7 ms·

> Before you "grounded" your system, the AC voltage in general acted pretty safe for your customers. The only way they could get a shock was if they touched bot

by nkurz 7y ago

> Before you "grounded" your system, the AC voltage in general acted pretty safe for your customers. The only way they could get a shock was if they touched both wires at the same time. This was a fairly rare occurrence. One single wire acted as if it was "safe," and it did not deliver shocks. 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?

colanderman 7y ago
The statement is predicated on the entire system being ungrounded (i.e., everything on your side of the transformer). Voltage is relative; if there's no prior connection between two systems, when one gets connected to the other, the systems become referenced to each other at that connection, through which no current can flow (since it's the only connection), and hence exhibits zero voltage across it. In this example, the other half of the power outlet is then 120 V relative to whatever the person is touching (presumably Earth ground). (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.)
- nateguchi 7y ago
  In fact, you can buy transformers that are ungrounded to isolate the supply and make it safe to touch either wire. When combined with an RCD, this makes a very safe system
  - isostatic 7y ago
    In the UK, isolated transformers are how we allow shaver sockets in bathrooms. You touch one of the wires and (via wet feet a pipes) you create the path to ground, which is bad. But because the wire is only live when there's a complete circuit, it's safe. You'd have to grasp both sides of the circuit with different hands to cause major problems
    - flyinghamster 7y ago
      A bonus for the shaver socket is that it is often switchable, so a 120V shaver can be plugged in. This is less of an issue now, with newer shavers having circuitry to allow them to run on anything from 100-240V.
    - deleted 7y ago
      [deleted]
  - jakobegger 7y ago
    But the ungrounded transformer prevents the fault currents that the RCD would detect, so unless I am mistaken there is no scenario where a RCD would provide additional protection beyond an ungrounded transformer.
    - nateguchi 7y ago
      You can feasibly combine an RCD and an isolated transformer, not sure I've seen this done before though or if it gives any real benefit.
- flyinghamster 7y ago
  Note also that the typical residential electric service in the US is actually 240V split-phase; the distribution transformer secondary winding is center-tapped, with the center tap grounded and brought in as the neutral. Phase-to-ground is 120V, phase-to-phase is 240V. Normally, 240V is reserved for heavy-draw appliances like large air conditioners, ovens, arc welders, and so on. 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.]
  - Blackthorn 7y ago
    > Note also that the typical residential electric service in the US is actually 240V split-phase; the distribution transformer secondary winding is center-tapped, with the center tap grounded and brought in as the neutral. 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.
    - cr0sh 7y ago
      > 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.
- 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.
- amluto 7y ago
  > The statement is predicated on the entire system being ungrounded (i.e., everything on your side of the transformer). 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 https://circuitglobe.com/peterson-coil-grounding.html [1] https://www.pglifelink.com/line-isolated-monitors https://www.pglifelink.com/line-isolated-monitors
dfox 7y ago
Potential difference between two otherwise not-connected systems is not well defined. In practice the system will not be totally ungrounded, but there will be some high-impedance path to ground, and the impedance is hopefully large enough to not allow any significant fault current to flow.
- nkurz 7y ago
  I think this is getting close to the right answer, but I still don't understand the details. Let's say that we set up a large ground isolated system, and have it driving a resistance heater (20A at 120VAC). Standing barefoot on wet concrete, I tightly grab one of the leads to the heating element. With my other hand, I use a hammer smash the heating element so it's no longer a complete circuit. What happens and why? 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?
  - pjc50 7y ago
    > When the circuit is broken, it's current limited. 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)
  - kragen 7y ago
    > because the transformer used is small enough that the maximum current is not life threatening. 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.
klodolph 7y ago
> 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? 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.
LeifCarrotson 7y ago
> 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? 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.
nkurz 7y ago
I've gotten some helpful answers, but it's apparent that I've failed make clear my actual question, which is about the relative safety of a system with an isolated ground, such as the "razors only" plugs discussed in the article. The article seems to claim that such systems are safe for any single point of contact. 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.
- deleted 7y ago
  [deleted]
- thebluehawk 7y ago
  A good example are the helicopters with workers inside that work on high voltage lines while they are charged. 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.
  - nkurz 7y ago
    It's a great example, but a very different one. In the case of the helicopter, there truly is no path to ground. In the case of a person standing barefoot on wet ground, they (potentially) become the ground. If the human touching the wires is ungrounded (as in the case of the helicopter), there is no difference in this case whether or not the system itself is ground connected. The question is how much difference it makes for the system to be grounded if the human is also acting as a ground.

by nkurz 7y ago

colanderman 7y ago

The statement is predicated on the entire system being ungrounded (i.e., everything on your side of the transformer). Voltage is relative; if there's no prior connection between two systems, when one gets connected to the other, the systems become referenced to each other at that connection, through which no current can flow (since it's the only connection), and hence exhibits zero voltage across it. In this example, the other half of the power outlet is then 120 V relative to whatever the person is touching (presumably Earth ground). (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.)

nateguchi 7y ago
In fact, you can buy transformers that are ungrounded to isolate the supply and make it safe to touch either wire. When combined with an RCD, this makes a very safe system
- isostatic 7y ago
  In the UK, isolated transformers are how we allow shaver sockets in bathrooms. You touch one of the wires and (via wet feet a pipes) you create the path to ground, which is bad. But because the wire is only live when there's a complete circuit, it's safe. You'd have to grasp both sides of the circuit with different hands to cause major problems
  - flyinghamster 7y ago
    A bonus for the shaver socket is that it is often switchable, so a 120V shaver can be plugged in. This is less of an issue now, with newer shavers having circuitry to allow them to run on anything from 100-240V.
  - deleted 7y ago
    [deleted]
- jakobegger 7y ago
  But the ungrounded transformer prevents the fault currents that the RCD would detect, so unless I am mistaken there is no scenario where a RCD would provide additional protection beyond an ungrounded transformer.
  - nateguchi 7y ago
    You can feasibly combine an RCD and an isolated transformer, not sure I've seen this done before though or if it gives any real benefit.
flyinghamster 7y ago
Note also that the typical residential electric service in the US is actually 240V split-phase; the distribution transformer secondary winding is center-tapped, with the center tap grounded and brought in as the neutral. Phase-to-ground is 120V, phase-to-phase is 240V. Normally, 240V is reserved for heavy-draw appliances like large air conditioners, ovens, arc welders, and so on. 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.]
- Blackthorn 7y ago
  > Note also that the typical residential electric service in the US is actually 240V split-phase; the distribution transformer secondary winding is center-tapped, with the center tap grounded and brought in as the neutral. 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.
  - cr0sh 7y ago
    > 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.
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.
amluto 7y ago
> The statement is predicated on the entire system being ungrounded (i.e., everything on your side of the transformer). 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 https://circuitglobe.com/peterson-coil-grounding.html [1] https://www.pglifelink.com/line-isolated-monitors https://www.pglifelink.com/line-isolated-monitors

dfox 7y ago

Potential difference between two otherwise not-connected systems is not well defined. In practice the system will not be totally ungrounded, but there will be some high-impedance path to ground, and the impedance is hopefully large enough to not allow any significant fault current to flow.

nkurz 7y ago
I think this is getting close to the right answer, but I still don't understand the details. Let's say that we set up a large ground isolated system, and have it driving a resistance heater (20A at 120VAC). Standing barefoot on wet concrete, I tightly grab one of the leads to the heating element. With my other hand, I use a hammer smash the heating element so it's no longer a complete circuit. What happens and why? 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?
- pjc50 7y ago
  > When the circuit is broken, it's current limited. 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)
- kragen 7y ago
  > because the transformer used is small enough that the maximum current is not life threatening. 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.

klodolph 7y ago

> 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? 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.

LeifCarrotson 7y ago

> 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? 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.

nkurz 7y ago

I've gotten some helpful answers, but it's apparent that I've failed make clear my actual question, which is about the relative safety of a system with an isolated ground, such as the "razors only" plugs discussed in the article. The article seems to claim that such systems are safe for any single point of contact. 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.

deleted 7y ago
[deleted]
thebluehawk 7y ago
A good example are the helicopters with workers inside that work on high voltage lines while they are charged. 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.
- nkurz 7y ago
  It's a great example, but a very different one. In the case of the helicopter, there truly is no path to ground. In the case of a person standing barefoot on wet ground, they (potentially) become the ground. If the human touching the wires is ungrounded (as in the case of the helicopter), there is no difference in this case whether or not the system itself is ground connected. The question is how much difference it makes for the system to be grounded if the human is also acting as a ground.