This version is obsolete. Try this one: http://amasci.com/amateur/whygnd.html (c)1996 William Beaty http://amasci.com/amateur/whygnd.txt Q: WITH AC POWER, AREN'T BOTH THE WIRES OF THE PAIR INTERCHANGABLE? WHY IS ONE WIRE CALLED "NEUTRAL?" WHAT'S ALL THIS STUFF ABOUT "GROUNDING?" __________________________ / \ / _ \ | | | _ | | | | | | | | | | | | | | | | | | | | | | | | | | | | |_| | | |_| | | ____ | | / \ | | | | | | |______| | \ / \__________________________/ A: Yes, in an Alternating Current system there is no "plus" and "minus," so in theory the two wires should be interchangable. However, the real world is a bit more complex than the theory. Other issues crop up and cause trouble. If you were to build your own electric power distribution system, you'd quickly discover some unexpected and strange effects. Sometimes your appliances would fail for no apparent reason. And sometimes when you reached for a light switch, a foot-long spark would leap out to your hand and knock you senseless! This happens because all the world is a gigantic electrostatic generator, and there is a flow of charge going on vertically everwhere on earth. Thunderstorms pump negative charge downwards, and everywhere else on earth the charge filters upwards. Depending on the height of the circuitry above the earth's surface, depending on the area covered by your wires, and depending on whether there was a thunderstorm above you at the time, there might be a fairly large DC charge on your electrical distribution system. This charge might be several hundred volts, enough to zap computers and delicate electronics. Or it might be many tens of thousands of volts, enough to create enormous sparks which jump across switches and leap out of wall outlets, wall switches, across transformer windings, etc. Your electric power system is acting like a sort of capacitive "antenna" which intercepts the sky current and builds up a large potential difference with respect to the earth. In addition to the above, you would find an unsettling phenomenon whenever lightning strikes your electrical distribution system. The lightning impulse voltage instantly spreads throughout your whole network, which not only can explode every single appliance plugged in at the time, but can create lethal arcs many feet in length that reach out to your customers should they be anywhere near those wires within the walls. There is a simple solution to these problems: connect your system to the earth. Drive some long metal rods into the earth, and connect them to your wires. Of course you cannot connect BOTH wires to ground, since that would also connect the wires to each other and short out the system. So, you must close your eyes and pick one wire. Connect that wire to ground. Do this at many points throughout your system, so if one part is disconnected for any reason, it won't present a lightning or static discharge hazard. Now the clear weather sky-voltage will be discharged to earth before it can build up. And during a lightning strike, the huge current will be diverted into ground at many points, and hopefully will find very few customers' bodies on its way into the earth. The story isn't over. Since you've made a change to your system, Murphy's Law crops up and informs you that for every problem you cure, another one is created. Before you grounded your system, the AC voltage in general acted pretty safe for customers. The only way they could get a shock was if they touched both wires at the same time. This was a fairly rare occurrence. One single wire acted "safe," and did not deliver shocks. If a miswiring inside an appliance caused one wire to accidentally touch the metal case of the device, your customers could still touch that metal case without danger. Curious kids might still stick their finger in a light socket and receive a shock, but the current was directed through the length of the finger and caused no danger of heart-stoppage. Now that you've grounded your system, you'll find that suddenly your customers are occasionally dying! One wire of your system is now almost totally safe because it is connected to ground. But the other wire has developed a new hazard, because when the occasional customer comes into contact with it, that foolish customer is usually STANDING ON THE GROUND! By grounding half of your electric network, you've accidentally connected that half of your network indirectly to everyone's feet. When someone stands barefoot on a damp floor, this connects that person into the system. If they touch the other non-grounded wire, this applies the AC voltage between feet and hand. The current path within their body then includes the heart muscle, and the resulting current induces rapid, tail-chasing heartbeat waves called 'fibrillation' in their main blood pump, and so your new, half-grounded AC system has developed a lethal characteristic. One solution would be to insist that all customers wear dry, insulating footwear, never walk on wet floors, never sit in bathtubs, etc . This guarantees that they are not connected to one half of your system, and makes the other half act safe again should they touch it. Professional electricians might enjoy the challenge of learning all these rules, but your new requirements would cause some negative repercussions in selling your service to non-experts. Some other solution is needed. The solution: guarantee that no one touch the non-grounded wire. Get into the schools and pound into everone's mind that AC wires are dangerous. Teach all electricians and technical people that one of the wires is now to be called "Hot", and that this wire can be lethal if touched. Choose differing colors for the two wires (black is "hot" in the US, brown is "hot" most everywhere else.) Make manufacturers treat the wires differently inside appliances, designing with careful wire positioning and adding extra insulation to the "hot" wire. Another problem springs up. Some appliance manufacturers INTENTIONALLY connect the outside of their metal products to one of the power wires. This must be stopped. But economic concerns prevent making massive, instant changes. You can't force a recall of half the appliances in the world, and can't force manufacturers to instantly redesign all their products. The economic upheaval from this would wreck far more lives than the dangerous circuitry does. So instead you decide to change the power outlets of all new homes and the plugs of all new appliances to force customers to always stick the plug in the "right" way. For appliances with one wire connected to the metal case, this connects the case to the grounded or "neutral" side instead of to the "hot" side. Make one slot of the two-slot electric outlet longer than the other. Do the same with the appliance plugs. Choose the neutral side to be the long slot, the hot side to be short. Apply legal pressure to get manufacturers to stop connecting their metal cases to the power wires. Make electricians preserve the polarity of the wiring when they install outlets in new homes. Thus we enter the "Age of the Electrical Outlet with One Long Slot." Things now seem much improved, but there are still problems. If an appliance is dropped into water, and if that water is in a grounded container (such as a basement floor, a standing pool outdoors, or a bathtub,) then any human sticking more than one appendage into the water will be in serious trouble. Humans are salt water, and they present a low-resistance path for current, which preferentially directs it through their bodies rather than through the water. Another thing: sometimes an appliance with a metal case will suffer internal wear or damage, and the "hot" wire will wiggle around inside and end up touching the metal case. Anyone standing on wet ground will feel pain and death if they should grab that metal case. Some unnamed genius realizes that if we could somehow permanently connect all the metal cases of appliances to the "neutral" wire, then if the "hot" wire should ever accidentally touch the case, a short circuit would blow the fuses in the building and quickly remove the electrical connections, and the hazard. However, occasionally an electrician will accidentally wire an outlet backwards. This can't be helped, because Perfect Electricians are far more expensive than the normal human variety. And so we cannot intentionally wire appliance cases to the Wide Prong of the plug, this would cause a lethal hazard if the appliance was plugged into a miswired wall outlet. The solution? Why, a Third Prong! Connect this prong to the neutral side of the network, but do it only in one place, and run a third wire to all the wall outlets. Give this wire a third color, different from the other two. Make this third prong shaped very differently as well, so even Highly Imperfect Electricians will rarely connect this special prong to the wrong wire. Inside metal-cased appliances, insist that manufacturers connect this third wire to the case. And like magic the faulty appliances start blowing the fuses, and power tools dropped into water will create a current path to the metal case rather than to nearby humans. We've entered the "Age of Electrical Outlets Having a Little Face and Different Sized Eyes." And still, every once in a while a Customer will get a really nasty shock from a cheap plastic appliance that's wet. Or perhaps an electric hair dryer will fall into the bathtub but the short circuit current won't be enough to blow the fuses in the house, and the bathtub water will become lethal. In theory there is a way to prevent this. These Customer shocks are happening because the customer's body offers a path for current between the appliance and the lightning safety earth grounds. During the shocking event, some of the flowing charge is going in (and out) of the "hot" wire, but it is NOT going back into (and out of) the "neutral" wire as it's supposed to. It's going through the human instead, and then into the grounded pipes of the plumbing. If we could measure the current in the "hot" wire, measure the current in the "neutral" wire, then subtract them, we would know what level of current was flowing in the "illegal" path through the human to ground. The subtraction should normally give a zero result, since there never should be a current path to ground that isn't using the neutral wire. If we amplify the subtraction's result and use it to trip a circuit breaker, we'll have a new type of appliance which turns itself off immediately when a human gets into the electrical path. These devices are now required in wet areas of homes (bathrooms.) They're called Ground Fault Interrupters. And so we've finally entered modern times, the "Age of the Electric Outlet with the Little Red Button Which Pops Out!" - Bill Beaty