Question About AC Plug Polarity - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: February 24, 2023, 04:33:54 am
content: Polarity with DC is obvious (reverse it and things blow up), but not sure I understand when an AC plug should have polarity and when it shouldn't; and should know at this stage of my career.I've seen two-prong lights with plugs that have polarity (one prong is larger than the other). When I see this, I assume the neutral needs to be larger for some compliance standard because an incandescent light bulb doesn't have polarity.In the case of say a generic/basic full-wave rectifier I'd assume polarity doesn't matter.What about other electronics? I can't think of anything off the top of my head, but I can imagine lots of electronics will get damaged with reversed AC.On a side note, I think AC motors spin in a specific direction based on the windings because I've dealt with AC motors lately that don't require polarity (although I need to make sure to wire the hot side to the switch to avoid the motor sitting "hot" when the switch is off).

author: fchk
author_rank: Frequent Contributor
timestamp: February 24, 2023, 05:09:16 am
content: In Germany and all other countries with Type F Schuko plugs there is no polarisation at all. Works fine.The reason to have polarisation is safety. Usually you have a Live and a Neutral wire, and Neutral is basically on Earth potential and causes no harm. Touching Live, however, might end yours. This obviously doesn't work with split phase systems where you have two Live pins with half the voltage and no Neutral.fchk

author: bostonman
author_rank: Super Contributor
timestamp: February 24, 2023, 05:21:30 am
content: I should add, I'm talking about single phase. i.e. one hot and one neutral.I lack experience with multi-phase systems, however, to keep things simple, I'm talking US 120V AC single phase.

author: IanB
author_rank: Super Contributor
timestamp: February 24, 2023, 05:47:49 am
content: AC supplies do not have polarity in the sense of DC supplies. If you swap the wires of an AC supply it makes no difference to the load.Polarity of mains plugs is a matter of safety. In the mains supply "neutral" generally has a voltage close to ground beneath your feet, so in principle touching it will not cause an electric shock. On the other hand, the "hot" conductor has the full mains voltage on it relative to ground, so if you touch that you have a strong risk of electric shock. This means that equipment like lamp sockets should be wired internally such that the hot conductor goes to the least accessible parts (the base of the bulb, not the side of the bulb).Lamp sockets are however the exception when it comes to accessible parts. Most appliances should be constructed with good insulation so that no live conductors can be touched.

author: Whales
author_rank: Super Contributor
timestamp: February 24, 2023, 05:57:04 am
content: The "neutral can be safer than hot" argument is almost entirely theoretical, I'm yet to see a real life scenario where it's advisable to exploit.House wiring can be wrong, extension cord wiring can be wrong and some countries supply both wires as active (eg most household 240V in America has both wires hot).> (the base of the bulb, not the side of the bulb).Eep! I guess it's better than nothing if you're already in a situation of having exposed metal.Bayonet type fittings are a little better in this regard; but they seem to be more delicate.

author: Marco
author_rank: Super Contributor
timestamp: February 24, 2023, 05:59:52 am
content: You can avoid some capacitive ground loops with consistent neutral, you can also kill yourself carelessly relying on it. Win some, lose some.

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2023, 06:00:22 am
content: It's just a safety thing, functionally it doesn't matter. Neutral is tied to earth, ideally a lamp socket should have the shell on the neutral side to reduce the risk of shock, and if a single pole switch is used, the switch should be on the hot side. A polarized plug enforces this and offers a marginal improvement to safety for negligible added cost.

author: CosteC
author_rank: Regular Contributor
timestamp: February 24, 2023, 07:18:01 am
content: Not sure if you refer to mains AC plug/socket or wall wart AC power supplies. Latter are dying out, so I will go with mains.Answer is country dependent.Here is catalogue of solutions:https://en.wikipedia.org/wiki/AC_power_plugs_and_socketsGenerally one shall not rely safety on polarity (Live vs Neutral). Correct connection of PE can/shall be assumed.Europe, USA, China, India - almost always plugs are not polarised by design. Assuming polarisation imminently will lead to issues.UK - wiring regulation are super strict and in theory Live and Neutral have fixed locations. I however would not trust safety to such assumption. It is too easy to swap them without notice. Mandatory controls rarely are done on EACH and EVERY socket in home also extension cords may swap them.

author: rstofer
author_rank: Super Contributor
timestamp: February 24, 2023, 04:25:35 pm
content: Many years ago, things like radios and TVs had what was called a "hot chassis" where the neutral was bonded to the metal chassis. It mattered which wire this was so they 'identified' the neutral wire. First, the wire in the house wiring will be white then the slot in the outlet will be the wide one and, in the case of two conductor flat cable (zip cord) there will be ribs along the strand that should be neutral ('identified conductor'). Should the end device have a 'hot chassis' design, the identified conductor is the one to bond.In later years we moved to the common U-ground (3 pin) device with 3 wire cable.https://www.geojohn.org/Radios/MyRadios/Safety.html

author: TimFox
author_rank: Super Contributor
timestamp: February 24, 2023, 04:47:46 pm
content: In the US, two-blade male plugs with one blade wider than the other are called "polarized", but rarely is the term "polarity" applied to them.The wider blade should be connected to neutral: see the above discussion for what that means and why you should be careful about if it is wired properly.

author: Ian.M
author_rank: Super Contributor
timestamp: February 24, 2023, 05:01:35 pm
content: As IanB has already noted, its obviously undesirable for the shell of an Edison Screw bulb to be connected to an AC mains Line / Phase conductor if it can instead be connected to Neutral, which is normally held at a near Earth/Ground potential, by bonding somewhere between the final distribution transformer and the consumer's main breaker panel. Some countries used to care about this and similar issues (which are now vanishingly rare) so had polarised plugs and sockets, and attempted to enforce their correct use, by inspection and testing of installations and products.Of course, they couldn't actually guarantee the shell would be touch-safe, as a Neutral fault or miswiring could make it 'hot' live, but the idea was the stupid would survive unless there was also a fault. However the increasing prevalence of GFCI/RCD protection on all circuits makes actively enforcing safe use of polarised connections less critical - on a protected circuit, touching a 'hot' live conductor *probably* wont kill you.

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2023, 05:49:33 pm
content: Quote from: rstofer on February 24, 2023, 04:25:35 pmI never saw a hot chassis radio that came with a polarized plug, although it's fairly common for people to add one in modern times. By the time TVs came along the safety thing had been figured out to the extent that the metal chassis itself was not directly connected to either side of the line.

author: Zero999
author_rank: Super Contributor
timestamp: February 24, 2023, 06:24:27 pm
content: Quote from: Whales on February 24, 2023, 05:57:04 amIn the UK the phase is always switched and fused, never the neutral. The circuit breakers in the consumer unit all interrupt the phase.QuoteThe house wiring should be correct. It's always inspected. I've never seen the live and neutral reversed in the UK mains wiring.

author: TimFox
author_rank: Super Contributor
timestamp: February 24, 2023, 06:29:02 pm
content: Quote from: james_s on February 24, 2023, 05:49:33 pmAs I remember, back in the "hot chassis" era, normal two-prong wall outlets were not polarized.The current 3-prong grounded outlet has polarized flat-prong female contacts along with the round ground contact.Postwar 5-tube AC-DC radios did not usually connect the chassis to one prong on the power cord: UL had a requirement for minimum 60 Hz impedance (maximum 60 Hz current) between the metal chassis and power line, something like 0.05 uF in parallel with 220 k ohms.

author: TimFox
author_rank: Super Contributor
timestamp: February 24, 2023, 06:32:16 pm
content: Quote from: Zero999 on February 24, 2023, 06:24:27 pmYes, house wiringshould becorrect.However, I moved into a newly-wired condominium (converted from a pre-war three-story apartment) in the US where I found several of the outlets to be miswired by a non-union cheap electrician.I had been warned to check for this and borrowed one of the simple three-neon-bulb outlet checkers.When I complained to the seller, he had the miswired outlets corrected.Chicago code requires metal conduit, so the ground contacts were correct.

author: themadhippy
author_rank: Super Contributor
timestamp: February 24, 2023, 06:52:35 pm
content: QuoteApart from double pole devices.Theres a load of regs that determine whether you need to switch the neutral or not for safe isolation.

author: rstofer
author_rank: Super Contributor
timestamp: February 24, 2023, 06:56:54 pm
content: Quote from: TimFox on February 24, 2023, 06:29:02 pmThe polarized receptacle was introduced in 1916. It took a long time for it to become anywhere near 'universal'.I remember getting knocked on my butt when I touched a radio volume control that was missing a plastic knob. That was some time prior to 1958, probably around 1954. Bare foot on the garage floor.By 1925, only half of homes even had electricity! I remember some relative's house that had gas lighting and this was around 1950. I certainly remember gas street lighting.

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2023, 07:05:36 pm
content: Quote from: james_s on February 24, 2023, 05:49:33 pmIndeed hot-chassis radios did not have polarized plugs and in many cases, the chassis could be hot with the power switch either on or off depending on which way you plugged them in. So plugged one way, the chassis is hot when on, plugged the other it was hot when off. The thing to do was to try it both ways and leave it plugged in the position with the lowest hum and noise. I have one of these in my living room and I've added in a transformer.IIRC, on some TVs the chassis was not connected to either line, but it was connected to the negative side of a line-level bridge rectifier. The result was that the chassis potential to ground was a complex and energetic mess (and quite hazardous).

author: Zero999
author_rank: Super Contributor
timestamp: February 24, 2023, 07:22:05 pm
content: Quote from: TimFox on February 24, 2023, 06:32:16 pmThe same could be said about anything. How do you know it's earthed? Are you sure they've not skimped and used too thin cable? There are far more dangerous faults than reverse polarity.

author: TimFox
author_rank: Super Contributor
timestamp: February 24, 2023, 07:39:56 pm
content: Is it grounded? With respect to Chicago code:With metal conduit, metal outlet boxes, and the third contact of the outlet mechanically connected to the box, ground should be reliable.The usual outlet checker senses that the "line" contact sees appropriate voltage to each of the neutral and ground connections.If the connection is broken between the third contact and the ground connection back at the breaker box, then that light will not illuminate.

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2023, 07:52:07 pm
content: Quote from: Zero999 on February 24, 2023, 06:24:27 pmIt's rare, but I've seen it a few times here. The inspectors do random spot checks, they don't test every single receptacle, although many home inspectors will do that when you buy a used home. I've also seen the switch on neutral a few times, it's pretty common for homeowners to do unpermitted work themselves that is not inspected. They're not supposed to do that but it's impossible to completely prevent.

author: bostonman
author_rank: Super Contributor
timestamp: March 07, 2023, 07:49:20 pm
content: Lots of good feedback, as usual on this forum.Basic AC circuits I sensed wouldn't matter about polarity since it's AC (i.e. going positive to negative) such as say the input section of a wall wart transformer, but I had rattled my brain thinking of different circuits where maybe polarity was necessary.I had one outlet in my house wired backwards (also found by way of those cheap LED polarity testers). I've also seen switches for lights wired so the bulb socket is "hot" all the time (i.e. neutral is on the switch).If I'm understanding correctly, unless there is some two-wire piece of (old?) electronics where neutral could be tied to chassis, or someone recreated such a wired system, a two or three prong plug doesn't have polarity in all cases (ignoring the Earth ground prong)? If the chassis was wired to neutral, then I imagine the issue with reversing the outlet would be: the chassis is hot, the electronics works fine, but someone touches chassis (such as an old radio with a metal case), and the other hand to a water faucet, then they can be electrocuted and why AC polarity would be important.As for a typical three-prong modern plug, is the neutral wider for less resistance since polarity doesn't matter because I've seen two-prong stuff such as electric leaf blowers where the wider prong causes me to flip the plug a few times trying to figure out if the prongs are slightly bent or I need to flip it.Honestly, I don't know why anything is two-prongs when Earth ground is so important; except stuff that goes through a wall wart I guess.

author: TimFox
author_rank: Super Contributor
timestamp: March 07, 2023, 07:52:45 pm
content: The wider blade for neutral is just a polarization marker: it stops you from inserting a two-prong plug into a socket the wrong way.With a three-prong socket, the wider blade is to be compatible with a polarized two-prong plug.I find that "polarization" or "polarity" in this context leads to confusion.Apolarizedplug is one that can be inserted only one way, it does not imply positive or negativepolarity.A good reason to distinguish the "line" and "neutral" connections is to allow the switch and/or fuse to be in series with the "line" wire for safety reasons.

author: bostonman
author_rank: Super Contributor
timestamp: March 07, 2023, 07:56:55 pm
content: QuoteThis is a great point. Until now, I was thinking about circuit function, not the fact that the fuse blowing would cause the circuit (i.e. device) to be hot.

author: james_s
author_rank: Super Contributor
timestamp: March 07, 2023, 08:05:59 pm
content: In every case I can think of aside from perhaps antique AM radios, there is absolutely no difference in terms of circuit function, the polarization is entirely for safety reasons. If you know which wire is hot, you can design the product such that controlling and protective devices de-energize the majority of the circuit.

author: shapirus
author_rank: Super Contributor
timestamp: March 07, 2023, 08:33:43 pm
content: There is certain equipment such as some models of gas boilers that requires a specific arrangement of line and neutral connection. They won't work for example from a battery-powered AC inverter, unless you connect one of its output wires to mains earth, thus emulating neutral. As far as I understand, it's because of the way their flame detection circuitry works.

author: james_s
author_rank: Super Contributor
timestamp: March 07, 2023, 09:42:19 pm
content: Quote from: shapirus on March 07, 2023, 08:33:43 pmI suspect that's because the entire metal chassis is earth grounded. Even then though I don't know why the line polarity would matter, unless it's using a transformerless non-isolated supply? It could also be a safety feature where it detects wrong polarity and shuts down.

author: shapirus
author_rank: Super Contributor
timestamp: March 07, 2023, 09:51:06 pm
content: Quote from: james_s on March 07, 2023, 09:42:19 pmI'm not sure really. I just know they exist and that people who have them need to use workarounds to make them work from inverters. And *maybe* they actually need an earth connection rather than a specific line/neutral polarity, but all mentions of this that I encountered told that those boilers required a neutral, not earth.My gas boiler doesn't care about earthing or polarity and has no problem running from a DC->AC inverter. Maybe someone who has one of those which require a neutral will eventually come to comment.

author: james_s
author_rank: Super Contributor
timestamp: March 07, 2023, 09:53:35 pm
content: I've installed a few gas forced air furnaces recently and now that you mention it I think they did say they required correct polarity but that was never a concern since they are hardwired and it's easy enough to get the wiring right.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 07, 2023, 10:33:09 pm
content: Just a plus one on not counting on the correct connections. In half of all of the houses encountered in my family one or more sockets was incorrectly wired. Some were homeowner botches but some of the worst were "professionals" though I have no idea whether they were union pros or not.It goes beyond that. I recently installed a couple of 6 bulb chandeliers that SWMBO selected for appearance. These came with each of the sockets installed and the wires from the sockets run to the central housing, but it was left to the owner to do final connection. The instructions included repetitive warnings to connect appropriate colors and match when attaching to the supply. But when I checked I found exactly half of the sockets the white (nominally neutral) wire was connected to the tip.That left me with three options. Completely rewire the lamp, wire it electrically correctly and leave a mish mosh of interconnected colors, or connect the appropriate colors and have a slightly less safe chandelier. I elected the latter. I started down the first route and found that the sockets were not reworkable, and decided it wasn't worth the effort to redo the work I had done connecting it color matched.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 09, 2023, 03:47:43 pm
content: Quote from: james_s on March 07, 2023, 09:42:19 pmYes, it uses flame rectification and leakage between live and the chassis (which is grounded) to detect the presence of absence of the flame. The cathode is generally the burner tube itself so it isn't desirable to connect that to the line.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 09, 2023, 04:21:45 pm
content: Flame rectification sensors are also used on some gas cooktops which are likely to be plugged into a normal outlet rather than hard wired.It's still a relatively uncommon scenario and one that is pretty easily addressed when necessary as long as the installer knows what they are doing.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 09, 2023, 06:23:31 pm
content: I've found seeing how power distribution is wired can be helpful. There are few good site, but this one is OK. Scroll to the bottom for typical US wiring.http://waterheatertimer.org/How-to-identify-transformer-wiring.html