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WHY THREE PRONGS?
©1996 William Beaty, BSEE

QUESTION:
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?"
Why are three prongs needed?

Also see below: WHY "GROUND" IS CONFUSING

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ANSWER:
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 customers' 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! What the heck?! It takes a huge DC voltage to make a foot-long spark. Why are high DC voltages appearing on your AC power lines?

The sparks occur because of a little-known fact: all the world is a gigantic electrostatic generator. There is a flow of charge going on vertically everwhere on earth. Thunderstorms pump negative charge downwards, and the charge filters upwards everywhere else on earth. Depending on the height of your 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 huge 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 feeble current coming from the sky and builds up a huge potential difference with respect to the earth.

In addition to the above, you would find an unsettling phenomenon whenever lightning directly strikes your electrical distribution system. The lightning impulse-voltage spreads instantly 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 "touch" 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 dirt, and connect them to your wires. That way, lightning currents will be directed into the Earth rather than spreading throughout your power lines. Also, the clear-weather sky current can no longer build up a high voltage, if any excess charge immediately leaks into the earth.

Of course you cannot connect BOTH wires to ground, since that would also connect your 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 of power lines, so if one part is disconnected for any reason, it won't present a lightning hazard or static discharge hazard. Now the clear weather sky-voltage will be discharged to Earth, 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 that you cure, another one is created. Before you "grounded" your system, the AC voltage in general acted pretty safe for your 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 as if it was "safe," and it 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 unwanted current was directed through the length of their 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 whenever 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 one entire half of your network indirectly to everyone's feet. Most of the time the floor is a pretty terrible conductor, so most of the time the hazard is small. However, when someone stands barefoot upon a damp floor, this electrically connects that person into the system. If they touch the grounded ("neutral") wire in the AC system, nothing happens. But if they touch the other, non-grounded wire, this applies the full AC voltage between their feet and finger. The new unwanted current path within their body then includes the heart muscle, and the resulting AC 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. Unfortunately, removing the ground connection causes even more danger, so you can't go back to a "floating" system where Hot and Neutral don't exist.

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 it makes the other half of the system act safe again should they touch it by accident. 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. To say the least. Some other solution is needed.

The solution: guarantee that no one touches 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.) Force manufacturers to treat the wires differently inside appliances, designing with careful wire positioning and adding extra insulation to the "hot" wire.

Another problem springs up. At present, 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 entire world, and you 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, as well as changing the plugs of all new appliances, in order to force customers to always stick the plug in the "right" way. Manufacturers must use the new types of cords, but at least they don't have to redesign all of their products. 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. Now 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 wide slot, the hot side to be the shorter slot. 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."


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Things now seem much improved, but there are still problems. If an appliance is dropped into water, and if that water is touching a grounded container (such as a bathtub, a kitchen sink, or even a basement floor or a standing pool outdoors) 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 then 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, this is not entirely safe. 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, since it would cause a lethal hazard if the appliance was plugged into a miswired wall outlet. Miswired outlets look exactly the same as the normal ones.

The solution? Why, add a Third Prong! Connect this prong to the neutral side of the network, but do it only in one place in the circuit, and run a new third wire out to all of the wall-outlets. Give this wire a new color, one which is different from the other two. Give this this third prong a very different shape as well, so even Highly Imperfect Electricians will rarely connect the special prong to the wrong wire. And inside all the metal-cased appliances, insist that manufacturers connect this third wire to the case.

The idea works! Like magic the faulty metal-cased appliances start blowing their fuses to indicate trouble. And power tools dropped into water will create a current path to the metal case rather than to nearby humans standing in the puddle. We've entered the "Age of Electrical Outlets Having a Little Face and Different Sized Eyes."


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But still, every once in a while a Customer will get a really nasty shock from a cheap 2-prong plastic appliance that's wet. Or perhaps an electric hair dryer will fall into the bathtub, but the short-circuit current in the water 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 customers' bodies offer a path for current between the appliance and the lightning-safety earth grounds. During the shocking event, some of the AC 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. Instead, it's going through the human, and also going into the grounded pipes of the plumbing.

If we could measure the current that's taking the "wrong" path, maybe we could detect the problem and turn off the power before anyone dies. We can't measure the current in the plumbing, but we can measure it in the "hot" wire, measure the current in the "neutral" wire, then subtract them. This tells us the value of electric current escaping via 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 any 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

Other articles here:


 



MORE ABOUT "GROUNDING" AND "EARTH"

In electrical circuits, the word "ground" can be very confusing. The word has several different meanings. Your teacher might know which meaning he or she is using in any spoken sentence. But this spreads confusion, because students aren't even aware of the multiple meanings. Or, even when students know that "ground" is a multi-facted term, they may lack skill in connecting up each definition. It's always a bad idea to let science terms have more than one meaning. Perhaps it's best to simply forbid use of the word "ground" in the classroom. That forces everyone to use accurate and less-confusing words.

The word "ground" means:

  1. A 'common' connection, but not connected to Earth.
  2. A direct connection to the power supply (usually to the DC negative terminal)
  3. A connection to the inside of a shielded metal box
  4. A connection to a metal stake driven into the earth (or a connection to a metal water pipe which extends out of the house into dirt.)
Only number four is actually connected to ground!

STUPID BILL B. VIDEOS! educational too. But no less stupid!

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