19 ms·

Why Three Prongs? (1996)

249 points by nkurz 7y ago

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.
dang 7y ago
A thread from 2012: https://news.ycombinator.com/item?id=4058834 https://news.ycombinator.com/item?id=4058834
dforrestwilson 7y ago
Perhaps an ignorant question, but given that when I travel I have to bring a seemingly endless array of plug adapters, which power standard is the "best" and how do we get rid of the rest?
- nateguchi 7y ago
  I may be biased, but I think British plugs are great. They have integrated fuses and are switchable too! The sockets have shutters as well to stop children sticking paperclips or forks in and electrocuting themselves. Not great to stand on, however
  - danieltillett 7y ago
    They are absolutely massive though.
    - jjoonathan 7y ago
      Also, half the shutters always seem to be stuck shut on public outlets.
      - jacobush 7y ago
        At least failing in a safe mode?
        ken 7y ago
        Or, depending on your perspective, failing in a way that incentivizes users to jab things in the holes to try to "fix" it. This is another reason I'm a fan of TRUE1: the moving parts are in the plug, not the socket.
  - martin-adams 7y ago
    Tom Scott did a wonderful video breaking down the design of the British plug. https://www.youtube.com/watch?v=UEfP1OKKz_Q https://www.youtube.com/watch?v=UEfP1OKKz_Q
linsomniac 7y ago
Short form: Neutral is grounded. The "mouth" part of the plug is the ground and is isolated from the neutral to prevent inadvertent carrying of potential. Aside: it is generally considered safer to have the outlets ("receptacles") installed with the ground at the top, but we don't tend to do this in NA because ground at the bottom makes it look like a little face and humans like faces. Edit: Adding link to EETimes: https://www.eetimes.com/document.asp?doc_id=1272972# https://www.eetimes.com/document.asp?doc_id=1272972#
- count 7y ago
  I recently remodeled a house (in the US), and installed every receptacle with the ground at the top, because of that. Every person who comes over that knows anything about electrical stuff instantly says 'your plugs are upside down'...and then I have to explain to them how/why it works :) The inspector had no issue with it though, thankfully! In health/saftey critical installs (like hospitals), the ground MUST be on top, according to our local code. It's optional for residential.
  - throw0101a 7y ago
    > I recently remodeled a house (in the US), and installed every receptacle with the ground at the top, because of that. Have you run into any appliances or devices that are "optimized" for a ground-down configuration?
    - kevin_thibedeau 7y ago
      Low profile plugs intended to have the cord exit downward at 45-degrees don't work as well with upside down receptacles.
    - count 7y ago
      Nothing so far. Most all appliances these days are single-device circuits (in the new kitchen, literally every appliance required a dedicated circuit - fridge, gas range, microwave, ovens, dishwasher, garbage disposal, etc.). So, lots of single-plug receptacles (code says a dedicated circuit has to have a special single plug receptacle here). No issues so far with anything.
phasetransition 7y ago
For those outside the US, or generally confused how US domestic power distribution works, let me explain in the context of the 3 prong plug and NFPA 70 NEC (National Electrical Code) for the lay reader. We'll start outside the house and go all the way to the plug: 0. Before the transformer secondary on the pole outside the house, the wiring is dictated by the NESC, which is like the NEC for utilities. We'll leave that there. 1. Most US homes have a single phase, "center tapped" transformer secondary. This gives three voltage potentials to feed the home: (Nominal) 0V - from the center tap; +120Vrms - from one "end" of the transformer; -120Vrms from the other end of the transformer. And, yes, I am glossing over the phase relationship here. All three potentials are connected through the chunky copper of the transformer, which provides a current path to complete the circuit. 2. The three potentials feed the house meter and disconnect. The 0V potential may, or may not, be insulated on the path back to the transformer secondary, and often doubles as the mechanical support for the incoming lines (the "service"). The other two potentials will be insulated wiring. 3. Inside the main service panel the +120V and -120V inputs are split between the breakers. Every other breaker is connected to +120V or -120V. Typically a breaker that spans multiple slots is connected from -120V to +120V. That is the "240Vrms" for ovens, hot water heaters, dryers, air conditioners, etc. 4. Adjacent to the metal bits that fan out the +120V and -120V is a strip of metal that allows multiple connections to 0V potential. Colloquially this the "neutral bus," but in the NEC this is termed a "grounded service conductor." All of the white wires from the circuits in the house ("branch circuits") are tied to the spaces in this metal bus bar. This metal bar is also electrically connected to the 0V potential entering the house from the transformer center tap. 5. Also inside the main service panel there is strip of metal that allows connection for a number of "grounds." It looks just like the "neutral" connection bar, but instead has a number of green wires attached to it. Colloquially these wires a called "ground," but the NEC calls them "Equiment Grounding Conductor" or EGC. 6. Between the grounded conductor ("neutral") bar, and the EGC ("ground") bar there is a removable conductive link. In the main panel this link remains installed, but in secondary panels it is removed. The link is removed in "subpanels" to insure correct operation in the event of a fault. Fault conditions are discussed below. 7. Also connected to the EGC and grounded conductor is a third wire. The NEC terms this the "grounding electrode conductor" or GEC. The GEC then goes to a water pipe or "grounding" rod(s). Thus the GEC is the connection between physical Earth voltage potential and the power panel. It is unfortunate that the EGC and GEC are such similar abbreviations. In the event I'm out of characters, I'll pause and reply to myself now.
nck4222 7y ago
For some reason, this reminds me of the Laryngeal nerve. It's like a natural process repeating itself in different systems. Because re-engineering the solution from the ground up to be more efficient isn't an option, there has to be small incremental steps to arrive at a solution that works. https://en.wikipedia.org/wiki/Recurrent_laryngeal_nerve https://en.wikipedia.org/wiki/Recurrent_laryngeal_nerve
- jedimastert 7y ago
  > Because of the inefficiencies of the routing the nerve takes, it's often hailed as one of the most striking cases against intelligent design — or at least calls into question the "intelligence". Man, what an obnoxious way to start an article. EDIT: link was removed, this comment no longer needed.
  - nck4222 7y ago
    Oh my bad, didn't mean to suggest it was intelligently designed. I didn't read that full page and didn't pick up on that context. I'll remove that link.
    - jedimastert 7y ago
      My comment was a bit rude as well, I apologize. The website seems to be a little argumentative and it rubbed me the wrong way.
ken 7y ago
The obvious follow up question, then, is: why does Japan use 2-prong outlets even today? Are Japanese people dying from electrical shock at a much higher rate than the rest of the world? Is Japan full of only Perfect Electricians? Or is this not actually the significant problem that this myth (in the etiological sense, not the fictional sense) would lead us to believe? The only 2-prong device I own is a (Japanese-made, of course) rice cooker. You'd think that if any device warranted 3-prong safety, it'd be a metal container which combines water and mains electricity.
- deleted 7y ago
  [deleted]
- throw0101a 7y ago
  > The obvious follow up question, then, is: why does Japan use 2-prong outlets even today? Because JP is behind the times: > Japan also uses a grounded plug similar to the North American NEMA 5-15.[49] However, it is less common than its NEMA 1-15 equivalent. Since 2005, new Japanese homes are required to have class I grounded sockets for connecting domestic appliances. This rule does not apply for sockets not intended to be used for domestic appliances, but it is strongly advised to have class I sockets throughout the home.[52] * https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#JIS_C_8303,_Class_I_earthed https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#JIS... So as things are re-built, you'll see more. In the same way the old 'best practices' in automobiles didn't mandate ABS or air bags, some country's electrical best practices aren't written to the state of the art. Everyone is probably looking at IEC 60364 for overall guidelines: * https://en.wikipedia.org/wiki/IEC_60364 https://en.wikipedia.org/wiki/IEC_60364 See: * https://en.wikipedia.org/wiki/Act_on_Product_Safety_of_Electrical_Appliances_and_Materials https://en.wikipedia.org/wiki/Act_on_Product_Safety_of_Elect...
  - ralfd 7y ago
    What about the Europlug?
    - sitharus 7y ago
pkaye 7y ago
Why do a majority of electronic devices has only 2 prong?
- MisterTea 7y ago
  These are called CLASS II devices and they are designed with double or reinforced insulation. In plain terms it means that the current carrying components are themselves insulated AND surrounded by a second layer of insulation. That or the current carrying components are protected by reinforced insulation like a hard plastic case. A practical example is the humble power brick or wall wart. These are class II power supplies which are enclosed in a hard plastic case which protects the user from electrical shock. Reference: https://www.cui.com/blog/class-2-vs-class-ii-power-supplies https://www.cui.com/blog/class-2-vs-class-ii-power-supplies
haberman 7y ago
If GFI sockets and breakers had been widely available at the time, would the third prong have been added? Would a GFI along with the "one long slot" plugs make the ground wire obsolete?
- Bjartr 7y ago
  Possibly, GFI mechanisms are good, but they're not as good as a true ground since they take time to activate and require a minimum current draw to trigger (you can still be injured or killed below that current draw)
- blattimwind 7y ago
  No
- leni536 7y ago
  Probably. AFAIK GFI measures the current difference live and neutral, so a current between live and ground (as in the third slot) would trip it. Imagine if someone forms a path for current between a "grounded" metal case and live. If there is no third slot then "grounding" means connecting to neutral, so GFI wouldn't trip.
- xyzzyz 7y ago
  GFI outlets are $15+, and normal outlets are $2. This adds up when you're building a house: a typical house has something on the order of 30-50 outlets, so you're looking at $400-$700 vs $60-$100. It might not seem like much in the grand scheme of things, especially to software engineers making lots of money, but if we had this approach not only for outlets, but also for everything else, building literally anything would cost 5 times as much (and in fact it does these days, to a large degree because we can afford spending 5 times as much on building).
  - magduf 7y ago
    >GFI outlets are $15+, and normal outlets are $2. Where are you getting your prices? Last time I checked, GFCI outlets were indeed around $15, but the normal ones are less than a dollar when you buy them in bulk packs (which you would for building a house). Otherwise, your point is sound, but the price difference is even greater than you said. Those standard outlets are really dirt-cheap, and it's annoying that builders are so cheap about installing them. New construction has gotten better though, but really there should be an outlet every 5-6 feet in my opinion, or maybe even less.
jhallenworld 7y ago
Three prongs was a good idea before the existence of ground fault interrupters, but now that they do exist, ground fault interrupters with two prongs provides a much better solution. The problem with ground is that it's a conductor. If there's a fault on it, you don't know exactly what it's connected to, so now you have a situation where the metal chassis is connected to something. You touch the chassis and the really-grounded a sink and you have a problem. Here's a real world example of this. The ground is bonded to neutral at the panel. But what happens when neutral between the pole and the house breaks? This happened to my neighbor. There is still 240V with neutral/ground floating somewhere in the middle. All of his appliances on the less loaded side of the 240V blew up and "ground" was very not at earth ground.
- blattimwind 7y ago
  That's a problem with incorrect installation / code. In a proper TN-C-S system the combined PEN will be split up at the main panel, and PE will be local ground and pipework is typically connected to that.
kazinator 7y ago
The article isn't not really on point. A two-prong system still has an earthed neutral. The third prong is purely for redundancy. If neutral happens to be disconnected, there is still a return path. It also protects against mixed up neutral and hot. If the chassis of a device is grounded to the third conductor, and that one is properly routed, then an inadvertent reverse wiring of hot/neutral is mitigated. A third conductor also makes possible ground fault detection: situations when a device is not returning all current through the neutral, because its safety ground has become energized. How might a neutral end up disconnected? Not just by accident; for instance, someone would incorrectly wire a cold-side switch instead of hot-side (E.g. wall panel switch that controls an outlet) so that when the outlet is off, the neutral has been cut off, but hot is still live. In that situation, the third conductor would still be grounded. Nobody would mistakenly implement the switch on the ground line unless they didn't bother testing it, because the switch then wouldn't work; the ground line is implicitly protected from having switches accidentally planted on it by the fact that interrupting it does nothing, functionally.
bookofjoe 7y ago
What I learned from reading every single one of the 184 comments (as of 6:28 pm ET) on this post: call an electrician for anything more complex than changing a light bulb.
komali2 7y ago
> There is a simple solution to these problems: connect your system to the Earth. Drive some long metal rods into the dirt, and connect them to your wires. That way, lightning currents will be directed into the Earth rather than spreading throughout your power lines. I have tried and failed many times to understand the concept of "ground." Why does electricity "want to go to the ground?" What constitutes "ground?" Can I touch wire to concrete, and that counts as "ground?" No? Must I then sink it? How deep? Until it touches dirt? What about sand? Where does the electricity "go" then? I've read plenty of articles, never got it.
- sansnomme 7y ago
  Study the Maxwell equations and thou shalt be enlightened.
- mantap 7y ago
  Obviously rock doesn't conduct electricity but water+impurities does. When people talk about "ground" they are talking about the water in the ground.
  - saalweachter 7y ago
    Incidentally, this is why if you're running an electric fence, and you have rocky or dry soil, it is advisable to run alternating hot & ground wires, and to not rely on the ground to complete the circuit.
- ZainRiz 7y ago
  There are two different mental models I find really useful when thinking about electricity. The second one will answer your question more directly, but the first one helps set the stage better. Mental Model #1 (useful for most work with electricity): Think of electricity moving through circuits as water going through pipes. Electricity = water Voltage = water pressure Battery/Voltage Source = water pump Wires = Water pipes High/Low resistance = Wide/narrow water pipes Voltage is like water pressure. You have a pump creating high water pressure is like a power source with a high voltage. The pump is always on and creating water pressure, but there's a cap placed on it that blocks the water from escaping. Similarly, in an open circuit the air acts as this cap and blocks the electricity from proceeding. The wires in a circuit act as pipes channeling the water to different areas. Resistance in a circuit is analogous to the thickness of the pipes (low resistance == wide pipes, high resistance == thin pipes). The above analogy sets part of the mental model. The twist is that in reality electrons want to actually travel from a place of low voltage to a place of high voltage (classical electrical diagrams show this process in reverse). Which leads us to... Mental Model #2 Voltage sources are powerful electron vacuum cleaners trying to to suck up electrons. (You can apply this on top of mental model #1, just reverse the direction of the water flow) Open circuits generally don't conduct electricity since within a nano-second the voltage source will suck up all the free electrons in the circuit. The negative/ground pole in circuits are where the free electrons get supplied from for the voltage source to "suck in" the electrons from. The ground (by virtue of it being so large and having lots of moisture/materials which conduct electricity) happens to have a virtually limitless source of electrons available for the voltage source to suck into itself. This provides the circuit with all the electrons required to successfully operate. Hope that helps :)
CapitalistCartr 7y ago
Past posting on HN: https://news.ycombinator.com/item?id=4058834 https://news.ycombinator.com/item?id=4058834
js2 7y ago
Since this article was written, the NEC has also started requiring arc fault circuit interrupters. https://en.wikipedia.org/wiki/Arc-fault_circuit_interrupter https://en.wikipedia.org/wiki/Arc-fault_circuit_interrupter BTW, if you have a portable generator for emergency purposes and it's connected via a transfer switch, take note of the important grounding and bonding requirements: https://cpower.com/PDF/InfoSheets/44.pdf https://cpower.com/PDF/InfoSheets/44.pdf
jonprobably 7y ago
Why is the metal chassis wired at all?
deleted 7y ago
[deleted]
peter_d_sherman 7y ago
Excerpt: "The sparks occur because of a little-known fact: all the world is a gigantic electrostatic generator. There is a flow of charge going on vertically everwhere on earth. Thunderstorms pump negative charge downwards, and the charge filters upwards everywhere else on earth. Depending on the height of your circuitry above the earth's surface, depending on the area covered by your wires, and depending on whether there was a thunderstorm above you at the time, there might be a fairly huge DC charge on your electrical distribution system. This charge might be several hundred volts; enough to zap computers and delicate electronics. Or... it might be many tens of thousands of volts, enough to create enormous sparks which jump across switches and leap out of wall outlets, wall switches, across transformer windings, etc. Your electric power system is acting like a sort of capacitive "antenna" which intercepts the feeble current coming from the sky and builds up a huge potential difference with respect to the earth."

249 points 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.

dang 7y ago

A thread from 2012: https://news.ycombinator.com/item?id=4058834 https://news.ycombinator.com/item?id=4058834

dforrestwilson 7y ago

Perhaps an ignorant question, but given that when I travel I have to bring a seemingly endless array of plug adapters, which power standard is the "best" and how do we get rid of the rest?

nateguchi 7y ago
I may be biased, but I think British plugs are great. They have integrated fuses and are switchable too! The sockets have shutters as well to stop children sticking paperclips or forks in and electrocuting themselves. Not great to stand on, however
- danieltillett 7y ago
  They are absolutely massive though.
  - jjoonathan 7y ago
    Also, half the shutters always seem to be stuck shut on public outlets.
    - jacobush 7y ago
      At least failing in a safe mode?
      - ken 7y ago
        Or, depending on your perspective, failing in a way that incentivizes users to jab things in the holes to try to "fix" it. This is another reason I'm a fan of TRUE1: the moving parts are in the plug, not the socket.
- martin-adams 7y ago
  Tom Scott did a wonderful video breaking down the design of the British plug. https://www.youtube.com/watch?v=UEfP1OKKz_Q https://www.youtube.com/watch?v=UEfP1OKKz_Q

linsomniac 7y ago

Short form: Neutral is grounded. The "mouth" part of the plug is the ground and is isolated from the neutral to prevent inadvertent carrying of potential. Aside: it is generally considered safer to have the outlets ("receptacles") installed with the ground at the top, but we don't tend to do this in NA because ground at the bottom makes it look like a little face and humans like faces. Edit: Adding link to EETimes: https://www.eetimes.com/document.asp?doc_id=1272972# https://www.eetimes.com/document.asp?doc_id=1272972#

count 7y ago
I recently remodeled a house (in the US), and installed every receptacle with the ground at the top, because of that. Every person who comes over that knows anything about electrical stuff instantly says 'your plugs are upside down'...and then I have to explain to them how/why it works :) The inspector had no issue with it though, thankfully! In health/saftey critical installs (like hospitals), the ground MUST be on top, according to our local code. It's optional for residential.
- throw0101a 7y ago
  > I recently remodeled a house (in the US), and installed every receptacle with the ground at the top, because of that. Have you run into any appliances or devices that are "optimized" for a ground-down configuration?
  - kevin_thibedeau 7y ago
    Low profile plugs intended to have the cord exit downward at 45-degrees don't work as well with upside down receptacles.
  - count 7y ago
    Nothing so far. Most all appliances these days are single-device circuits (in the new kitchen, literally every appliance required a dedicated circuit - fridge, gas range, microwave, ovens, dishwasher, garbage disposal, etc.). So, lots of single-plug receptacles (code says a dedicated circuit has to have a special single plug receptacle here). No issues so far with anything.

phasetransition 7y ago

For those outside the US, or generally confused how US domestic power distribution works, let me explain in the context of the 3 prong plug and NFPA 70 NEC (National Electrical Code) for the lay reader. We'll start outside the house and go all the way to the plug: 0. Before the transformer secondary on the pole outside the house, the wiring is dictated by the NESC, which is like the NEC for utilities. We'll leave that there. 1. Most US homes have a single phase, "center tapped" transformer secondary. This gives three voltage potentials to feed the home: (Nominal) 0V - from the center tap; +120Vrms - from one "end" of the transformer; -120Vrms from the other end of the transformer. And, yes, I am glossing over the phase relationship here. All three potentials are connected through the chunky copper of the transformer, which provides a current path to complete the circuit. 2. The three potentials feed the house meter and disconnect. The 0V potential may, or may not, be insulated on the path back to the transformer secondary, and often doubles as the mechanical support for the incoming lines (the "service"). The other two potentials will be insulated wiring. 3. Inside the main service panel the +120V and -120V inputs are split between the breakers. Every other breaker is connected to +120V or -120V. Typically a breaker that spans multiple slots is connected from -120V to +120V. That is the "240Vrms" for ovens, hot water heaters, dryers, air conditioners, etc. 4. Adjacent to the metal bits that fan out the +120V and -120V is a strip of metal that allows multiple connections to 0V potential. Colloquially this the "neutral bus," but in the NEC this is termed a "grounded service conductor." All of the white wires from the circuits in the house ("branch circuits") are tied to the spaces in this metal bus bar. This metal bar is also electrically connected to the 0V potential entering the house from the transformer center tap. 5. Also inside the main service panel there is strip of metal that allows connection for a number of "grounds." It looks just like the "neutral" connection bar, but instead has a number of green wires attached to it. Colloquially these wires a called "ground," but the NEC calls them "Equiment Grounding Conductor" or EGC. 6. Between the grounded conductor ("neutral") bar, and the EGC ("ground") bar there is a removable conductive link. In the main panel this link remains installed, but in secondary panels it is removed. The link is removed in "subpanels" to insure correct operation in the event of a fault. Fault conditions are discussed below. 7. Also connected to the EGC and grounded conductor is a third wire. The NEC terms this the "grounding electrode conductor" or GEC. The GEC then goes to a water pipe or "grounding" rod(s). Thus the GEC is the connection between physical Earth voltage potential and the power panel. It is unfortunate that the EGC and GEC are such similar abbreviations. In the event I'm out of characters, I'll pause and reply to myself now.

nck4222 7y ago

For some reason, this reminds me of the Laryngeal nerve. It's like a natural process repeating itself in different systems. Because re-engineering the solution from the ground up to be more efficient isn't an option, there has to be small incremental steps to arrive at a solution that works. https://en.wikipedia.org/wiki/Recurrent_laryngeal_nerve https://en.wikipedia.org/wiki/Recurrent_laryngeal_nerve

jedimastert 7y ago
> Because of the inefficiencies of the routing the nerve takes, it's often hailed as one of the most striking cases against intelligent design — or at least calls into question the "intelligence". Man, what an obnoxious way to start an article. EDIT: link was removed, this comment no longer needed.
- nck4222 7y ago
  Oh my bad, didn't mean to suggest it was intelligently designed. I didn't read that full page and didn't pick up on that context. I'll remove that link.
  - jedimastert 7y ago
    My comment was a bit rude as well, I apologize. The website seems to be a little argumentative and it rubbed me the wrong way.

ken 7y ago

The obvious follow up question, then, is: why does Japan use 2-prong outlets even today? Are Japanese people dying from electrical shock at a much higher rate than the rest of the world? Is Japan full of only Perfect Electricians? Or is this not actually the significant problem that this myth (in the etiological sense, not the fictional sense) would lead us to believe? The only 2-prong device I own is a (Japanese-made, of course) rice cooker. You'd think that if any device warranted 3-prong safety, it'd be a metal container which combines water and mains electricity.

deleted 7y ago
[deleted]
throw0101a 7y ago
> The obvious follow up question, then, is: why does Japan use 2-prong outlets even today? Because JP is behind the times: > Japan also uses a grounded plug similar to the North American NEMA 5-15.[49] However, it is less common than its NEMA 1-15 equivalent. Since 2005, new Japanese homes are required to have class I grounded sockets for connecting domestic appliances. This rule does not apply for sockets not intended to be used for domestic appliances, but it is strongly advised to have class I sockets throughout the home.[52] * https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#JIS_C_8303,_Class_I_earthed https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets#JIS... So as things are re-built, you'll see more. In the same way the old 'best practices' in automobiles didn't mandate ABS or air bags, some country's electrical best practices aren't written to the state of the art. Everyone is probably looking at IEC 60364 for overall guidelines: * https://en.wikipedia.org/wiki/IEC_60364 https://en.wikipedia.org/wiki/IEC_60364 See: * https://en.wikipedia.org/wiki/Act_on_Product_Safety_of_Electrical_Appliances_and_Materials https://en.wikipedia.org/wiki/Act_on_Product_Safety_of_Elect...
- ralfd 7y ago
  What about the Europlug?
  - sitharus 7y ago

pkaye 7y ago

Why do a majority of electronic devices has only 2 prong?

MisterTea 7y ago
These are called CLASS II devices and they are designed with double or reinforced insulation. In plain terms it means that the current carrying components are themselves insulated AND surrounded by a second layer of insulation. That or the current carrying components are protected by reinforced insulation like a hard plastic case. A practical example is the humble power brick or wall wart. These are class II power supplies which are enclosed in a hard plastic case which protects the user from electrical shock. Reference: https://www.cui.com/blog/class-2-vs-class-ii-power-supplies https://www.cui.com/blog/class-2-vs-class-ii-power-supplies

haberman 7y ago

If GFI sockets and breakers had been widely available at the time, would the third prong have been added? Would a GFI along with the "one long slot" plugs make the ground wire obsolete?

Bjartr 7y ago
Possibly, GFI mechanisms are good, but they're not as good as a true ground since they take time to activate and require a minimum current draw to trigger (you can still be injured or killed below that current draw)
blattimwind 7y ago
No
leni536 7y ago
Probably. AFAIK GFI measures the current difference live and neutral, so a current between live and ground (as in the third slot) would trip it. Imagine if someone forms a path for current between a "grounded" metal case and live. If there is no third slot then "grounding" means connecting to neutral, so GFI wouldn't trip.
xyzzyz 7y ago
GFI outlets are $15+, and normal outlets are $2. This adds up when you're building a house: a typical house has something on the order of 30-50 outlets, so you're looking at $400-$700 vs $60-$100. It might not seem like much in the grand scheme of things, especially to software engineers making lots of money, but if we had this approach not only for outlets, but also for everything else, building literally anything would cost 5 times as much (and in fact it does these days, to a large degree because we can afford spending 5 times as much on building).
- magduf 7y ago
  >GFI outlets are $15+, and normal outlets are $2. Where are you getting your prices? Last time I checked, GFCI outlets were indeed around $15, but the normal ones are less than a dollar when you buy them in bulk packs (which you would for building a house). Otherwise, your point is sound, but the price difference is even greater than you said. Those standard outlets are really dirt-cheap, and it's annoying that builders are so cheap about installing them. New construction has gotten better though, but really there should be an outlet every 5-6 feet in my opinion, or maybe even less.

jhallenworld 7y ago

Three prongs was a good idea before the existence of ground fault interrupters, but now that they do exist, ground fault interrupters with two prongs provides a much better solution. The problem with ground is that it's a conductor. If there's a fault on it, you don't know exactly what it's connected to, so now you have a situation where the metal chassis is connected to something. You touch the chassis and the really-grounded a sink and you have a problem. Here's a real world example of this. The ground is bonded to neutral at the panel. But what happens when neutral between the pole and the house breaks? This happened to my neighbor. There is still 240V with neutral/ground floating somewhere in the middle. All of his appliances on the less loaded side of the 240V blew up and "ground" was very not at earth ground.

blattimwind 7y ago
That's a problem with incorrect installation / code. In a proper TN-C-S system the combined PEN will be split up at the main panel, and PE will be local ground and pipework is typically connected to that.

kazinator 7y ago

The article isn't not really on point. A two-prong system still has an earthed neutral. The third prong is purely for redundancy. If neutral happens to be disconnected, there is still a return path. It also protects against mixed up neutral and hot. If the chassis of a device is grounded to the third conductor, and that one is properly routed, then an inadvertent reverse wiring of hot/neutral is mitigated. A third conductor also makes possible ground fault detection: situations when a device is not returning all current through the neutral, because its safety ground has become energized. How might a neutral end up disconnected? Not just by accident; for instance, someone would incorrectly wire a cold-side switch instead of hot-side (E.g. wall panel switch that controls an outlet) so that when the outlet is off, the neutral has been cut off, but hot is still live. In that situation, the third conductor would still be grounded. Nobody would mistakenly implement the switch on the ground line unless they didn't bother testing it, because the switch then wouldn't work; the ground line is implicitly protected from having switches accidentally planted on it by the fact that interrupting it does nothing, functionally.

bookofjoe 7y ago

What I learned from reading every single one of the 184 comments (as of 6:28 pm ET) on this post: call an electrician for anything more complex than changing a light bulb.

komali2 7y ago

> There is a simple solution to these problems: connect your system to the Earth. Drive some long metal rods into the dirt, and connect them to your wires. That way, lightning currents will be directed into the Earth rather than spreading throughout your power lines. I have tried and failed many times to understand the concept of "ground." Why does electricity "want to go to the ground?" What constitutes "ground?" Can I touch wire to concrete, and that counts as "ground?" No? Must I then sink it? How deep? Until it touches dirt? What about sand? Where does the electricity "go" then? I've read plenty of articles, never got it.

sansnomme 7y ago
Study the Maxwell equations and thou shalt be enlightened.
mantap 7y ago
Obviously rock doesn't conduct electricity but water+impurities does. When people talk about "ground" they are talking about the water in the ground.
- saalweachter 7y ago
  Incidentally, this is why if you're running an electric fence, and you have rocky or dry soil, it is advisable to run alternating hot & ground wires, and to not rely on the ground to complete the circuit.
ZainRiz 7y ago
There are two different mental models I find really useful when thinking about electricity. The second one will answer your question more directly, but the first one helps set the stage better. Mental Model #1 (useful for most work with electricity): Think of electricity moving through circuits as water going through pipes. Electricity = water Voltage = water pressure Battery/Voltage Source = water pump Wires = Water pipes High/Low resistance = Wide/narrow water pipes Voltage is like water pressure. You have a pump creating high water pressure is like a power source with a high voltage. The pump is always on and creating water pressure, but there's a cap placed on it that blocks the water from escaping. Similarly, in an open circuit the air acts as this cap and blocks the electricity from proceeding. The wires in a circuit act as pipes channeling the water to different areas. Resistance in a circuit is analogous to the thickness of the pipes (low resistance == wide pipes, high resistance == thin pipes). The above analogy sets part of the mental model. The twist is that in reality electrons want to actually travel from a place of low voltage to a place of high voltage (classical electrical diagrams show this process in reverse). Which leads us to... Mental Model #2 Voltage sources are powerful electron vacuum cleaners trying to to suck up electrons. (You can apply this on top of mental model #1, just reverse the direction of the water flow) Open circuits generally don't conduct electricity since within a nano-second the voltage source will suck up all the free electrons in the circuit. The negative/ground pole in circuits are where the free electrons get supplied from for the voltage source to "suck in" the electrons from. The ground (by virtue of it being so large and having lots of moisture/materials which conduct electricity) happens to have a virtually limitless source of electrons available for the voltage source to suck into itself. This provides the circuit with all the electrons required to successfully operate. Hope that helps :)

CapitalistCartr 7y ago

Past posting on HN: https://news.ycombinator.com/item?id=4058834 https://news.ycombinator.com/item?id=4058834

js2 7y ago

Since this article was written, the NEC has also started requiring arc fault circuit interrupters. https://en.wikipedia.org/wiki/Arc-fault_circuit_interrupter https://en.wikipedia.org/wiki/Arc-fault_circuit_interrupter BTW, if you have a portable generator for emergency purposes and it's connected via a transfer switch, take note of the important grounding and bonding requirements: https://cpower.com/PDF/InfoSheets/44.pdf https://cpower.com/PDF/InfoSheets/44.pdf

jonprobably 7y ago

Why is the metal chassis wired at all?

deleted 7y ago

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peter_d_sherman 7y ago

Excerpt: "The sparks occur because of a little-known fact: all the world is a gigantic electrostatic generator. There is a flow of charge going on vertically everwhere on earth. Thunderstorms pump negative charge downwards, and the charge filters upwards everywhere else on earth. Depending on the height of your circuitry above the earth's surface, depending on the area covered by your wires, and depending on whether there was a thunderstorm above you at the time, there might be a fairly huge DC charge on your electrical distribution system. This charge might be several hundred volts; enough to zap computers and delicate electronics. Or... it might be many tens of thousands of volts, enough to create enormous sparks which jump across switches and leap out of wall outlets, wall switches, across transformer windings, etc. Your electric power system is acting like a sort of capacitive "antenna" which intercepts the feeble current coming from the sky and builds up a huge potential difference with respect to the earth."