why this seemingly unsafe behavior of sticking my finger in a live socket is actually safe.
Theoretically, no. Your generator isn't going to pick up large static voltage from the atmosphere the way a power grid does, so it's theoretically safe to leave it ungrounded.
However, I would strongly advise against actually doing this experiment unless you are absolutely certain that your generator is isolated from the ground, and even then I would advise against it. Some risks are not worth taking even in the name of science.
A safe version of the experiment would be to take a standard incandescent lightbulb and run one wire to an earth ground and one wire to one side of the plug on the generator. My guess is still (although with more doubt based on how many intelligent people here have opined otherwise) that because of the enormous capacitance of the earth, the light bulb would still light up even though no clear circuit is formed.
In my quite possibly flawed mental model, electrons will be pulled and pushed from the earth across the filament, causing it to heat up. Whether this actually works (in my mental model) depends on the frequency and the capacitance of the generator, but I'm guessing that at 60Hz whatever is inside the generator is enough to get an effect. Unless someone has already done this, I guess I'll have to get a generator and try it.
Your statement implies that you are viewing the earth as a huge, perfect, capacitor. And by "perfect" I mean no series resistance.
In reality, the earth is a relatively poor electrical conductor (i.e., a high resistance [1]) and so where your mental model breaks is omitting that resistance from consideration in your model.
I.e., the equivalent circuit your 'experiment' produces would be:
G-----bulb------R--C
The workers touch the lines to bring themselves (and the whole helicopter) to the same voltage and then get to work. That high of voltage should be fatal, but voltage is relative. They are at the same voltage and there's no path for the electricity to follow.
In a truly ungrounded system, if you touch one of the wires in the system, your body will match the voltage of that wire and nothing "bad" will happen. Only if that voltage has a connection back to the other side of the system will current flow through you, either by touching the other wire or if one of the wires is grounded and you touch the "hot" side.
You are correct that there will be at least 60V that exists between your body and at least one of the wires from the generator. What isn’t guaranteed to exist is a low impedance loop for that current to flow through your body back to the generator. You’re in bare feet on the concrete, but the generator has a plastic body with rubber feet.
People repeat that old “it’s the current that kills you” line and often in a misguided and dangerous way, but it’s actually true in this case. If there’s no way for the current to flow back to the generator, the voltage on its own will likely not do much.
If you were to touch both wires while wearing rubber gloves, you’d be in a similar situation. 120V exists across your body, but because the gloves have a very high resistance, no current can flow (I=V/R)
Edit: kragen has good overview of this below from the generator in a puddle.
Now in practice, most small generators that you buy as a consumer are "floating neutral," which means that the generator chassis, EGC, and generator GSC (i.e. neutral) are NOT bonded together. Floating neutral generators are another, very long post.
---
Source: 6 years ago, when the NEC changed substantially for small portable generators, I wrote a multi-part series on all of this for a trade magazine in the pro audio industry, who make prolific use of portable power distribution systems, and generators, in the 3kW to 500kW size range.
In the course of writing that series, I dealt directly with the EE who works for PGMA, the portable generator manufacturer's trade organization.
It turns out that that current is precisely equal to the current flowing through the impedance between the other side of the circuit ("neutral", if you're connected to the "hot" side) and true ground. If we suppose that that impedance is purely capacitive, the question is just how big the capacitance is, because that tells us how big the impedance ((2πfC)⁻¹) is, and thus the current.
If it's 0.1 pF, the impedance at 60 Hz is 27 GΩ, and the current is 4.5 nA.
If it's 100 pF, 27 MΩ and 4.5 μA, which is still too small for you to feel. Megohms to tens of megohms is a typical resistance range for protection in the kind of neon-bulb testers that use your body to detect hot wires, because microamps are still enough to light up a neon bulb visibly.
If it's 0.1 μF, 27 kΩ and 4.5 mA. At this point you will probably yell and let go rapidly, but you will retain control over your muscles; this is still uncomfortable rather than hazardous.
If it's 1 μF, 2.7 kΩ and 45 mA. At this point your muscles will spasm and, depending on how you happen to be connected, you may be unable to let go until somebody else pulls you off. You will probably not die or suffer any permanent damage but it may be a few days before you feel okay again.
If it's over about 3 μF, and your body's resistance has become the limiting factor in the situation; for the purposes of electrical safety, it's not really any safer than if you had the generator grounded to start with.
So, the question is, how much capacitive coupling are you going to have between your gasoline generator and the actual earth, or rather, the wet concrete you're standing on? And this depends on geometry. Let's suppose the generator has a steel base, connected to its neutral wire, that is ¼m² and suspended on rubber feet of 10 mm above that same wet concrete floor. In this situation C = εA/d ≈ 220 pF, so you're down in the microamp range, three orders of magnitude away from an uncomfortable shock. If you instead have the generator on a dry wooden pallet or a dry concrete floor, you'll have about an order of magnitude less capacitive coupling. If it's on a dry wood floor elsewhere in the house and you're down in the basement standing on the wet concrete, you'll have about two orders of magnitude less capacitive coupling.
So in this situation what you need to worry about is not really the capacitive coupling; it's that maybe one of the rubber feet has a steel screw in the middle that has made conductive contact with the puddle.
Higher voltages make capacitive coupling more dangerous; you've probably seen the video of the lineman coming in on a helicopter to repair a live 100kV line, finishing by flying away while a meter-long arc connects his helicopter to the line. That's tens of milliamps of displacement current flowing across the helicopter's capacitance to the Earth, even though the helicopter is considerably further from the Earth than our hypothetical generator is from your wet concrete.
Is that helpful?
Yes, this is tremendously helpful for putting numbers on my intuitions. To understand the details, I'll need to think about it more tonight when I'm less distracted. As a starting question, does the capacitance internal to the generator matter, and what level might this typically be?
Also, how did you estimate the 1KΩ resistance to ground? Mostly as an aside, I presume you are familiar with the possibly apocryphal story of electrocution with a 9V battery? https://darwinawards.com/darwin/darwin1999-50.html
It might, but it can only make the situation safer. For example, if the "neutral" wire's only connection to the chassis is through its capacitive coupling to that chassis, then the impedance from that capacitance is in series with whatever the chassis-puddle capacitance is. Typically such capacitances are going to be much smaller, because the chassis and hypothetical puddle have much larger surface areas than anything else in the generator. Some can be larger, but their impedance isn't going to go below zero. (A sufficiently large tuned inductance could go below zero, but it would have to be very carefully tuned to lower the impedance rather than increase it.)
> Also, how did you estimate the 1KΩ resistance to ground? Mostly as an aside, I presume you are familiar with the possibly apocryphal story of electrocution with a 9V battery?
Mostly by sucking on ohmmeter leads in the past, although it's also common in textbooks. Yes, I've heard the story; if I had heard it earlier, I might have been more reluctant to suck on ohmmeter leads, although modern digital VOMs use smaller currents that are probably not dangerous.
Yes, it does, but that's fine. The moment you touch a wire in an ungrounded system, that wire is connected to ground through your body, and the voltage between it and ground "floats" harmlessly and instantly to zero, with negligible current passing through you.
Practically, the system is always weakly grounded through mega-ohms of resistance in the insulation. Typically, this means that one wire is better grounded than the other (through a ground plane or part case), and it floats closer to one wire or the other. You can measure the voltage of the "ungrounded" system with a high-impedance multimeter or oscilloscope probe; I've probably done this more times by accident ("105 volts!?? Now 100 volts....Why is my motor ground at 95 volts and falling without turning the motor?") than on purpose.
I suspect that I'll get a terrible shock and likely die, because some substantial current will start flowing through my body. You seem to be saying that I'll be fine. Why do you believe this?
My guess is that the typical ground isolated system is safe not because of the isolation per se, but because the transformer used is small enough that the maximum current is not life threatening. When the circuit is intact, I'm unlikely to get anything other than a static shock. When the circuit is broken, it's current limited. My intuition is that this is the same as having a very small generator with only a few coils of wire. What am I missing?
No, this is definitely not the case. 8mA is 1W at 120V, and although 8mA will not actually kill you, it will be a very nasty experience you will not want to repeat. 80mA, I wouldn't want to sell you life insurance. 160mA can definitely kill you.
Efficient transformers are also not effective current-limiting devices; their coils do have some resistance, and their magnetics have some leakage inductance which limits the current, but these only produce a small voltage loss at their rated output. Say you have a transformer designed for a 50-watt razor (400mA at 120V, several times more than you can survive). If its coils are dissipating 10 watts, it's going to get pretty hot there inside the wall while you're shaving, so this means that its coil resistance is already impractically high for this application. (Think about how hot a 10-watt compact fluorescent or LED bulb gets, even though it's exposed to airflow.) Yet that's only 62.5Ω, which means that if you short out its output, you're going to get 1900 mA through the short, almost five times its rated output and 19 times as much current as is needed to kill you. Using a more realistic coil resistance gives even larger peak currents.
(Also, using 20% of the total circuit power means that the razor sees 20% lower voltage, which may cause it to fail to work.)
The bottom line is, you can't depend on the transformer burning itself up or otherwise limiting current to protect you. The correct answer is given in my other response above.
Special transformers like neon-sign transformers and arc-welding transformers do have sufficiently high leakage inductance to not burn up if you short out their output, but such transformers are not used for general power application. Generally their maximum currents are still far too high to provide much protection for squishy human bodies.
When the circuit is broken .. by definition, there isn't any current flowing!
You need a loop somehow from one of the terminals of the transformer through you to the other terminal in order to get a current flow.
(There might be some transient effect caused by the inductance of the system within one half-cycle, which may give you an arc as you smash the heater, but that can't continue and is unlikely to pass through you)
One specific wire is “safe.” One wire (neutral) is at 0V, and the other wire (hot) is at 120V. So it is “safe” to touch the neutral wire. In practice, there are various fault conditions which will cause the neutral wire to become unsafe, which is why we now have a separate earth ground.
I'm confused by the assertion that you could not get a shock from a single wire in an ungrounded system. If there is 120V AC between the two wires, doesn't this mean that there has to be at least 60V AC potential between at least one of the wires and earth ground? Is he depending on the "halved" voltage for safety, or depending on having an intact circuit so the other connected wire is always "better" than an earth ground, or is he right in some way that I'm not seeing?