7 ms·

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 jus

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.
        shreddish 7y ago
        Yes instinctivly it wasn't making sense as I knew the circuit needs to be "completed". So if that's the case then what is the point of the ground rod that the neutral bar/wires in your breaker box are connected to?
        NobodyNada 7y ago
        This article posted in a comment above explains it well: http://amasci.com/amateur/whygnd.html http://amasci.com/amateur/whygnd.html Essentially, the ground rod acts as an "anchor" holding the neutral wire and the ground at the same electric potential. If there was no ground rod, the earth and the circuit would be "floating" relative to one another, and a dangerously large voltage could develop between them.
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?

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.
      - shreddish 7y ago
        Yes instinctivly it wasn't making sense as I knew the circuit needs to be "completed". So if that's the case then what is the point of the ground rod that the neutral bar/wires in your breaker box are connected to?
        NobodyNada 7y ago
        This article posted in a comment above explains it well: http://amasci.com/amateur/whygnd.html http://amasci.com/amateur/whygnd.html Essentially, the ground rod acts as an "anchor" holding the neutral wire and the ground at the same electric potential. If there was no ground rod, the earth and the circuit would be "floating" relative to one another, and a dangerously large voltage could develop between them.

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?