11 ms·

Why we ground electrical systems (2015)

119 points by terminalhealth 7y ago

vvillyd 7y ago
I'm actually the person who wrote this Reddit post. I'd never heard of Hacker News before, but someone messaged me on Reddit telling me this 4 year old post just surfaced over here. Let me know if you have any questions about electrical theory or installation, or anything else!
- shreddish 7y ago
  Question with myth #1 that "electricity is trying to get to ground". Would that be an accurate statement if you replaced "ground" with a place of lower electrical potential? Or can you elaborate but the meaning that it's trying to get back to it's source?
  - vvillyd 7y ago
    Well, electrical potential is only defined as a relation between two points. So the neutral point on a transformer, for example, doesn't have any electrical potential in and of itself. It has electrical potential with relation to, say, the termination point of one of the phases. The electrical potential between any two given points is dependent on the impedance along the pathway between the two. The lower the impedance, the lower the potential (or work/energy needed to move current). So, yes, in a way electricity wants to get to the point with the lowest potential, but that point will always be the point where the electricity originates. I was a little vague with "where the electricity originates" because that could be a number of things. It could be a battery, a transformer, a generator, a turbine, a dynamo, etc. Whatever is creating the difference in potential between two points is the "source" of the electricity.
    - throwaway542134 7y ago
      I've always preferred to think of it as pumping water from the bottom of a (practically infinite) reservoir and dumping it at the top of a mountain then doing some work as it flows downhill, back to the reservoir. It provides a nice visual for why current always makes it back to ground, just like water always flow downhill. It also removes the tendency to anthropomorphize electrical current and say things like it "seeks out" ground. When something falls from the sky we don't say it's trying to find the ground!
      - shreddish 7y ago
        Yes water is a great way to describe electricity in a lot of ways. I'm just struggling with the "returning to where it originates". For instance the electricity didn't originate from the grounding rod it came from the transformer. Earth is providing a very low electrical potential that the electricity is "attracted" to or "falling" to in the reservoir example. Am I wrong in thinking that all electricity is flowing back into earth?
        bsder 7y ago
        > Am I wrong in thinking that all electricity is flowing back into earth? Yes, sadly, you are wrong. But it's not a strange error. Electricity always flows in a circle(circuit). And, in fact, in medical devices, transformers are often used to completely isolate devices from the line that they are plugged into. Electrons on the device side of the transformer will NOT try to flow back into the line side or an earth. They only want to flow back to the device side of the transformer. Now, the issue is that when you want to create really strict isolation like this, suddenly all manner of things that normally you don't pay attention to suddenly become relevant. Is the device side of that transformer really not connected to the line side anywhere? No goop on the board? No water vapor? No lines that are a little too close? Is the hospital bed not connected to anything? Guitar players who use tube amps and vocalists who use condenser microphones wind up with this issue all the time. Both the amps and the mics are "isolated" with relatively high voltage signals floating around--300-400V for amps:48V for mics--and consequently strange paths cause lots of "buzz" in the signal.
- Stratoscope 7y ago
  Here is an entertaining take on some related questions, going into the history of how our modern ground systems evolved: http://amasci.com/amateur/whygnd.html http://amasci.com/amateur/whygnd.html I would be interested in your comments about that article, thanks!
  - yesenadam 7y ago
    Bill Beaty's site has a load of great articles on electricity (and much else), linked to here: http://amasci.com/elect/elefaq.html http://amasci.com/elect/elefaq.html Also misconceptions about electricity taught in school: http://amasci.com/miscon/elect.html http://amasci.com/miscon/elect.html Or pages on generally believed science myths: http://amasci.com/miscon/miscon.html http://amasci.com/miscon/miscon.html How transistors really work: http://amasci.com/amateur/transis.html http://amasci.com/amateur/transis.html How capacitors really work: http://amasci.com/emotor/cap1.html http://amasci.com/emotor/cap1.html There's loads more great stuff. He's a pleasure to read - a guy who thinks for himself, and experiments - tries things out for himself.
- amriksohata 7y ago
  So in the UK we have 3 wires, red, blue and green, the green is ground or earth, what purpose does this serve? Is it the same as ground as you call it in the US
  - deleted 7y ago
    [deleted]
  - bardworx 7y ago
    I’m not an electrician but will take a stab at your answer. Please see if you can verify what I said so you don’t fry yourself. If the green wire is connected between the receptacle and the outlet/switch then it’s “bonding” the two mental items. (Using the terminology from post). Most people refer to this as the ground wire or grounding the receptacle. The purpose, from my understanding, is to have a stable system. In the event the hot wire touches the receptacle. If the red/blue/green are coming from the same cable (3 wires inside another sheath), then your green is the ground/neutral that connects to the electrical box itself, while the other two are hot/lead. I posted a similar question above which may clarify things if the OP answers.
  - vvillyd 7y ago
    I'm from the US, so I'll add the caveat that this is answer is based on my foreign understanding and a British electrician may be better suited to answer this. Your green wire would be what the US calls a ground wire, and what is more accurately called a Equipment Grounding Conductor (EGC). It's purpose is to bond all normally non-current carrying conductive parts of a system together to provide a ground-fault current path back to the source so that the overcurrent protective device can operate properly to open the circuit in the case of a ground fault. It is 100% a safety measure, and will operate as intended to open a breaker whether or not it is connected to the Earth. Your red and black wires are both ungrounded conductors, and are the conductors which actually create the circuit that allows current to flow to power whatever you connect to the circuit. The circuit will work to provide current without the green wire. However, without the green wire, if either the red or black shorts to something, the breaker will not open and you have an enormous potential to harm someone or burn your house down.
- temporaryvector 7y ago
  I don't have a specific question, but can you recommend a good, authoritative resource on learning to work with household electrical installations correctly, both theory and actual installation? Something like The Art of Electronics and Learning the Art of Electronics, but for general household electrical. I do quite a bit of DIY electrical in my home, both because I enjoy tinkering and because I live in a developing country where most of the electricians you can hire is better described as "practiced amateur" rather than "professional," and I've gotten by with online resources (and the fact that my house is made from non-combustible materials) but as you point out in that post, there's a lot of conflicting or downright wrong information. I don't want to become an actual electrician, but aside from just being interested in the topic, I'd at least want to know for sure that what I'm doing is safe and to be able to check the work of anyone I hire for obvious faults. I'm planning a solar installation in the near future, so I'd like to be well informed on how everything is supposed to work.
- Unsimplified 7y ago
  For a machine, is it generally a good idea to distribute a common power bus voltage like 48VDC to independent modules (embedded computers, servo controllers) for them to stepdown inside the application module (12vdc, 24vdc) or should the power be stepped down early at the distributor module? What are the advanced tradeoffs?
  - chendragon 7y ago
    Generally the higher the voltage you distribute, the lower the current in the wires, allowing you to use thinner wire and reducing I^2R losses. With this said, having multiple point of load converters may have impacts on efficiency and will add to the cost of the system.
- tomatotomato37 7y ago
  There's one thing I don't understand between hot and neutral. How do the electric turbines upstream at the power plant only act on what becomes the hot wire? By my understanding of how motors work the magnet causes a positive current on the whole coiling/wire, meaning a positive current on one side is matched by an equally powerful negative current on the other. As both wires on the consumer end eventually join to the ends of that coiling, than wouldn't that mean both wires switch between hot and neutral everytime the current changes direction? Also, as all household wiring is in parallel off the mains, wouldn't that mean a closed circuit on one branch would turn the neutral wire on another branch hot as they effectively become one wire? How does the electric "push/pull" get separated onto only the hot line?
  - ThrustVectoring 7y ago
    The electric generators aren't connected to your neutral wire. They deliver three phase power with three different "hot" lines, and then only one of those phases gets pulled down into your house electricity.
  - madengr 7y ago
    Yes, just as you describe, but the addition of a transformer allows you to reference the neutral to ground, therefore the hot swings above and below ground. It’s a little more complicated than what I described, as your breaker box is 240V split phase, but the transformer outside your house allows for a ground reference at its center tap. If you have ever made the mistake of connecting a diode rectifier directly to AC mains, then grounding the negative output, you’ll blow a breaker. A 1:1 isolation transformer is still needed.
  - taneq 7y ago
    > than wouldn't that mean both wires switch between hot and neutral everytime the current changes direction? The neutral line is connected to Earth at some point. The voltage difference is created by a transformer upstream which just pushes a 240V (or whatever) difference between live and neutral, and since neutral is tied to Earth, it stays at ~0V while the live line stays at ~240VAC. You're right, though, that any current pulled from the live line goes back through the neutral line and they both end up with alternating current going through them. The voltage only alternates between live (or 'hot') and neutral relative to each other. This voltage difference between the two lines is generated by a transformer and there doesn't have to be any connection between live/neutral and anything else (at which point you can just think of them as live-1 and live-2) but the neutral line is connected to Earth at some point (although depending on load and the distance to the earthing point your neutral will actually have some AC ripple on it).
- esaym 7y ago
  Here's a good one: A couple of years ago I borrowed a small camper trailer from my grandparents as I was going on a week camping trip. I towed the camper over to my house and left it hitched to the truck. I then plugged the trailer into an outlet on the side of the house so I could run the air conditioner while I packed it. There was an issue with the brake lights not always working on trailer also. I crawled under the truck to trace the wires (they had been spliced under there before so I wanted to check that area first). While on my back I rested my forearm against the frame of the truck and then felt a small "pinch". I thought a bug bit me at first, but seeing nothing there, I touched my forearm back again to the frame and felt it again. I realized it was electricity. I had a multi meter handy and I stuck one end of the probe in my mouth and touched the other end to the frame. It read 50 volts a/c. I went and unplugged the trailer from the outdoor outlet and ran it through a window and plugged it into an outlet inside and the issue went away. So what's up with that?
  - muzika 7y ago
    Inproperly wired outlet? Hot swapped with neutral?
    - muzika 7y ago
      Or floating ground?
  - Gibbon1 7y ago
    > So what's up with that? Usually 50 VAC means the ground is open. A thing about most ordinary transformers[1] is they wrap the primary and secondary together. Fast, cheap, and more efficient. The result is there is a large amount of capacitive coupling between the primary and secondary. Since one side of the secondary is grounded it's like connecting 1000-5000pf between hot and chassis ground. If chassis isn't grounded then it floats at 60VAC. [1] For transformers with more isolation they wrap the primary and secondary on top of each other. Less capacitance that way. Medical grade transformers have physically separate windings.
- simonebrunozzi 7y ago
  Well, seems like you should do an AMA on Reddit!
- taneq 7y ago
  This is a great introduction to protective earthing. The only nit I'd pick is with the bit at the end about AC. The voltage on the neutral wire isn't due to inductive coupling, it's due to voltage drop along the neutral since neutral carries the same current as live. You'll always get this if anything's pulling significant current, earthing can't completely fix it, which is why you can't ever depend on neutral being the same voltage as earth, or as another neutral elsewhere (that's why we use dry contact relays to send signals between different pieces of kit - if they're getting neutral from different places then you can have a significant voltage difference between "neutral" in one cabinet and "neutral" in another, enough to blow up ELV gear!) There's also the fact that, any time you use a transformer and don't earth both sides, you end up with a 'floating' AC circuit that will end up at some arbitrary voltage compared with earth. I've made this mistake before and been bitten by a stray (high impedance) 50VAC, not deadly but enough to get your attention. :P
- sunstone 7y ago
  How do you test that the "earth" connection is really good enough to redirect the current in that direction when need be?
ganonm 7y ago
"Since the ground-fault current path has inherently much higher resistance than the neutral wire, the amount of current flowing through the circuit jumps enormously during a ground-fault." This is incorrect (probably a typo) - the GFC ensures there is a very low resistance path to source, which ensures a current spike that is sufficient to blow the fuse.
- vvillyd 7y ago
  Yeah, that was a typo on my part. I didn't notice it at first, and it's now been so long since I posted it that I would feel disingenuous editing it. I think someone pointed out the typo in the comments.
  - watersb 7y ago
    I encourage you to fix the typo; lots of HN exposure means lots of new readers without context to any time that has elapsed since your (really good) article was first posted.
  - penagwin 7y ago
    It's very common to edit your typo but still leave it there either crossed out, and/or leave an EDIT: and explain what/why you changed it. It's perfectly okay to ensure your text is communicating accurate information - in fact leaving it may undermine your explanation as certain folks will focus on the error (or others won't notice it and learn something incorrect).
ryanmarsh 7y ago
This blew my mind. So the white wire (neutral) is actually what I thought the ground was (connected to earth) and the “ground” wire (usually bare or green) is a conductor for clearing ground faults and isn’t connected to earth. Marvelous explanation.
- vvillyd 7y ago
  Thanks! In my 11 year experience in the electrical industry I have come to realize that there is a LOT of misinformation about how electrical systems work, and even the most experienced veterans in the field often don't really understand it.
  - ryanmarsh 7y ago
    So what would be the potential consequence if I didn’t connect the “ground” wire (green or bare) to the little brass ground screw on (let’s say) a ceiling fan when installing it?
    - shreddish 7y ago
      From my understanding, if the hot wire (black) were to come in contact with a piece of metal on the fan it would build up electrical charge and or allow for current to pass through the piece of the fan. Then if you were on a ladder fixing the fan and touched that piece of metal you would become a path for electricity to pass through to a lower potential.
      - vvillyd 7y ago
        That's exactly correct. You would get shocked if you touched the fan.
        chrisdhoover 7y ago
        The way you bond is codified too. The source feed is wire-nutted with a pigtail connected to the box, and to the ground of the device in parallel. If the bonding was daisy chained it could be interrupted when servicing or changing.
Ives 7y ago
The details of this also vary based on where you're at. In the US, neutral is bonded to earth at every house I think. In Europe, neutral is only bonded to earth at a local power station.
- vvillyd 7y ago
  Yeah, this is 100% based on the US electrical system. I am not familiar enough with how other countries do it to speak intelligently on their systems.
- robocat 7y ago
  The US system is somewhat unique and lacks heaps of safety features: I am guessing mostly because a 110V shock is far less dangerous than a 240V shock (Although higher amperages mean higher risk of fire?). For example for UK, EU and NZ etc: sockets and pins have a variety of features to avoid touching the phase (live) wire with say a screwdriver or knife in hands of a child. For example, all new circuits installed in New Zealand since 209 must have an Residual Current Detector (the actual rules lead to multiple RCDs per main power board). This will cut the circuit if someone does manage to touch a live wire somehow (current from phase to neutral doesn't match, because some of it is grounded through your body, and the mismatch triggers the breaker. Wayyyyyyy safer than a fuse!
  - pkaye 7y ago
    In the US there is tamper proof outlets and GFCI is equivalent to RCD. Its pretty much required for new houses though older houses may vary on what they implement.
    - robocat 7y ago
      Are tamper proof outlets mandatory? The UK has had shutters on the phase that is opened by the earth pin forever. Which is why their plugs are caltrops, and have a plastic earth pin if double insulated (only phase and neutral wired, with no earth wire). I don't think I have ever seen a US plug where the pins have a non-conductive protective covering at the base of the pin (I think all modern UK and NZ male plugs have that. Although I'll be honest that I think the NZ solution makes plugs less safe due to risk of pins bending then breaking and leaving a live metal pin exposed in the socket). EU sockets are mostly recessed from what I have seen (some NZ sockets are, but the recess is still is often unusable with old plugs, so is not common yet, but will become so as more plugs are sold that fit properly).
bardworx 7y ago
Why are there three wires in new receptacles? From my understanding, new code has you have to have a 3+ground but why is that safer? Did they just add an extra wire so you can have a hot, lead, and neutral to ground all the receptacle? (Plus the green/bare to bond the receptacle to switch/outlet) Also, for home owners, what’s the best resource to learn common sense basics of electrical work (besides reading the city/states codes). Are there classes for such cases?
- vvillyd 7y ago
  The three wires in any typical household circuit are your Ungrounded Conductor (hot wire), Grounded Conductor (neutral wire), and Equipment Grounding Conductor (EGC/Ground Wire). The hot and neutral are what actually makes the circuit work. The EGC/ground is a safety measure that creates a ground fault current path which enables overcurrent protective devices (like breakers and fuses) to operate. As far as learning common electrical, I'm sure local organizations in your area will have some kind of classes. Probably local hardware stores? I don't know. I learned through trade school and on-the-job training. As an electrician, I will self-servingly tell you that you should always hire a licensed electrician.
  - ars 7y ago
    > The EGC/ground is a safety measure that creates a ground fault current path which enables overcurrent protective devices (like breakers and fuses) to operate. This is NOT TRUE!! You've said it so many times in this thread, and it's just not true. A lot of people are tying to correct your mistakes, but you're not fixing them. Please do so, you are misleading a lot of people.
    - chrisdhoover 7y ago
      Perhaps you could explain it correctly?
      - wtallis 7y ago
        You can trip a breaker without the ground wire being involved. The ground wire does help with one failure case, but that's not the only way that your circuit breakers/fuses protect you.
fsh 7y ago
This section is plain wrong: "The ONLY purpose for the EGC (or green wire) is to clear a ground-fault (clearing a ground-fault means tripping a breaker or blowing a fuse) in the 'oh shit moments'. It has absolutely NOTHING to do with the ground or the Earth and will work exactly as it is intended to regardless of whether it is connected to the Earth or not." If the EGC is floating, no current will flow in a ground fault and the breaker never trips.
- jhayward 7y ago
  If the EGC is floating, it's not an EGC. EGC is tied back to Neutral at the distribution point. Its sole purpose is to carry fault current.
  - fsh 7y ago
    And the neutral is tied to (earth) ground. Otherwise all metal devices would shock you if you touched something earth referenced, such as a radiator, at the same time. Edit to clarify: In principle, only the voltage difference between Live and Neutral matters for running a device. So theoretically one could construct a building supply where Neutral is not at earth potential. However, EGC has to be at earth potential, otherwise there would be a voltage between any metal electronic device and earthed objects, such as radiators or your feet standing in a puddle. This means that in practice both Neutral and EGC are always physically connected to the earth in some way and have to be for the system to work properly.
- vvillyd 7y ago
  I'm not trying to be argumentative but you are misinformed. "Floating" means that the system is not connected to ground. That is, there is no system/main bonding jumper. There is no connection to a grounding electrode conductor. This has nothing to do with the Equipment Grounding Conductor, though. I don't know what you mean by a "floating EGC". The entire purpose of the EGC is to bond all normally non-current carrying conductive parts together to provide a ground-fault current path back to the source of the circuit. If the EGC is not connected to some non-current carrying conductive part, then it is not installed correctly, is against code, and is a safety hazard. If the EGC is bonded to everything properly and there is a fault (say an ungrounded wire touches a metal box), then the current will flow through the ungrounded conductor, through the metal box, through the EGC (which is bonded to the box), back to the source (usually a transformer), across the windings, and eventually back to the breaker that controls the circuit. This will build up enough current (usually VERY fast, like milliseconds) and the breaker will open. The Earth has nothing to do with this.
mikestew 7y ago
One of the few times I wish I had a reddit account, just so I could give an old post some $REDDIT_KARMA_POINTS (whatever currency they use). That is an outstanding explanation that cleared up some things for me that I do because electrical code, not because "knows the 'why' of what he's doing".
- nemetroid 7y ago
  Maybe it offers some solace to know that old posts (older than six months) cannot be voted on.
watersb 7y ago
I have been studying Grounding vs Bonding these past few weeks, great timing to see this HN discussion. Some training videos for pro electricians and US National Electric Code compliance helped me. https://youtu.be/qNZC782SzAQ https://youtu.be/qNZC782SzAQ Basically, bond everything together if it can carry a charge. Ground that bond at only one place in an electrical service. More than one "ground" sets up a situation where a potential difference can develop between the various "grounds". Ground Fault detection may no longer work in that case. Which is not good. (The presentation is weirdly sexist in offhand comments, which I often encounter in trade work of this kind, but I do believe that's changing. Slowly. The USA needs more people who know how to build things.)
- simonebrunozzi 7y ago
  > The USA needs more people who know how to build things. USA? The WORLD needs more people who know how to build things.
- deleted 7y ago
  [deleted]
- mindslight 7y ago
  Watching this video (myth #3) brings up a bit of a semantic peeve. And that peeve begins with how the NEC narrowly defines the term "grounding" as only applying to the electrical system itself. Then electricians apply that narrow definition to every use of the term, acting like someone is wrong because they say they're "grounding" something by hooking up it up to an EGC. But don't worry, they'll teach you this special term "bonding" that refers to the latter. Except it doesn't. I already used a synonym of "bonding" - "hooking up". You "bond" the black wire to the gold terminal on a receptacle by tightening the screw. > Basically, bond everything together if it can carry a charge It is perfectly fine to say that you are grounding those things together. Connecting something to an EGC is indeed "grounding it" - just not in NEC land which is focused on getting the EGC grounded. If you say "this washing machine needs to be better grounded", that doesn't mean it needs an immediate connection to earth, but is rather talking about its path to earth via the building's electrical system.
- devwastaken 7y ago
olouv 7y ago
Beware as this explanation does not apply to every country in the world, in France for residential systems at least, the protection (green/yellow) wire is never connected to the neutral (blue) wire, only to earth with a metal rod at the house. You would get a shock if it was somehow disconnected from the rod/earth. Neutral is also not bounded to earth inside the house. Be safe!
js2 7y ago
See also "Why Three Prongs (1996)" discussed recently: https://news.ycombinator.com/item?id=20378852 https://news.ycombinator.com/item?id=20378852 Also, be aware of grounding requirements when using a generator: https://cpower.com/PDF/InfoSheets/44.pdf https://cpower.com/PDF/InfoSheets/44.pdf

I'm actually the person who wrote this Reddit post. I'd never heard of Hacker News before, but someone messaged me on Reddit telling me this 4 year old post just surfaced over here. Let me know if you have any questions about electrical theory or installation, or anything else!

shreddish 7y ago
Question with myth #1 that "electricity is trying to get to ground". Would that be an accurate statement if you replaced "ground" with a place of lower electrical potential? Or can you elaborate but the meaning that it's trying to get back to it's source?
- vvillyd 7y ago
  Well, electrical potential is only defined as a relation between two points. So the neutral point on a transformer, for example, doesn't have any electrical potential in and of itself. It has electrical potential with relation to, say, the termination point of one of the phases. The electrical potential between any two given points is dependent on the impedance along the pathway between the two. The lower the impedance, the lower the potential (or work/energy needed to move current). So, yes, in a way electricity wants to get to the point with the lowest potential, but that point will always be the point where the electricity originates. I was a little vague with "where the electricity originates" because that could be a number of things. It could be a battery, a transformer, a generator, a turbine, a dynamo, etc. Whatever is creating the difference in potential between two points is the "source" of the electricity.
  - throwaway542134 7y ago
    I've always preferred to think of it as pumping water from the bottom of a (practically infinite) reservoir and dumping it at the top of a mountain then doing some work as it flows downhill, back to the reservoir. It provides a nice visual for why current always makes it back to ground, just like water always flow downhill. It also removes the tendency to anthropomorphize electrical current and say things like it "seeks out" ground. When something falls from the sky we don't say it's trying to find the ground!
    - shreddish 7y ago
      Yes water is a great way to describe electricity in a lot of ways. I'm just struggling with the "returning to where it originates". For instance the electricity didn't originate from the grounding rod it came from the transformer. Earth is providing a very low electrical potential that the electricity is "attracted" to or "falling" to in the reservoir example. Am I wrong in thinking that all electricity is flowing back into earth?
      - bsder 7y ago
        > Am I wrong in thinking that all electricity is flowing back into earth? Yes, sadly, you are wrong. But it's not a strange error. Electricity always flows in a circle(circuit). And, in fact, in medical devices, transformers are often used to completely isolate devices from the line that they are plugged into. Electrons on the device side of the transformer will NOT try to flow back into the line side or an earth. They only want to flow back to the device side of the transformer. Now, the issue is that when you want to create really strict isolation like this, suddenly all manner of things that normally you don't pay attention to suddenly become relevant. Is the device side of that transformer really not connected to the line side anywhere? No goop on the board? No water vapor? No lines that are a little too close? Is the hospital bed not connected to anything? Guitar players who use tube amps and vocalists who use condenser microphones wind up with this issue all the time. Both the amps and the mics are "isolated" with relatively high voltage signals floating around--300-400V for amps:48V for mics--and consequently strange paths cause lots of "buzz" in the signal.
Stratoscope 7y ago
Here is an entertaining take on some related questions, going into the history of how our modern ground systems evolved: http://amasci.com/amateur/whygnd.html http://amasci.com/amateur/whygnd.html I would be interested in your comments about that article, thanks!
- yesenadam 7y ago
  Bill Beaty's site has a load of great articles on electricity (and much else), linked to here: http://amasci.com/elect/elefaq.html http://amasci.com/elect/elefaq.html Also misconceptions about electricity taught in school: http://amasci.com/miscon/elect.html http://amasci.com/miscon/elect.html Or pages on generally believed science myths: http://amasci.com/miscon/miscon.html http://amasci.com/miscon/miscon.html How transistors really work: http://amasci.com/amateur/transis.html http://amasci.com/amateur/transis.html How capacitors really work: http://amasci.com/emotor/cap1.html http://amasci.com/emotor/cap1.html There's loads more great stuff. He's a pleasure to read - a guy who thinks for himself, and experiments - tries things out for himself.
amriksohata 7y ago
So in the UK we have 3 wires, red, blue and green, the green is ground or earth, what purpose does this serve? Is it the same as ground as you call it in the US
- deleted 7y ago
  [deleted]
- bardworx 7y ago
  I’m not an electrician but will take a stab at your answer. Please see if you can verify what I said so you don’t fry yourself. If the green wire is connected between the receptacle and the outlet/switch then it’s “bonding” the two mental items. (Using the terminology from post). Most people refer to this as the ground wire or grounding the receptacle. The purpose, from my understanding, is to have a stable system. In the event the hot wire touches the receptacle. If the red/blue/green are coming from the same cable (3 wires inside another sheath), then your green is the ground/neutral that connects to the electrical box itself, while the other two are hot/lead. I posted a similar question above which may clarify things if the OP answers.
- vvillyd 7y ago
  I'm from the US, so I'll add the caveat that this is answer is based on my foreign understanding and a British electrician may be better suited to answer this. Your green wire would be what the US calls a ground wire, and what is more accurately called a Equipment Grounding Conductor (EGC). It's purpose is to bond all normally non-current carrying conductive parts of a system together to provide a ground-fault current path back to the source so that the overcurrent protective device can operate properly to open the circuit in the case of a ground fault. It is 100% a safety measure, and will operate as intended to open a breaker whether or not it is connected to the Earth. Your red and black wires are both ungrounded conductors, and are the conductors which actually create the circuit that allows current to flow to power whatever you connect to the circuit. The circuit will work to provide current without the green wire. However, without the green wire, if either the red or black shorts to something, the breaker will not open and you have an enormous potential to harm someone or burn your house down.
temporaryvector 7y ago
I don't have a specific question, but can you recommend a good, authoritative resource on learning to work with household electrical installations correctly, both theory and actual installation? Something like The Art of Electronics and Learning the Art of Electronics, but for general household electrical. I do quite a bit of DIY electrical in my home, both because I enjoy tinkering and because I live in a developing country where most of the electricians you can hire is better described as "practiced amateur" rather than "professional," and I've gotten by with online resources (and the fact that my house is made from non-combustible materials) but as you point out in that post, there's a lot of conflicting or downright wrong information. I don't want to become an actual electrician, but aside from just being interested in the topic, I'd at least want to know for sure that what I'm doing is safe and to be able to check the work of anyone I hire for obvious faults. I'm planning a solar installation in the near future, so I'd like to be well informed on how everything is supposed to work.
Unsimplified 7y ago
For a machine, is it generally a good idea to distribute a common power bus voltage like 48VDC to independent modules (embedded computers, servo controllers) for them to stepdown inside the application module (12vdc, 24vdc) or should the power be stepped down early at the distributor module? What are the advanced tradeoffs?
- chendragon 7y ago
  Generally the higher the voltage you distribute, the lower the current in the wires, allowing you to use thinner wire and reducing I^2R losses. With this said, having multiple point of load converters may have impacts on efficiency and will add to the cost of the system.
tomatotomato37 7y ago
There's one thing I don't understand between hot and neutral. How do the electric turbines upstream at the power plant only act on what becomes the hot wire? By my understanding of how motors work the magnet causes a positive current on the whole coiling/wire, meaning a positive current on one side is matched by an equally powerful negative current on the other. As both wires on the consumer end eventually join to the ends of that coiling, than wouldn't that mean both wires switch between hot and neutral everytime the current changes direction? Also, as all household wiring is in parallel off the mains, wouldn't that mean a closed circuit on one branch would turn the neutral wire on another branch hot as they effectively become one wire? How does the electric "push/pull" get separated onto only the hot line?
- ThrustVectoring 7y ago
  The electric generators aren't connected to your neutral wire. They deliver three phase power with three different "hot" lines, and then only one of those phases gets pulled down into your house electricity.
- madengr 7y ago
  Yes, just as you describe, but the addition of a transformer allows you to reference the neutral to ground, therefore the hot swings above and below ground. It’s a little more complicated than what I described, as your breaker box is 240V split phase, but the transformer outside your house allows for a ground reference at its center tap. If you have ever made the mistake of connecting a diode rectifier directly to AC mains, then grounding the negative output, you’ll blow a breaker. A 1:1 isolation transformer is still needed.
- taneq 7y ago
  > than wouldn't that mean both wires switch between hot and neutral everytime the current changes direction? The neutral line is connected to Earth at some point. The voltage difference is created by a transformer upstream which just pushes a 240V (or whatever) difference between live and neutral, and since neutral is tied to Earth, it stays at ~0V while the live line stays at ~240VAC. You're right, though, that any current pulled from the live line goes back through the neutral line and they both end up with alternating current going through them. The voltage only alternates between live (or 'hot') and neutral relative to each other. This voltage difference between the two lines is generated by a transformer and there doesn't have to be any connection between live/neutral and anything else (at which point you can just think of them as live-1 and live-2) but the neutral line is connected to Earth at some point (although depending on load and the distance to the earthing point your neutral will actually have some AC ripple on it).
esaym 7y ago
Here's a good one: A couple of years ago I borrowed a small camper trailer from my grandparents as I was going on a week camping trip. I towed the camper over to my house and left it hitched to the truck. I then plugged the trailer into an outlet on the side of the house so I could run the air conditioner while I packed it. There was an issue with the brake lights not always working on trailer also. I crawled under the truck to trace the wires (they had been spliced under there before so I wanted to check that area first). While on my back I rested my forearm against the frame of the truck and then felt a small "pinch". I thought a bug bit me at first, but seeing nothing there, I touched my forearm back again to the frame and felt it again. I realized it was electricity. I had a multi meter handy and I stuck one end of the probe in my mouth and touched the other end to the frame. It read 50 volts a/c. I went and unplugged the trailer from the outdoor outlet and ran it through a window and plugged it into an outlet inside and the issue went away. So what's up with that?
- muzika 7y ago
  Inproperly wired outlet? Hot swapped with neutral?
  - muzika 7y ago
    Or floating ground?
- Gibbon1 7y ago
  > So what's up with that? Usually 50 VAC means the ground is open. A thing about most ordinary transformers[1] is they wrap the primary and secondary together. Fast, cheap, and more efficient. The result is there is a large amount of capacitive coupling between the primary and secondary. Since one side of the secondary is grounded it's like connecting 1000-5000pf between hot and chassis ground. If chassis isn't grounded then it floats at 60VAC. [1] For transformers with more isolation they wrap the primary and secondary on top of each other. Less capacitance that way. Medical grade transformers have physically separate windings.
simonebrunozzi 7y ago
Well, seems like you should do an AMA on Reddit!
taneq 7y ago
This is a great introduction to protective earthing. The only nit I'd pick is with the bit at the end about AC. The voltage on the neutral wire isn't due to inductive coupling, it's due to voltage drop along the neutral since neutral carries the same current as live. You'll always get this if anything's pulling significant current, earthing can't completely fix it, which is why you can't ever depend on neutral being the same voltage as earth, or as another neutral elsewhere (that's why we use dry contact relays to send signals between different pieces of kit - if they're getting neutral from different places then you can have a significant voltage difference between "neutral" in one cabinet and "neutral" in another, enough to blow up ELV gear!) There's also the fact that, any time you use a transformer and don't earth both sides, you end up with a 'floating' AC circuit that will end up at some arbitrary voltage compared with earth. I've made this mistake before and been bitten by a stray (high impedance) 50VAC, not deadly but enough to get your attention. :P
sunstone 7y ago
How do you test that the "earth" connection is really good enough to redirect the current in that direction when need be?

ganonm 7y ago

"Since the ground-fault current path has inherently much higher resistance than the neutral wire, the amount of current flowing through the circuit jumps enormously during a ground-fault." This is incorrect (probably a typo) - the GFC ensures there is a very low resistance path to source, which ensures a current spike that is sufficient to blow the fuse.

vvillyd 7y ago
Yeah, that was a typo on my part. I didn't notice it at first, and it's now been so long since I posted it that I would feel disingenuous editing it. I think someone pointed out the typo in the comments.
- watersb 7y ago
  I encourage you to fix the typo; lots of HN exposure means lots of new readers without context to any time that has elapsed since your (really good) article was first posted.
- penagwin 7y ago
  It's very common to edit your typo but still leave it there either crossed out, and/or leave an EDIT: and explain what/why you changed it. It's perfectly okay to ensure your text is communicating accurate information - in fact leaving it may undermine your explanation as certain folks will focus on the error (or others won't notice it and learn something incorrect).

ryanmarsh 7y ago

This blew my mind. So the white wire (neutral) is actually what I thought the ground was (connected to earth) and the “ground” wire (usually bare or green) is a conductor for clearing ground faults and isn’t connected to earth. Marvelous explanation.

vvillyd 7y ago
Thanks! In my 11 year experience in the electrical industry I have come to realize that there is a LOT of misinformation about how electrical systems work, and even the most experienced veterans in the field often don't really understand it.
- ryanmarsh 7y ago
  So what would be the potential consequence if I didn’t connect the “ground” wire (green or bare) to the little brass ground screw on (let’s say) a ceiling fan when installing it?
  - shreddish 7y ago
    From my understanding, if the hot wire (black) were to come in contact with a piece of metal on the fan it would build up electrical charge and or allow for current to pass through the piece of the fan. Then if you were on a ladder fixing the fan and touched that piece of metal you would become a path for electricity to pass through to a lower potential.
    - vvillyd 7y ago
      That's exactly correct. You would get shocked if you touched the fan.
      - chrisdhoover 7y ago
        The way you bond is codified too. The source feed is wire-nutted with a pigtail connected to the box, and to the ground of the device in parallel. If the bonding was daisy chained it could be interrupted when servicing or changing.

Ives 7y ago

The details of this also vary based on where you're at. In the US, neutral is bonded to earth at every house I think. In Europe, neutral is only bonded to earth at a local power station.

vvillyd 7y ago
Yeah, this is 100% based on the US electrical system. I am not familiar enough with how other countries do it to speak intelligently on their systems.
robocat 7y ago
The US system is somewhat unique and lacks heaps of safety features: I am guessing mostly because a 110V shock is far less dangerous than a 240V shock (Although higher amperages mean higher risk of fire?). For example for UK, EU and NZ etc: sockets and pins have a variety of features to avoid touching the phase (live) wire with say a screwdriver or knife in hands of a child. For example, all new circuits installed in New Zealand since 209 must have an Residual Current Detector (the actual rules lead to multiple RCDs per main power board). This will cut the circuit if someone does manage to touch a live wire somehow (current from phase to neutral doesn't match, because some of it is grounded through your body, and the mismatch triggers the breaker. Wayyyyyyy safer than a fuse!
- pkaye 7y ago
  In the US there is tamper proof outlets and GFCI is equivalent to RCD. Its pretty much required for new houses though older houses may vary on what they implement.
  - robocat 7y ago
    Are tamper proof outlets mandatory? The UK has had shutters on the phase that is opened by the earth pin forever. Which is why their plugs are caltrops, and have a plastic earth pin if double insulated (only phase and neutral wired, with no earth wire). I don't think I have ever seen a US plug where the pins have a non-conductive protective covering at the base of the pin (I think all modern UK and NZ male plugs have that. Although I'll be honest that I think the NZ solution makes plugs less safe due to risk of pins bending then breaking and leaving a live metal pin exposed in the socket). EU sockets are mostly recessed from what I have seen (some NZ sockets are, but the recess is still is often unusable with old plugs, so is not common yet, but will become so as more plugs are sold that fit properly).

bardworx 7y ago

Why are there three wires in new receptacles? From my understanding, new code has you have to have a 3+ground but why is that safer? Did they just add an extra wire so you can have a hot, lead, and neutral to ground all the receptacle? (Plus the green/bare to bond the receptacle to switch/outlet) Also, for home owners, what’s the best resource to learn common sense basics of electrical work (besides reading the city/states codes). Are there classes for such cases?

vvillyd 7y ago
The three wires in any typical household circuit are your Ungrounded Conductor (hot wire), Grounded Conductor (neutral wire), and Equipment Grounding Conductor (EGC/Ground Wire). The hot and neutral are what actually makes the circuit work. The EGC/ground is a safety measure that creates a ground fault current path which enables overcurrent protective devices (like breakers and fuses) to operate. As far as learning common electrical, I'm sure local organizations in your area will have some kind of classes. Probably local hardware stores? I don't know. I learned through trade school and on-the-job training. As an electrician, I will self-servingly tell you that you should always hire a licensed electrician.
- ars 7y ago
  > The EGC/ground is a safety measure that creates a ground fault current path which enables overcurrent protective devices (like breakers and fuses) to operate. This is NOT TRUE!! You've said it so many times in this thread, and it's just not true. A lot of people are tying to correct your mistakes, but you're not fixing them. Please do so, you are misleading a lot of people.
  - chrisdhoover 7y ago
    Perhaps you could explain it correctly?
    - wtallis 7y ago
      You can trip a breaker without the ground wire being involved. The ground wire does help with one failure case, but that's not the only way that your circuit breakers/fuses protect you.

fsh 7y ago

This section is plain wrong: "The ONLY purpose for the EGC (or green wire) is to clear a ground-fault (clearing a ground-fault means tripping a breaker or blowing a fuse) in the 'oh shit moments'. It has absolutely NOTHING to do with the ground or the Earth and will work exactly as it is intended to regardless of whether it is connected to the Earth or not." If the EGC is floating, no current will flow in a ground fault and the breaker never trips.

jhayward 7y ago
If the EGC is floating, it's not an EGC. EGC is tied back to Neutral at the distribution point. Its sole purpose is to carry fault current.
- fsh 7y ago
  And the neutral is tied to (earth) ground. Otherwise all metal devices would shock you if you touched something earth referenced, such as a radiator, at the same time. Edit to clarify: In principle, only the voltage difference between Live and Neutral matters for running a device. So theoretically one could construct a building supply where Neutral is not at earth potential. However, EGC has to be at earth potential, otherwise there would be a voltage between any metal electronic device and earthed objects, such as radiators or your feet standing in a puddle. This means that in practice both Neutral and EGC are always physically connected to the earth in some way and have to be for the system to work properly.
vvillyd 7y ago
I'm not trying to be argumentative but you are misinformed. "Floating" means that the system is not connected to ground. That is, there is no system/main bonding jumper. There is no connection to a grounding electrode conductor. This has nothing to do with the Equipment Grounding Conductor, though. I don't know what you mean by a "floating EGC". The entire purpose of the EGC is to bond all normally non-current carrying conductive parts together to provide a ground-fault current path back to the source of the circuit. If the EGC is not connected to some non-current carrying conductive part, then it is not installed correctly, is against code, and is a safety hazard. If the EGC is bonded to everything properly and there is a fault (say an ungrounded wire touches a metal box), then the current will flow through the ungrounded conductor, through the metal box, through the EGC (which is bonded to the box), back to the source (usually a transformer), across the windings, and eventually back to the breaker that controls the circuit. This will build up enough current (usually VERY fast, like milliseconds) and the breaker will open. The Earth has nothing to do with this.

mikestew 7y ago

One of the few times I wish I had a reddit account, just so I could give an old post some $REDDIT_KARMA_POINTS (whatever currency they use). That is an outstanding explanation that cleared up some things for me that I do because electrical code, not because "knows the 'why' of what he's doing".

nemetroid 7y ago
Maybe it offers some solace to know that old posts (older than six months) cannot be voted on.

watersb 7y ago

I have been studying Grounding vs Bonding these past few weeks, great timing to see this HN discussion. Some training videos for pro electricians and US National Electric Code compliance helped me. https://youtu.be/qNZC782SzAQ https://youtu.be/qNZC782SzAQ Basically, bond everything together if it can carry a charge. Ground that bond at only one place in an electrical service. More than one "ground" sets up a situation where a potential difference can develop between the various "grounds". Ground Fault detection may no longer work in that case. Which is not good. (The presentation is weirdly sexist in offhand comments, which I often encounter in trade work of this kind, but I do believe that's changing. Slowly. The USA needs more people who know how to build things.)

simonebrunozzi 7y ago
> The USA needs more people who know how to build things. USA? The WORLD needs more people who know how to build things.
deleted 7y ago
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mindslight 7y ago
Watching this video (myth #3) brings up a bit of a semantic peeve. And that peeve begins with how the NEC narrowly defines the term "grounding" as only applying to the electrical system itself. Then electricians apply that narrow definition to every use of the term, acting like someone is wrong because they say they're "grounding" something by hooking up it up to an EGC. But don't worry, they'll teach you this special term "bonding" that refers to the latter. Except it doesn't. I already used a synonym of "bonding" - "hooking up". You "bond" the black wire to the gold terminal on a receptacle by tightening the screw. > Basically, bond everything together if it can carry a charge It is perfectly fine to say that you are grounding those things together. Connecting something to an EGC is indeed "grounding it" - just not in NEC land which is focused on getting the EGC grounded. If you say "this washing machine needs to be better grounded", that doesn't mean it needs an immediate connection to earth, but is rather talking about its path to earth via the building's electrical system.
devwastaken 7y ago

olouv 7y ago

Beware as this explanation does not apply to every country in the world, in France for residential systems at least, the protection (green/yellow) wire is never connected to the neutral (blue) wire, only to earth with a metal rod at the house. You would get a shock if it was somehow disconnected from the rod/earth. Neutral is also not bounded to earth inside the house. Be safe!

js2 7y ago

See also "Why Three Prongs (1996)" discussed recently: https://news.ycombinator.com/item?id=20378852 https://news.ycombinator.com/item?id=20378852 Also, be aware of grounding requirements when using a generator: https://cpower.com/PDF/InfoSheets/44.pdf https://cpower.com/PDF/InfoSheets/44.pdf