Mains AC, why doesn't neutral shock you? And other black magic. - Page 1
Forum section: electronics / beginners

author: kyle
author_rank: Contributor
timestamp: October 09, 2013, 04:43:42 am
content: Assuming that things are properly wired because when I searched I found cases where neutral would give you a jolt. I live in US and have a 3 wires leading to service panel, 1 is a center tap and other 2 are 2 phases. Center tap is used as a neutral wire and it is grounded to earth with use of a rod. I have problems understanding few things:1. Why doesn't neutral wire give you a jolt? Consider an outlet with disconnected wires and you touch neutral, my reasoning: you should get shocked since for 1/2 of an AC cycle current flows from the transformer toward you -> through you -> into earth, the other 1/2 of a cycle current retreats with no harm since you can't source current from nearby environment?2. Does a coil+magnetic field on secondary transformer winding need a circuit for current to flow or can current flow straight into earth? I learned some time ago that with batteries circuit is needed for chemical reactions to keep on going to create current. If this was true, would for 1/2 of a cycle the current from transformers go through load -> neutral -> earth? Into earth since neutral is bond to earth and not back into the transformer's changing-magnetic-field?3. Related to the above, wouldn't current on the other 1/2 of cycle go from transformer straight to earth and dissipate all energy because neutral is bonded to earth and not even reach the load/device?4. After some reasoning, I came to conclusion that since neutral and earth are bound, at any instant they posses same potential therefore there is no difference and no current flows though you? But wouldn't that energize objects that are connected to ground? And wouldn't earth drop some voltage, leading to voltage difference, and possibly zapping you? Wait...what happens when you have a transformer and only connect center tap connected to earth? Maybe that might clear up my issues.Also was wondering why grounding is encouraged as in safety, I read about examples talking about how grounded device posses low impedance path so in case of a short it can trip breaker easily, others so a microphones doesn't blow up in your face, but I still don't get full picture. Does anybody have good explanation?thanks in advance.

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 09, 2013, 05:08:00 am
content: This is almost exactly the same as another thread from today, so you may want to read these answers:https://www.eevblog.com/forum/beginners/question-on-electricity/1. Neutral doesn't give you a jolt because it's at the same potential as ground. Typically, you're either well insulated, in which case Kirchoff's current law would allow you to touch either hot or neutral (but not both!) without getting a jolt, or else you're standing in a wet puddle and well grounded, in which case touching the hot wire would give you a shock, but touching the neutral wire wouldn't expose you to any voltage difference.2. Kirchoff's current law says that current always flows in circles. If it flows into something, it flows out. If it flows out of something, it flows in. The transformer secondary obeys this rule, so if current is flowing out of one terminal of the secondary, it must flow into the other terminal. There must be a conductive path from one end to the other in order for current to flow.3. Nope. There are two wires involved. One wire goes from terminal "A" of the secondary to terminal "X" of the load. The other goes from terminal "B" of the secondary to terminal "Y" of the load. (I just made up those A/B X/Y terminals arbitrarily). Draw just that picture by itself, and see if it makes sense how the current flows in a loop, in opposite directions as the cycle switches. Once you understand that picture, connect one of those wires to earth ground at one point. Nothing vital about the picture changed, did it? It still works exactly the same way? If so, now you understand.

author: ve7xen
author_rank: Super Contributor
timestamp: October 09, 2013, 05:14:38 am
content: Quote from: kyle on October 09, 2013, 04:43:42 amAs you note in your intro, neutral is shorted to earth. Unless there's a large earth potential between you and the grounding rod, you will be at the same voltage as earth, and thus the same voltage as neutral. The voltage between you and the neutral wire will always be very small.QuoteI'm not sure how low impedance the ground is or how easy it is to get a low impedance connection there, but in theory if the impedance is low enough it should work. The return current would flow through the ground, probably back to where your ground rod is to return to the transformer. I believe some transmissions lines intentionally exploit the earth as a return path to save a conductor. There's always a circuit though, the current must return bysomepath.QuoteIt's AC, so the current alternates direction, (and its voltage alternates +/- about the neutral) but unless there are diodes or other nonlinear components, the path doesn't change. In both cases the return current would flow from the transformer, through the load, through earth, and back to the transformer.QuoteEarth or ground is used as a reference against which the rest of the system pushes. It takes a huge amount of current to induce an appreciable voltage across the ground, the distributed resistance is very low. Sinking the ground rod essentially anchors this neutral point to the same potential as the earth, and anything else in good contact with it, like you. If a voltage (versus ground) ever developed on the neutral wire, it would cause a current to flow into the earth via the ground rod, which can sink a (for practical purposes) unlimited amount of current. A very large current in a single neutral wire back to the ground rod may allow enough voltage to develop to be dangerous, but this would probably also cause fire-hazard levels of heating in the neutral wire, and your breakers would trip long before.QuoteThe ground wire of appliances is attached to their case, if done properly, by a solid connection that should come disconnected last in a failure condition as well. The idea is that the ground wire never carries current in normal operation, so should always be at a safe potential. In a failure mode where say the hot gets shorted to the case, normally the case would become energized. However, the low impedance path directly to ground shorts hot directly to ground, causing a large current to flow that will trip the breaker, cutting off the power and preventing the case of the device from becoming energized. Having that low impedance path directly to ground ensures that something fails (a fuse, breaker, wire...) if the case ever gets energized, instead of sitting there waiting for someone to touch it and get a shock.Neutral could theoretically be used this way as well, but would not be safe. Many plugs are wired with hot/neutral reversed, which would energize such an appliance even when no fault occurs!

author: kyle
author_rank: Contributor
timestamp: October 09, 2013, 10:12:37 pm
content: Quote from: ve7xen on October 09, 2013, 05:14:38 amThank you for the explanations.Quote from: AG6QR on October 09, 2013, 05:08:00 amThank you, the youtube video was very helpful because it explained the history and how things came about.Ok to test out my understanding for the last time. Going back to the old days where only 2 conductors where used and no grounding whatsoever, where you could get zapped by static electricity or lighting when being near an outlet. Back then touching either live wire was a non issue correct? Since the current could not flow through a person due to no return path.But as the electricity providers started grounding one of the wires to combat static electricity and lighting strikes suddenly that gave transmission lines opportunity to be able to source or sink theoretically infinite amounts of electrons. So if a person had a device where a conductive enclosure had one of the wires connected to itself, in case of touching netural (at this point I'm still assuming service panel is not using a grounded center tap neutral, only 1 wire being grounded at substation) yielded no harm, but touching case where hot was connected would zap you because electrons would get sourced/sinked at substation, flow through transmission wires, through a person since he/she would be in series within that circuit, and back to earth? (So effectively 1 transmission line in series with generator, person, and then earth at both ends for sourcing/sinking)

author: Nerull
author_rank: Frequent Contributor
timestamp: October 09, 2013, 10:51:36 pm
content: If the circuit was truly floating, yes. Through touching both wires would still give you a jolt.I'm not sure that is a safe assumption to make, though. If anything connected to the circuit has a path to ground, the danger is still there.

author: bgsteiner
author_rank: Contributor
timestamp: October 10, 2013, 04:18:15 am
content: This reminded me of the time I was shocked by 60v on the ground wire. Tingled and surprised me luckily the next outlet I converted to 3 prong was the problem child.

author: Zero999
author_rank: Super Contributor
timestamp: October 10, 2013, 04:19:32 pm
content: In the fixed wiring, neutral should always be neutral but you shouldn't rely on it being 0V, because it could become life if a fault develops (broken neutral).

author: MauriceS
author_rank: Contributor
timestamp: October 17, 2013, 04:26:51 am
content: Also, the neutral wire in the US is bonded (connected) with earth at the first mains panel only. The earth is a (number) of long copper (clad) spikes pounded into the soil.What could happen if there is a neutral break between you and the panel, there could be voltage on it. For example when a transformer or bulb is turned on / connected - the low resistance will connect the neutral to almost 110V AC..