AC power, I think I finally understand it! (Maybe?) - Page 1
Forum section: electronics / beginners

author: edy
author_rank: Super Contributor
timestamp: July 10, 2014, 02:05:30 am
content: Hey folks,I think I'm finally understanding AC but maybe not? I drew the picture below to figure out if I'm correct, please let me know. Feel free to modify or add other images as needed.In picture A, I drew the 3 phases generated at a source on the left and one phase going right to a local transformer. I imagine a potential "loop" that is available via the live and neutral wires. The only difference is the neutral wire is grounded.So in picture B you touch neutral but since it is also grounded closer to the source and better than you are, you do not get current. In picture C you touch live and since you are the only ground path (instead of neutral), the ground acts as a huge free electron source through you and the AC is able to oscillate through you, giving you a shock?In picture D the appliance like a fan is plugged in and it completes the loop from live and neutral lines so it works fine as intended. The AC oscillates in the circuit despite part of the loop being grounded (on neutral side) because the circuit creates the easiest loop path for oscillating electron movement?So first, is my diagram even correct? Why can you just ground one side of the circuit ( like the neutral wire) without issues yet grounding the other wire (called live) is a big deal? If live and neutral are really just the two ends of a loop, and shorting them (through a device) just completes the loop passing through the transformer... then the choice of which one we call "live" and which "neutral" only depends on which is tied to ground? Yet grounding one 100 feet away at the utility pole and grounding another in your house (which may sit on various soil/rock types) can still complete the loop through ground?Any help understanding this better is most appreciated. Thanks!

author: Falcon69
author_rank: Super Contributor
timestamp: July 10, 2014, 02:09:54 am
content: Forgive me if I am wrong,that first circuit is a 3 phase circuit. You will not find an electric line rated for 3 phase in an area zoned for residential, and maybe not even commercial. 3 Phase is primarily for industrial applications so will usually be in areas zoned for industrial.Huge CNC and other machines require 3 phase power for their motors. 3 Phase is more reliable and makes the motors operate more smoothly and making motor life much much longer then that of 2 phase, which is for residential use.

author: edy
author_rank: Super Contributor
timestamp: July 10, 2014, 02:14:09 am
content: Yes, correct. I just drew that to show the power generation source. I am just illustrating how eventually it gets converted... one of the phase of high voltage low current ends up being transformed in an isolated transformer in your residential neighbourhood to your single phase 60 hz 120v high current (in Canada).

author: peter.mitchell
author_rank: Super Contributor
timestamp: July 10, 2014, 02:18:50 am
content: Quote from: Falcon69 on July 10, 2014, 02:09:54 amPlease remember, this is an international forum and what may apply to one country may not apply to others, in a lot of countries, 3phase power is available in residential environments.

author: Falcon69
author_rank: Super Contributor
timestamp: July 10, 2014, 02:22:25 am
content: I think you got it down though. But you mentioned a 'shock' for C when you touch that wire. I don't know about you, but even though it is only 110-120v, my hands/arm throbs with pain for a few minutes with a burning sensation when I touch it. LOL, not fun at all. I'd rather stick my tongue on a 9-volt battery.The earth ground is usually grounded from the panel box/meter to a rod that is pounded into the ground and is also connected to the main water line from the street connection or the connection FROM the street into your house. The neutral, or white wire, is connected to the neutral coming from the electric company's line.I live in USA, we have same 120v 60hz here.And Peter, you are right, I was just thinking of USA when I said that. I wish 3-phase was available here, at my house. My CNC machine I'm building, I will need to get something to change the 1 phase to 3 phase. I heard those devices are spendy, and the power from them are so so.

author: IanB
author_rank: Super Contributor
timestamp: July 10, 2014, 02:40:08 am
content: Quote from: peter.mitchell on July 10, 2014, 02:18:50 amTrue enough. Three phase is rare in the USA in domestic situations because of the split phase supply arrangement.But in the UK and many other places the domestic household supply is taken from one phase of a three phase transformer where the secondary is in a wye configuration with the center as neutral. So getting a three phase supply is as easy as taking all three phases from the transformer instead of just one. Although rare in homes, a three phase supply is exceedingly common in commercial or light industrial premises, in office buildings, in schools, almost anywhere you can think of with a non-negligible power demand. If you need three times as much power you just take the three phases from the transformer and spread them around.

author: edy
author_rank: Super Contributor
timestamp: July 10, 2014, 02:55:33 am
content: Thanks for the help. It is confusing and I thought I understood it better from reading this thread as well:http://www.physicsforums.com/showthread.php?s=50c503ab0ff7c4629897151c902268ab&t=395722&page=1But now I have even more questions.Especially when it comes to grounding, why it seems ground is such a good source/sink of electrons that a circuit can be completed through ground... or is it because neutral is also grounded somewhere nearby and grounding just completes the circuit?What if neutral wasn't grounded at all... Would you still get shocked? If the live and "neutral " were not tied to ground at all, and you could run appliances by completing the loop when you plug something in... then when you ground either wire live or neuttal) wouldn't you be just turning that wire into the "neutral". Would the ground still be a path?I guess I am trying to understand it by imagining a bunch of electrons being pushed one way, then the other, one way, then the other (like DC but switching directions). Instead of just thinking if potentials on the lines. Either way though, touching something with a huge potential does nothing to hurt you unless you are a path... you don't get electrons pushed into you, then sucked out of you, then pushed into you 60 x per second.... and causing you any harm unless you are connected to Earth which I figure does cause problems because now there are way more electrons available to move?

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 10, 2014, 03:05:39 am
content: Quote from: edy on July 10, 2014, 02:05:30 amCorrect.QuoteYes. Ground is ground. Some grounds are better than others, but you can consider them all the same for theoretical (and most practical) purposes.In my country, the "neutral" is grounded BOTH at the distribution transformer, AND ALSO at the power mains entrance to the premises.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 10, 2014, 03:21:34 am
content: Quote from: edy on July 10, 2014, 02:55:33 amYou seem to be using the term "ground" interchangably with "neutral". That might explain why you are confused. If you look at a typical modern mains plug, you will find three pins and wires: Black (or brown in IEC-land) is hot, white (blue in IEC) is neutral, and green (green/yellow in IEC) is safety ground. Properly designed equipment doesn't use the green-wire safety ground for ANY power current. All current is drawn between the neutral and hot leads. The green wire is ONLY for safety protection of the user (and equipment).QuoteIf the neutral were TRULY floating everywhere, then no, you probably wouldn't get shocked. There are a few power schemes that float both sides, but it is very uncommon and you wouldn't want to bet your life (literally!) on it.QuoteYes.QuoteYes, the ground would still be a path from the ungrounded side ("hot")QuoteAs far as conducting current through the load, through ground, and/or through YOU, it makes no difference whether you are talking about AC or DC. You may be over-thinking this and confusing yourself. The crust of the planet between two ground rods is simply another conductor. Not as good as a long skinny piece of copper, but easily more than enough to kill you.

author: Lee697
author_rank: Regular Contributor
timestamp: July 10, 2014, 03:24:20 am
content: Quote from: peter.mitchell on July 10, 2014, 02:18:50 amLots of houses in Australia, mine included, have a 3-phase supply (3x240V 100A each)....

author: edy
author_rank: Super Contributor
timestamp: July 10, 2014, 03:43:54 am
content: Thanks, those answers helped clarify a few things:If neutral wasn't tied to earth and was floating, it means it would only have a potential difference between itself and the live wire... but not to earth. Touching either live or neutral separately wouldn't shock you... Is it because when neutral is tied to earth you fix it's potential at ground, thereby making live potential different relative to you?Would it make a difference if you were 100 miles from where your neutral is earthed?

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 10, 2014, 03:48:33 am
content: Quote from: edy on July 10, 2014, 03:43:54 amCorrect.QuoteYou would likely receive a bit less of a shock. It might take a few more milliseconds to kill you. I'm not sure I understand WHY you are asking these questions. Do you have a death-wish?

author: edy
author_rank: Super Contributor
timestamp: July 10, 2014, 03:57:44 am
content: Ha Ha... No death wish.I have been thinking about grounding and AC generation and read about the basics... finally thought I had a Eureka moment . So different from DC Iam used to thinking about.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 10, 2014, 04:01:51 am
content: 3.1 Gigawatts of power is regularly sent through the crust of the earth by the "Pacific DC Intertie". One end is up here in Celilo, Oregon, near where I live now, and the other end is down in Sylmar, California near where I used to live. It uses a different grounding scheme at each end.http://en.wikipedia.org/wiki/Pacific_DC_Intertie"The grounding system at Celilo consists of 1,067 cast iron anodes buried in a two foot trench of petroleum coke, which behaves as an electrode, arranged in a ring of 3,255 m (2.02 mi) circumference at Rice Flats (near Rice, Oregon)." I wonder if the crops grown inside that circle are any different?"The Sylmar grounding system is a line of 24 silicon-iron alloy electrodes submerged in the Pacific Ocean at Will Rogers State Beach suspended in concrete enclosures about one meter above the ocean floor." I presume swimmers along that beach don't feel a tingle when they go swimming/surfing there.

author: onlooker
author_rank: Frequent Contributor
timestamp: July 10, 2014, 04:08:23 am
content: Even on a theoretical ground, Live or neutral are not something an end user can select; the electricity company made the choice for you. In the US, even the electricity company has only one choice on this. This is because a residential house is supplied with 2 lives of opposite phases. The center split tab of the transformer has to be the neutral.Again theoretically, even if both live and neutral are float, one can still get a "shock" when touching one of the conductors. This is due to the capacitances amongst/between you, the wires and the earth.

author: miguelvp
author_rank: Super Contributor
timestamp: July 10, 2014, 04:11:05 am
content: Someone linked this on some other thread about earthing standards:http://en.wikipedia.org/wiki/Earthing_system

author: Rerouter
author_rank: Super Contributor
timestamp: July 10, 2014, 07:46:30 am
content: Here in Australia we conform to a standard called MEN or multiple-earthed neutral, which means that an earth stake is hard wired to the neutral conductor at both the transformer and the residence, the reason why in an AC system we refer to one conductor as neutral comes more from the 3 phase delivery of it,To the left of the image is called a delta topology, (415V per phase), and on the right is Star (240V to neutral and 415V phase - phase) which you would be more familiar with, and if you wanted to redefine your active as your neutral, have a think on what happens to those other 2 phases?Even if your house only connects to a single phase, they still deliver it over 3 on the power lines out the front of your house, and they even try there best to balance the loads per phase between houses, as it means less current flows in the neutral conductor, (practically 0 current flows down the neutral wire when all 3 phase loads are balanced)getting back on subject, the MEN approach was so that if you have a ground stake fault, you still have the neutral conductor to carry the current, and in a worse case, if the neutral wire fails, the current only needs to flow to your nearest neighbors stakes, rather than the hundreds of meters the transformer is located at, if you measure your ground connection with a clamp ammeter you may even find some current flowing into your (current takes all paths),To answer the other implied question, floating the system can cause problems, as both mineral content, various leakages from underground wiring and static charge in clouds above you can shift your plot of dirt's potential up to a few thousand volt, from a fixed reference, and this can result in easily fatal equalization currents if you happen to form a path between the 2 (think of the mains conductors as a big capacitor, and the earth as a big capacitor, and you as a crowbar connecting a flat capacitor to a highly charged one)

author: G7PSK
author_rank: Super Contributor
timestamp: July 10, 2014, 08:08:43 am
content: The power distribution system Here in the UK is pretty much the same as in Australia. Another reason for trying to balance the phase loads is generation efficiency alternators work best when all the stator's are in balanced load when you get greatly imbalanced load there is also a torsional vibration set up on the drive shaft and power source, this can even be seen with smaller power plant with imbalance on the load as flicker on the lights, I have seen this myself.