6 ms·

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 c

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

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