Monday, March 12, 2012

What's the Ground Prong On Outlets For?

I almost didn't do this one, since getting all the way to a Ph.D in electrical engineering without fully understanding the concept of electrical grounding is embarrassing at best and probably evidence of some kind of larger issue with our higher education system at worst.  Judging by the random sampling of people I asked the titular question to though, I'm not the only one in this boat-- I got a lot of mostly tautological responses like "so you'll have a common reference" or "so there's always a path to ground," which are roughly the non-answers I'd probably give if you asked me out of the blue.  It's one of those things that makes less sense the more you think about it; yes, you do need a "ground" potential in order to have a circuit, but there's already a ground on two-prong power plugs (the voltage on the "line" prong cycles between +120V and -120V relative to the "neutral" prong), so why bother with a redundant third prong?  Most simpler electrical devices (lamps and such) don't even bother with a three-prong plug, and most of the time using a "cheater plug" to plug a three-prong plug into a two-prong outlet won't cause major issues. So what's the point? 

Before we start, we should probably define some terms for anyone who hasn't been blessed with a circuit theory course.  Voltage is a measure of the difference in electric potential between two points (the fact that there's no such thing as an absolute voltage is very confusing if you're not used to thinking like a physicist).  Since it's a completely relative measurement and math is always easier when there are some zeros involved, it usually makes sense to simplify things by calling one of the points in a circuit "zero volts," or more commonly, the "ground."  Then you can define all the other voltages in the circuit relative to that ground.  Again, it's a totally arbitrary thing, since all that matters is the difference between the chosen ground and the other parts of the circuit. 

A three-prong electrical socket has three wires in it: the "line" wire, the "neutral" wire (the two flat prongs), and the "ground" wire (the little round one), the last being the source of confusion here.  In normal circuit-theory talk, the neutral wire would be called the ground; it's the zero point that the voltage on the line wire gets defined in reference to.  To avoid/enhance confusion though, we don't call it the ground, because that would both make sense and mean we had to find a new name for the ground plug.  So the ground-like word "neutral" gets used instead.  I have a feeling misuse of naming conventions is one of the main things that makes this so unnecessarily confusing. 

I started with the few things I definitely knew about the ground prong:

  1. Its existence gives you a much more stable line voltage, which is important for things like audio, sensitive lab equipment, and anything where precise analog signals are used.
  2. Two-prong outlets are generally considered to be a shock and fire hazard; I'm not even sure if it's legal to build a house using two-prong outlets in the US anymore
So obviously the thing is pretty important, but nothing I'd learned in circuit theory could tell me why either of the above two facts were true.

It's easy to forget how idealized circuit theory is.  In a circuit diagram, wires have zero resistance and can conduct an infinite amount of current without seeing any kind of voltage drop along their length.  So in a circuit diagram, the part you've defined as "ground" stays at zero volts along its entire length no matter what happens.   Basically we're operating under the assumption that any resistance in the lines is negligible compared to resistance in the circuit elements themselves, which in most normal situations is a perfectly reasonable thing to assume.

In real life, of course, things get messy.  A two-prong electrical outlet isn't an isolated, magical voltage source; both prongs are connected to a series of power lines, transformers, and other things leading all the way back to the power plant where said voltage was generated.  All of this stuff has a low but finite resistance to it.  Run current through anything with resistance and you'll get a voltage differential between one side of the element and the other (V=IR, remember?)-- the higher the current, the more severe the voltage drop.  All that stuff between the electrical system's "ground" point (there's usually one at the transformer on your street, Americans) and your outlet means that when you plug something into it (running current through it, effectively), cumulative line resistance means that the voltage you see at the outlet will be less than the voltage at the originating point.
Circuit diagram of what goes on behind a two-prong outlet.  While the voltage at the (grounded) transformer out on the street is 120V, resistance in the lines between the transformer and the outlet (R1 and R2 here) will cause the actual voltage seen at the outlet to be lower, proportional to the amount of current flowing through the line.
This is problematic for two reasons.  The most obvious one is safety.  While under normal operation that line resistance is negligible, say the circuit gets hit by lightning and sees a brief 100,000-volt spike.  Way more voltage means way more current traveling down the lines which, as you'll recall, are slightly resistive.  Running current through a resistor generates heat.  Heat can make things catch on fire.  Do the math here.

The other, more subtle issue is voltage stability.  Not only are the lines resistive, but they can't even be relied on to be predictably resistive; things like temperature variations, stray electromagnetic interference, and load variations in the power grid will all affect line resistance slightly.  Again, this is ordinarily negligible, but if you're doing something with precise analog signals you really want a line voltage with as little fluctuation as possible.

The ground prong solves both of these problems by providing a low-resistance path directly to a ground right from the circuit, instead of locating the ground point all the way at the other end of the neutral line.  "Ground" (more accurately called "earth" in Europe) is usually exactly what it sounds like-- since the earth is a) quite large and able to sink a basically infinite amount of current, and b) everywhere, the best ground points are actually just big metal rods hammered into the ground.  For reasons that are probably obvious though, this can be impractical, so in a pinch anything metal that's big enough to run a lot of current into without much of a voltage drop will work; grounding to things like metal pipe systems and iron girders is pretty common when an earth ground isn't available, although that can create unique problems of its own (in one of my old labs, we actually drove a copper rod four stories down through the building and into the ground from our lab so our ultra-sensitive SQUIDs would have the most stable possible reference). 

Wiring diagram of a 3-prong (grounded) outlet.  By providing a common zero-voltage point at both ends of the neutral line, the third prong effectively removes the effects of neutral-line resistance from the circuit.  Since the outlet and transformer grounds are presumably both plugged into the earth, I drew them as connected here.

So now you've got a very direct path to ground on your circuit.  That means that if you tie that ground to the neutral wire, the neutral wire will be at zero volts both at your end and at its originating point.  For best results, the path to ground should be short and the conductor used to get there should be as large as possible to minimize line resistance.  In most cases, the ground prong is just sitting there holding your end of the neutral conductor at zero volts, so there's no actual current flowing through it; everything still goes back through the neutral wire, but now we don't have to worry (as much) about a voltage drop when it does so.  In extreme cases like our lightning strike, the fact that our ground wire is a much more direct path to ground than the neutral wire mean that all that excess voltage will "see" a much lower resistance along the ground conductor.  Electricity being as lazy as everything else in the universe, the excess current will travel to ground via the ground line instead of the neutral line, and since you designed your ground wiring with as low a line resistance as possible, it means something is way less likely to catch on fire.

There are other safety things to consider too.  If you build something with a metal chassis (like a refrigerator or computer), it's a good idea to ground the chassis so if a mis-wired "line" wire inside the device somehow came into contact with the chassis, it would just create a short circuit and blow your fuse/breaker instead of energizing the chassis to line-level and zapping the next person who touched it.  The neutral line will work perfectly well for this, but it's also really easy to accidentally cross-wire a two-prong outlet when you install it, and anything using the neutral line as a chassis ground that's plugged into a cross-wired will instead have its chassis permanently energized to line-level, turning it into a shock machine.  The three-prong design (with the third prong looking conspicuously different than the first two, both inside and outside the outlet) is much harder to screw up this way, so to be on the safe side it's generally a good idea to ground a metal chassis using the ground, rather than the neutral, wire.

As any of the electrical engineers shaking their fists at the screen right now will be happy to tell you, there are entire books' worth of grounding techniques and arcana that I've skipped over here, but strictly in terms of answering the question of "what's the third prong for?," those are most of the essentials. While the whole thing was a little more complicated than expected, this particular journey of discovery is a good demonstration of this blog's operating principle: that it's possible to get a Ph.D and still not know a large number of very basic things, including things in the field you got the Ph.D in.  It seems like the world's greatest scam until you remember that you still have to spend 4-7 years in grad school, something I wouldn't wish on my worst enemies.

EDIT: Both images had the line/neutral plugs reversed.  This has since been corrected.  


  1. Right after I graduated I was at a friend's graduation party and somewhere between 5 and 10 of his family members came over to us to ask various questions about the electricity in their houses. I repeatedly had to explain that I was an electrical engineer not an electrician and that I didn't actually learn any practical information in college.

    1. I don't think you should be an electrical engineer then either....btw the second image is wrong the line and neutral are reversed...this is coming from a seasoned pro out of college....I would not want to buy anything you ever design....

  2. I always tell people that if they need a resistor network solved I'm totally their guy. Useful things like setting up their electrical system so it doesn't burn the house down, not so much. Education!

  3. That is really valuable information mate. But I have a doubt. Why doesn't current from live wire directly enter the the ground which is connected at neutral wire? How or why does it flows from line-neutral?

    I'm sure after reading my stupid question you would want to kill me but I'm an illiterate when it comes to electrical concepts :(

  4. It's actually a really good question, and I had to think about it awhile to come up with an answer.

    Here's the best I could come up with, sourced from nothing except my own ass because it's Monday morning and I'm running around: under normal operating conditions, the easiest path back to the voltage source in the circuit is going to be the neutral line. It's just a wire connecting the two, where the ground connection goes into the earth, through the earth for who knows how long (the earth is not incredibly conductive), and then out of the earth at the originating point. So when line resistance is negligible, the neutral line is going to look pretty good to electricity trying to complete the circuit. It's only when line loss goes way up (over-current condition from a lightning strike or whatever) that the current "sees" a lower impedance on the ground line and goes that way.

    So the "ground" path back to the beginning of the circuit has a higher resistance than the neutral line usually, but that resistance is also much less sensitive to current levels. Make sense? Like I said, I didn't fact-check that but it seemed like it mostly made sense after one cup of coffee.

  5. This kind of bugs me. Why is your live/neutral reversed? Shouldn't the wide blade be neutral(black) to correctly accept polarized plugs? Please correct me if I'm wrong.

  6. Sorry, poorly worded --(black) in your picture, white in real life.

  7. No, you're right, it was a lazy drawing done in about two minutes of free time. Which prongs correspond to line and neutral on a polarized plug don't really change the point it's making though, which is basically that there's a bunch of crap behind the outlet that you have to take into account.

  8. You'd better revise your diagram before somebody get hurt. Not too many people read comments about the wrong drawing.

  9. If you're doing electrical work using only a random blog post about grounding as a reference, using the wrong polarity on your outlets is probably the least of your problems.

  10. Your three-hole outlet diagram has an error in it. On an American three-hole outlet, the nine millimeter slot on the left is neutral, and the seven millimeter slot on the right is hot.

  11. Yeah, I messed that up, as other posters have pointed out. It doesn't really change the point of the diagram and I'm a lazy, lazy person so I haven't fixed it. Hopefully nobody's died as a result.

  12. thanks for the post, really useful information, but, truly, wouldn't it be less trouble to correct your diagram than to keep defending it?

  13. Fine, you polarity nazis win. I'll fix it as soon as I figure out what I did with the original file.

  14. Excellent post.

    If you will forgive a long post, I wanted to comment on the April 8 question of why the current returns through the neutral wire instead of flowing directly to ground. ICanHasPhD explains it in his response, but I’d like to add a bit to it, from a slightly different perspective.

  15. First of all, the key point is worth stressing: The current doesn’t flow to local ground in preference to the remote ground at the power station because the current doesn’t flow to EITHER ground. Under normal circumstances, the current flows in a loop, from one side of the generator to the other. One side of the generator (the neutral) is connected to the earth, but that connection is not normally part of the circuit. Or rather, although the electrical charge moves into the ground rod as well as the neutral wire, once it hits the ground, it loses interest. A copper wire conducts electricity much more easily than the earth does, and so the current runs in the copper.

    Think of it this way. The power station generator pumps charge from its neutral terminal to its line terminal, creating an excess of charge on the line terminal and a deficit on the neutral. (Or vice versa.) The charge flows in a loop from the line terminal through the transmission lines, transformers, house wiring and appliances and back to the neutral terminal (or vice versa). Now, notice that the net charge in the circuit is zero. The excess charge on the line terminal is just balanced by the deficit on the neutral terminal (or vice versa). The circuit is flowing because the excess charge is returning to fill the vacancy. The ground connection at the neutral terminal, like the one at the transformer and the one(s) in the house, is ignored.

    On the other hand, if a lightning strike puts a huge charge on the transmission lines, that’s not part of the loop. That’s not charge that the generator has removed from the neutral terminal and forced onto the line terminal. It’s charge from the clouds (or ground) that has been forced onto the line, and the net charge is no longer zero. The excess charge from the generator will flow naturally to the neutral terminal but the rest will not. Once the vacancy has been filled, there’s no more motive.

    So now that connection to ground starts to look pretty good. The earth is a poor conductor, compared to copper, but it’s a lot better than air. The lightning charge flows through the grounding rod into the earth, and it’s easier for the charge in the lines at your house to flow to ground through the local connection than it is to go all the way back to the station and then to ground. Either way it has to fight the earth, but with local ground, all the line resistance and other impedances in the grid apparatus are bypassed. And notice that the lightning charge is not trying to get back to the power station. It’s not part of that circuit. Its circuit connects the earth and sky.

    By the way, we talk about current as though the electric charge were flowing like water through the lines and traveling from the power station to your house and back, and this is the common picture in circuit theory. But what flows is really the electrical force rather than the electrical charge.

    It is true that in a DC circuit, like in your car or in a flashlight, electric charge, electrons actually, do move around the loop, but they do so very slowly, whereas the force that pushes them moves at light-speed. And in an AC circuit, like in your house, the electrons make very little progress. They respond violently to the force from the excess electrons pushing on them from the power station, but they’re always crashing into each other, and into copper nuclei in the wires, and so they don’t get very far, perhaps a few microns, before the voltage reverses and they start bumping back in the other direction.

    So the physical picture is of zillions of electrons boiling in the line, pulsing back and forth 60 times a second, with very little net progress in either direction. But, again, when there is a change in the excess or deficit charge on the generator terminal, and it is constantly changing, the corresponding change in force travels all the way from the power station to your house and back in an eyeblink, so the jiggling electrons all pulse back and forth together all along the line.

  16. This may have been mentioned but THE HOT AND NEUTRAL ARE EFFING BACKWARDS!!!!!

  17. "Yeah, I messed that up, as other posters have pointed out. It doesn't really change the point of the diagram and I'm a lazy, lazy person so I haven't fixed it. Hopefully nobody's died as a result"

    Well then fix the drawing !!! are able to do that.....right ??

  18. I almost got hurt. I wired wrong based on a quick google image search. Lucky I thought to check another reference. Please get rid of this picture!

  19. still wrong, mate. when one googles "ac outlet line neutral" the first two images that show up are a correct one and YOURS. If for no other reason you should fix it, 1) so the world doesn't think you're an idiot, and 2) before some lawyer realizes he can get someone to sue you for getting shocked.

  20. Wow, I had no idea it was showing up on Google Image Search. Finally edited before someone gets killed.

  21. hey I have a push button switch with 3 prongs will it work if I don't use the third prong?

  22. I agree with all the people bringing to your attention your posting of wrong information, not only it;s dangerous but you could be held liable if somebody gets hurt. Do the right thing and correct your mistake.