Alternating current, based on my Physics 101 understanding of it, flows one way then turns around and flows the other, alternating 50 or 60 times per second. This is in contrast to direct current, which always flows the same way. Even so, an AC outlet still has a hot side and a neutral side. Good AC plugs force you to plug them in the correct way, either by having one large and one small prong, or by having a grounding prong. Devices with plugs that don’t enforce this can run backwards, deliver a small electric charge to their outside casing, or just work normally, depending on the device. What’s going on there?
I don’t believe that reversing neutral and hot would cause a motor to run backward. Sometimes houses that were poorly wired ended up reversed, and the status was ignored. I doubt that’d be the case if motors ran backwards.
Yes, a 115 volts AC outlet has a “hot” pin and a “neutral” pin, along with a “ground” pin. The neutral and ground pins are at ground voltage, but they are supposed to be separate wires all the way back to the breaker box in your basement (or wherever it is in your house).
The hot pin delivers a voltage that varies sinusoidally over time, reaching voltages that are sometimes above that of the ground pin, and sometimes below. When the hot pin’s voltage is above that of the ground pin, current flows from hot to ground; when the hot pin’s voltage is below that of the ground pin, current flow from ground to hot, i.e. the other direction.
Appliances that use a polarized two-prong plug (one fitted with one wide and one narrow blade) are depending on the neutral prong of the plug to be at ground voltage, usually for safety reasons. Suppose, for example, that your toast is jammed in your toaster. Smart toaster design will use a polarized plug, and will have the hot wire connected directly to the toast lever, and the neutral wire connected to the other end of the heating elements. When the toast lever is up (i.e. not toasting), then the switch is open, and the heating elements are at ground voltage; if you stick a knife in there at this point to free up your stuck toast, you won’t get zapped if your knife happens to touch one of the heating elements.
Bad toaster design skips the polarized plug, and if you plug it in the other way, then the hot pin of the outlet will be connected to the heating elements at all times, even when the toast lever is up (i.e. not toasting). Then, even if the toast lever is up, you may get a shock if you stick a knife in there to free up your stuck toast and make contact with a heating element.
(As an aside, it’s generally bad policy to stick metal implements into a toaster any time it’s plugged in, regardless of whether it utilizes a polarized plug or not. As Arkcon notes, sometimes houses are miswired, and sometimes the appliance is miswired - and you don’t want to bet your life on it.)
Devices using a polarized plug with no ground pin will often connect metal parts of the chassis to the neutral prong of the plug, and they are counting on that prong to be held at ground voltage when the device is plugged in. If for example you replace that polarized plug with an unpolarized one and then plug it in the wrong way, then the metal parts of the chassis will be at 115 volts, and you could get shocked if you touch it.
If you have a device in your house powered by 230 volts (e.g. an electric stove/oven), then it’s always a three-pin plug, and two of the pins are hot. Each of the hot pins has 115 volts on it - but the sinusoidally-varying voltages are 180 degrees out of phase with each other. So when pin A is at +115 volts, pin B is at -115 volts, so the voltage difference between A and B is +230 volts. And when pin A is at -115 and B is at +115, then the difference between A and B is -230 volts. Note that in this system some components of the appliance may be connected to only one of the hot pins and ground, supplying that component with just 115 volts AC.
No device should ever deliver an electrical shock, no matter which direction it’s plugged in. If it does the device is defective. I don’t believe it’s allowed to tie a metal shell to a neutral and depend on a polarized cord and properly wired receptacle for safety. Even if the neutral is the one tied to the shell, it would immediately trip a GFCI if you touch it due to the current imbalance.
It should also be noted that anything with a power supply (laptop, cell charger, whatnot) doesn’t care which way it’s plugged in so such plugs are normally unpolarized since one direction is just as good as the other. Modern lamps in North America are polarized, they’d function just fine if the polarity was reversed, but would be a safety hazard because if reversed the shell of the bulb would be hot no matter what position the switch was in.
Dang. I thought I understood this “in-phase, out-of-phase” business, but when I tried to explain it, I couldn’t. I’m going to try again, and we’ll see if I have it wrong. :smack:
OK, picture your hands on a tabletop, rolling a ball back and forth,** left, right, left, right**, etc. That’s AC. If you reverse the plug, it’s right, left, right, left.
For most devices, it doesn’t matter which phase it’s in. Some things plug into each other, though, or have occasional traffic between the two devices.
If you connect two devices on opposite phases, (here’s the part I don’t know) you could get shocked, create anti-matter, or flash through a wormhole into 1817 New Jersey.
There’s an anecdote from the Jefferson Airplane at Woodstock. The band members were holding electric guitars plugged into amps, and every time they touched a microphone, they got a shock. Did that have to do with phase? I don’t know.
There are two types of AC power systems, isolated and grounded. Grounded systems have one of the conductors grounded. Isolated systems have all of the conductors isolated from ground.
Isolated systems are safer, because you can touch either line while touching something grounded and you won’t get shocked. In a grounded system, you can touch the grounded conductor and ground and not get shocked, but if you touch the non-grounded (hot) conductor while touching earth ground you get shocked.
This might lead you to wonder why if isolated systems are safer, why don’t we use them for residential power distribution? The answer to this is that it is very difficult to keep an isolated system isolated, and that if you try to run a large power distribution system isolated, it almost always ends up being randomly grounded instead. Mother nature just tries to throw tree limbs against lines and that sort of thing far too often for a large system to reliably remain isolated.
Isolated systems are used, under much more controlled conditions. If you are ever in a hospital (hopefully not for anything too serious) look for the red outlets. Those are fed from isolated transformers. Hospital isolated power is tested and certified yearly, which is a big hassle for someplace as big as a hospital.
In the U.S., most homes are fed off of a split phase transformer. What this means is that a transformer comes off of the distribution line, and this transformer has a “center tap” which is then connected to earth ground. From either line to the grounded center tap is 110 volts. From one line to the other is 220 volts. (Note - some homes are fed by two legs of a three phase transformer, which is slightly different)
The grounded center conductor is your “neutral” wire, which is colored white. The two hot lines are colored black, although a red wire might be used if you have a 220 volt circuit. The lights, outlets, and other 110 volt things in your home will be connected between one line and neutral, and should be roughly balanced out so that half of them are on one hot line and half of them are on the other. 220 volt stuff like your dryer and oven connects to both hot lines.
A separate protective ground is run to all circuits as well. This ground is connected to the neutral close to the breaker box, and both the neutral and the protective ground are then connected to earth ground (literally with a copper rod driven into the ground, usually). The purpose of the protective ground is to provide a direct path to earth ground for safety, using a conductor that under normal circumstances carries no current. Not only does this prevent a small voltage from developing on the safety ground (which would happen if current went through it) but also under various failure scenarios, having a separate ground offers more protection. For example, take your typical oven. It has an outer case made out of metal. For safety, you want that metal to be grounded, so that if you won’t accidentally get shocked by it. If you use the neutral as your safety ground, if the neutral breaks and you turn on the oven, the case becomes electrically hot. If you have a separate safety ground and the neutral breaks, the oven simply stops working and the case does not become electrically hot. This is why we now use a separate safety ground.
If all you have is a single phase isolated AC line (no center tap or whatever) then connecting it backwards would make no difference at all. The split phase arrangement of residential electrical systems however does mean that the center tap and the lines are not interchangeable.
If you can somehow force a plug backwards, motors will not run backwards. Polarized plugs are polarized for safety reasons, not function. For example, polarized lamp plugs are connected in such a way that the exposed metal parts of the lamp are more likely to contact the neutral instead of the hot line.
Plugging things into different phases should not cause a shock hazard. Joe Blow doesn’t know what phases particular outlets on his house are on what phase, so it would be much too dangerous for devices that did. Devices that connect together, like AV equipment tend to use low voltage signalling.
RE woodstock: Before the days of equipment grounding (and polarized outlets) amplifiers would tie the chassis to the powerline for RF shielding through a capacitor. Unfortunately old time paper capacitors grew leaky over time, resulting in AC voltage on the chassis and by extension anything metal plugged into the equipment if it was plugged in so the chassis was tied to the hot rather than the neutral. This arrangement became known as the “death cap” because it has killed people.
old devices (before polarized or grounded plugs) might use the chassis as a part of the circuit. if plugged in so the chassis was hot then there could be a shock hazard. the chassis should be isolated from being touched but that depended on cardboard/paper/wood/rubber/plastic spacers/backing/knobs to be functioning; these would deteriorate or be lost and the device be a hazard to people who didn’t know that technology.
Bad protective ground. If the ground connection is broken, the amplifier “ground” tends to float, which can lead to a shock. Microphones and all the metal bits on guitars all connect to the amplifier ground, so if the guitar ground is broken and the microphone ground is grounded properly (or vice versa) anyone holding a guitar will get shocked if they also touch a microphone.
It’s not just an unpleasant annoyance. It can be quite dangerous. Keith Relf, former guitarist for the Yardbirds, was killed by an improperly grounded amplifier.
This reminds me of an incident that happened to me a few years ago.
I was at my mother’s house (the house I grew up in) and she asked me to replace the burned out bulb in the hallway. It’s in a ceiling fixture with one of those globes that are held in place by little thumbscrews that go through holes in the metal fixture and into a groove at the base of the globe.
I got a ladder, made sure the wall switch was in the off position, then climbed the ladder and started to loosen one of the screws. I immediately felt a tingling in that hand and let go. This was not static, it was the constant tingle of live current.
I told her that something was shorted or wired backwards and I was not going to change that bulb until she called an electrician. She was skeptical - “your father changed that bulb many times over the years” - but didn’t want to electrocute her loving son so she did what I asked although I could tell she was thinking “there goes worry-wart Dave again”.
After the electrician came she told me that he said it was indeed wired backwards and fixed it.
I’m thinking that it must have been that way since the day that hallway light was installed (I don’t think the house had electricity when it was built). I guess when I grabbed that screw I was somehow grounded in a way that neither my father nor any of the previous owners had ever been.
Come to think of it, I had changed that bulb myself several times before without getting shocked.
Some old amps were transformerless, and very dangerous.
Maybe you were previously using a wooden ladder? Or just had the ladder positioned in such a way that whatever it was grounding on in the floor that time it wasn’t before?
It had to be something like that, but the hallway has wall to wall carpet. 
Ah. There is a metal heater vent in one spot on the floor on the side of the hallway. Visualizing it, it doesn’t seem like the ladder could be touching it and be in the right place to change the bulb, but I could be mistaken. I’ll have to look the next time I’m there.
Too late to edit my previous post, but red is for emergency power, orange is for isolated…or sometimes just an orange triangle on the face.
And if it’s isolated AND emergency power, it would be red with the orange triangle, like so:
http://images.marketplaceadvisor.channeladvisor.com/hi/58/57750/L5262-IGR-EA-2.jpg
Oh, and all hospital outlets, whether normal, isolated, emergency power, or emergency and isolated, should have a green dot on them. This indicates they are “hospital grade.”
Excellent post, but it was my understanding that the red outlets in hospitals signify that the outlets in question are connected to on-site emergency backup generators.
ETA: What bouv said three days ago. :smack:
As for grounded versus ungrounded systems, another example where ungrounded systems are used is on U.S. Navy vessels. As I recall, having the system ungrounded made it less safe, but more robust. As a consequence, electrical operators manually checked for grounds hourly. Detecting a ground indicated that there was a short somewhere in the system, which then had to be tracked down.
In addition, if the device has an AC power switch, you want to switch on the *hot *side, not the neutral side. Using a polarized plug will guarantee the device’s power switch is connected to hot. Assuming the outlet is correctly wired, of course.
…and a “broken neutral” happens more often than you think.
Let’s say you build an electrical device with a metal chassis. You use a two wire (hot and neutral) power cord, and you connect the neutral wire to the chassis. While touching the metal chassis with one hand, you insert the AC plug into the wall receptacle using your other hand. As you insert the plug into a receptacle, there is a 50% chance the hot prong will make contact before the neutral prong. Though this condition may only last a few milliseconds, it is long enough to shock you if you happen to be grounded. Or… someone steps on the power cord, which pulls the plug halfway out of the receptacle. The hot prong is connected, but the neutral prong is not. If you are grounded, and touch the chassis, you will get zapped.
Besides being a safety ground, some devices require an independent ground in order to even operate:
- Some devices require an EMI shield to be connected to earth ground.
- Some fluorescent lamp fixtures rely on current in the ground wire in order to start. If you don’t connect the metal reflector to earth ground, the light will be unreliable.
It’s not your fault you’re confused. The terminology is faulty.
In my opinion, it’s a damn shame the electricity at your wall receptacle is called “alternating current” (AC). This is for two reasons:
-
If nothing is plugged into the outlet, the current at the outlet is not alternating. The current is zero. The only time there is current at the receptacle is when a device is plugged into it, and it is turned on.
-
Even if a device is plugged into the receptacle, the current does not have to be alternating. It might be direct current (DC), depending on the device.
Because of this, the electricity at your receptacle should have been called “alternating voltage” (AV), in my opinion. This is because, regardless of what’s plugged into the receptacle, the voltage is ***present ***and it is always alternating. This is true even when nothing is plugged into the receptacle.
- When I was a kid (read: impulsive asshole) the call button of an elevator was missing, and I poked my finger in. IIRC, I was thrown a foot backwards and my arm felt like a safe had dropped on it.
Is this the type of situation you’re talking about?
- This was mentioned briefly upthread. Like most people here, I’ve accumulated a trove of transformers (?) for computer speakers, printers, and other doo-dads. They’re lying around in archaeological layers, and each says “must be used with x company”–they don’t even say which device, to add more spice to the mix-'n-match game.
Is such 1-1 matching of electrical device to peripheral valid?
I apologize for the hijack in question 2).