This might be a silly question.. but can any of the AC mavens help? - Page 1
Forum section: electronics / testgear

author: harmo
author_rank: Newbie
timestamp: March 01, 2025, 12:45:21 pm
content: Hi all,I hope you're all well. Do you mind a sub-three-minute story?I'm just getting my ankle's wet (as opposed to just dipping my toes in) in this electronics stuff, and most "mains voltage" is beyond my experience. So please forgive me for not knowing the intricate dance moves of "the AC current jiggle".So, we're renovating, and got a sparky (electrician) to install some power points. He lays his coppery electron aquaducts joining point A to the designated point B, and his apprentice installs the GPO in the wall at the aforementiond point B. It works, we pay, and he's off on his merry way. Except the apprentice must be new....After a rather awful experience a different tradie with a prior renovation (bare live wires left dangling from the wall, and a jackhammer incident involving a wall we share with a neighbour), I've been.... err... skeptical of the "work quality" whenever we try a new tradie. So, I grabbed one of those GPO outlet testers just for my own piece of mind, and checked his work.I plug it in, and turned the GPO on... the tester told me that the live/neutrals were reversed. I called the sparky and he fixed it promptly. However, that inspired this question:If AC currents are going back and forth 50/60 times a second (depending on where you live), how do those GPO testersknowwhich line should be live and which is neutral? Even a reversed polarity GPO runs most simpler appliances (in our case, a lamp) fine. Is there some specific characteristic that identifies categorically, which wire is what?What is my GPO tester looking for, and how does that work? Is this a deep-dive physics question? I last did engineering at uni two decades ago and completely switched fields in my career promptly after graduation to the controlled fermentation of tasty consumables after... so assume I know nothing anymore.I know it's not exactly critical for me to know, but I'm curious and I'd like to understand my test gear a little better.Anyway, thanks for reading. Any guidance would be appreciated.Harmo.

author: Circlotron
author_rank: Super Contributor
timestamp: March 01, 2025, 12:52:47 pm
content: The neutral wire is connected to earth at your switchboard and also out in the street. So measuring from the power outlet earth pin to a correctly wired neutral pin should give essentially zero volts. If the active wire is wrongly on the neutral pin you will measure the full 240VAC between earth and "neutral".

author: themadhippy
author_rank: Super Contributor
timestamp: March 01, 2025, 02:09:02 pm
content: QuoteWithout getting too technical thats not always the case ,however its fairly safe to say at some point between the power station and your socket outlet neutral is connected to mother earth so the voltage between them will be much lower than between live and earth

author: csuhi17
author_rank: Frequent Contributor
timestamp: March 01, 2025, 02:55:45 pm
content: Yes, it depends on the area.In our country, the neutral and live are connected around the electricity meter too. And in a few places before that.Swapping the Neutral and Live in AC is a safety issue.There are devices in which the switch only cuts off one of the wires.If it is swapped, the part after the switch in the device will also remain live.Changing a bulb in a lamp can be dangerous, because when you unscrew it from the socket and touch the larger metal part of the bulb, you can get a shock, regardless of whether you have turned off the switch or not.A few of my friends have already been shocked by electricity because of this.There are a couple of videos on Youtube on this topic, you can understand better there, because they show you.

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 01, 2025, 04:26:46 pm
content: I don't know about Australia but here in the USA the neutral and ground are connected together at the main panel and thus whether the ground is really earth ground there should be no voltage between neural and ground. So the hot must be whatever line voltage is to ground.

author: Fungus
author_rank: Super Contributor
timestamp: March 01, 2025, 05:40:50 pm
content: Clue: There's three pins in the plug (or should be).One of the AC wires wiggles more than the otherwhen compared to the ground wire.If you disconnect the ground wire then you're correct, there's no way to tell.(apart from parasitic capacitance but we won't go there yet...)

author: benj38
author_rank: Frequent Contributor
timestamp: March 01, 2025, 07:04:57 pm
content: Quote from: csuhi17 on March 01, 2025, 02:55:45 pmNote, however, that unlike Australia, the US, etc., in most of Europe power outlets are symmetric, i.e., you can get live and neutral reversed simply by rotating the plug 180 degrees. Hence, safety is a question of Geography

author: wasedadoc
author_rank: Super Contributor
timestamp: March 01, 2025, 08:01:16 pm
content: Quote from: benj38 on March 01, 2025, 07:04:57 pmI think that would have been more accurate if you had typed UK (and some other places with historical connections) instead of USA.

author: harmo
author_rank: Newbie
timestamp: March 02, 2025, 07:13:08 am
content: Hi all (wasedadoc, benj38, Fungus, BeBuLamar, csuhi17, themadhippy, & circlotron, et al),Thanks to all, for taking the time to explain that, I really appreciate it. Alas, I have never had the opportunity to really delve into the mysteries of the mains "grid". It's also interesting to read about how various countries approach things differently, which provides a broader context.I am indeed in Australia, so never thought about reversible plugs either... since Aussie plugs really do only go in one way.... well, not without attacking the pins with a pair of pliers or something... which seems unwise....Glad I got that little GPO tester, I honestly wouldn't have known about the reversal otherwise. It's also handy to know that some breakers and appliance switches only disconnect one wire. As I've stated, I've seen some "interesting" electrician jobs... and I understand that there might be some legitimate reason for doing some things in a particular-yet-odd way (although I may not understand it myself). I don't want to belittle anyone, but sometimes I have to do a double-take and ask if it makes sense.. and if I can test it easily and get it fixed up promptly as needed.. then I've done what I can to help.Anyway, I'm a little more knowledgeable now. So I could have just used a DMM to check the voltage between Earth and neutral and effectively done the same thing? (I guess that assumes the Earth was indeed connected, otherwise it'd all read zero if I'm using it as a reference....) Now I'm starting to doubtallour GPOs. It's a slippery slope...Oh well, the tester was about $20 AUD. I don't regret it.Thanks once again!harmo.

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 02, 2025, 01:32:05 pm
content: In the US of course you can't plug the 3 prong plug in the wrong way. For the polarizedd 2 prong plug you can't either as one of the prong is larger however there are many devices with 2 prong, non polarized plug and you can plug it in either way. These devices are designed so that either way is safe.

author: Fungus
author_rank: Super Contributor
timestamp: March 03, 2025, 07:22:52 am
content: Quote from: BeBuLamar on March 02, 2025, 01:32:05 pmAre you telling us the neutral is connected to the earth inside the three-prong devices?

author: benj38
author_rank: Frequent Contributor
timestamp: March 03, 2025, 08:15:35 am
content: Quote from: Fungus on March 03, 2025, 07:22:52 amI believe he is saying that such instrument do not need the earth (usually plastic enclosures) and are indifferent to which is the line and which is the neutral. E.g., these arenotlamps with screw-in bulbs where it is safer when the neutral is connected to the thread of the bulb.

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 03, 2025, 10:54:34 am
content: Quote from: Fungus on March 03, 2025, 07:22:52 amThese devices have no connection to earth but they are made in such a way that the chance of you touching either wire is very unlikely.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 03, 2025, 11:33:00 am
content: Quote from: BeBuLamar on March 03, 2025, 10:54:34 am"Double Insulated" is the term used & the symbol is a square within a square.

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 03, 2025, 12:19:02 pm
content: With a 3 prong device the neutral is not connected to the earth inside the device. The earth wire is connect to the case of the device where you may touch it. The neutral and the ground (earth) only connected together at the main panel of the house.

author: themadhippy
author_rank: Super Contributor
timestamp: March 03, 2025, 01:51:12 pm
content: QuoteWot even if yer incoming supply arrangement is TT or TNS ?

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 03, 2025, 02:50:51 pm
content: Quote from: themadhippy on March 03, 2025, 01:51:12 pmYes! Even if the house uses its own ground rod it is only connected to the neutral at the main panel where power coming into the house. If they are connected together before that point they are still connected together at that point but not anywhere after that.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 03, 2025, 04:22:39 pm
content: Quote from: BeBuLamar on March 03, 2025, 12:19:02 pmNot in my case. Earth and neutral are connected together just before the supply side of the supply company's meter. That connection and the meter are in a box in an external wall. The main panel (distribution and fusebox) are inside the house and contains no connection between neutral and earth.

author: themadhippy
author_rank: Super Contributor
timestamp: March 03, 2025, 04:30:05 pm
content: QuoteMaybe thats the case in your little bubble of the world,but its a big world outside that bubble and not everybody does it like you.

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 03, 2025, 04:43:40 pm
content: Quote from: themadhippy on March 03, 2025, 04:30:05 pmSo where the neutral and ground be connected together? or never connected together? Never connect neutral to ground in the device or at the outlet.

author: Gyro
author_rank: Super Contributor
timestamp: March 03, 2025, 05:33:43 pm
content: Quote from: BeBuLamar on March 03, 2025, 04:43:40 pmRemember that our outlets have 3 dedicated contacts (even if continental Europe allows you to plug 2 pin plugs [Ed:into them] for double insulated devices). Same with Australia.Edit: Slightly longer answer: Yes we do have wiring protocols like TN-C-S where the ground connection comes into the property disguised as Neutral, but if the Ground comes into the property on a separate conductor (TN-S) or is grounded by an earth spike (TT) it is never bonded to Neutral at the 'panel'.

author: bdunham7
author_rank: Super Contributor
timestamp: March 03, 2025, 05:59:10 pm
content: Quote from: themadhippy on March 03, 2025, 04:30:05 pmThat bubble is the US/NA and that is specifically what he referred to. And yes, it is somewhat different than the rest of the world. One thing that you have to understand that outside of large industrial/commercial subsystems (delta), there is ALWAYS a non-arbitrary neutral that cannot be interchanged with the hot side. Neutral is referenced to ground at the service entrance,even in installations that don't have ground wiresin part or all of the house. In older houses with only two wires going to the outlets, it is acceptable to have a two-prong outlet or a GFCI 3-prong outlet with a label on it telling you there is no ground provided. And I'm not referring to or considering installations which are done in conduit or have some other grounding available.

author: Gyro
author_rank: Super Contributor
timestamp: March 03, 2025, 06:32:04 pm
content: Quote from: bdunham7 on March 03, 2025, 05:59:10 pmIt's the same everywhere else (afik), it's simply a matter ofwherethe Neutral is connected to ground...https://en.wikipedia.org/wiki/Earthing_system

author: bdunham7
author_rank: Super Contributor
timestamp: March 03, 2025, 06:56:00 pm
content: Quote from: Gyro on March 03, 2025, 06:32:04 pmIf you had a completely isolated pair of wires coming in with 240VAC across them (I belive this is called "IT") then you could declare either of them to be neutral, connect it to ground and the system would work. For 3-phase WYE and the US split-phase setups, even a completely isolated input has a natural neutral unlike a simple single-phase IT connection. That's the issue I was referring to with that comment.In most US systems the incoming power actually does come in this "IT" fashion from a local transformer and the neutral is connected to ground locally at or near the service entrance or panel. The most common way to do this for standalone buildings is to have a ground rod with a connection to the plumbing that then goes to a bus bar in the panel where both ground and neutral feed wires are then attached. One reason for local grounding is to prevent the possiblity that a significant differential could build between the distant neutral-ground connection and local grounding point or the local neutral. There's really no need to avoid multiple neutral-ground connections in the outside wiring. In fact, the transformer feeding my house has the neutral connected to ground at no less than 4 different points, those being at each of the 4 houses that serves. Then each of those ground connections will branch out and be connected to our individual ground rods, plumbing, HVAC devices and swimming pools.