Let me know if you have any questions about electrical theory or installation, or anything else!
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.
http://amasci.com/amateur/whygnd.html
I would be interested in your comments about that article, thanks!
http://amasci.com/elect/elefaq.html
Also misconceptions about electricity taught in school:
http://amasci.com/miscon/elect.html
Or pages on generally believed science myths:
http://amasci.com/miscon/miscon.html
How transistors really work:
http://amasci.com/amateur/transis.html
How capacitors really work:
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.
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).
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.
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
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?
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.
With this said, having multiple point of load converters may have impacts on efficiency and will add to the cost of the system.
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.
By having a separate ground, it's harder to mess that up, and devices can connect the body of the device to that.
Inside the electrical box (mains) the neutral and ground are connected together, and are at the same potential.
The only reason they run separate wires is in case of of mistakes.
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.
Or can you elaborate but the meaning that it's trying to get back to it's source?
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.
"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.
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.
Also the EGC does have more purposes than simply clearing fault current - for example carrying away leakage current from the chassis of something with a switching power supply where the output can't be completely isolated to meet emissions. Ever been shocked by a laptop with only a two-prong AC adapter?
IMO this whole subject is a minefield of disagreements due to terminology, when really all questions are answered by drawing out the schematic of a typical electrical system and looking at the loops (circuits). For instance in the typical ground fault, the fault current returns to the distribution transformer in parallel through all of: your service's neutral, your grounding rod, your neighbors grounding rods/service neutrals, and your other leg of the split phase via turned-on devices. This seems like a lot of unrelated details to memorize until one draws it out.
EGC is bonded to the panel enclosure at the ground bus. The neutral bus is also bonded to the enclosure with a ground screw. So, the EGC is directly connected to the neutral bus and a fault between hot and a bonded enclosure is routed via the EGC to neutral, tripping the breaker.
If you don't bond neutral and ground at the main, it's still bound at the transformer.
> then the current will flow through the ungrounded conductor,
No, it will flow through the grounded conductor.
> back to the source (usually a transformer), across the windings, and eventually back to the breaker that controls the circuit
The flow of power is the same via ground as it is via neutral.
> This will build up enough current (usually VERY fast, like milliseconds)
There is no such thing as "build up enough current", current does not "build up".
The milliseconds has to do with built in time delays at breakers, it's not a function of the electricity.
> The Earth has nothing to do with this
The actual Earth is used as a conductor, so yes, it definitely has something to do with this.
I've seen houses where the earth wiring is completely independent and terminated in a metal rod buried outside. Seemed to be up to local code, breakers worked as they should.
The MBJ/SBJ is what bonds the neutral. This doesn't have anything to do with the Earth. This bonding has to be done anywhere between the origination of the circuit and the first overcurrent protective device.
The GEC is what connects to the Earth. This is used to stabilize voltage and to eliminate a difference in potential between metal parts during a high frequency event (like a nearby lightning strike or a transformer blowing up). This has nothing to do with the operation of breakers.
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.
Marvelous explanation.
> Marvelous explanation.
It might be Marvelous, but it's not actually true. The ground wire IS connected to the earth!
I don't want to be rude, but I think the OP should take this post down, it has a LOT of mistakes.
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!
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?
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.
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.
For 240V, can use 3-wire circuits by getting 240V between the +120V and -120V legs. There are 3-wire grounded plugs (hot-hot-ground) and non-grounded (hot-hot-neutral). Modern 240V wiring uses 4 wires, hot-hot-neutral-ground, which allows making 120V for low-power electronics from one hot to neutral.
Some training videos for pro electricians and US National Electric Code compliance helped me.
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.)
USA? The WORLD needs more people who know how to build things.
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.
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.
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).
https://news.ycombinator.com/item?id=20378852
Also, be aware of grounding requirements when using a generator:
