WHY THREE PRONGS?
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,
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.
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
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."
__________________________ / \ / \ | _ | | | | _ | | | | | | | | | | | | | | | | | | | | | | | | | | | | |_| | | |_| | | | | | \ / \__________________________/
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."
__________________________ / \ / _ \ | | | _ | | | | | | | | | | | | | | | | | | | | | | | | | | | | |_| | | |_| | | ____ | | / \ | | | | | | |______| | \ / \__________________________/
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
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
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
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
- 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
Perhaps it's best to simply forbid use of the word "ground"
in the classroom. That forces everyone to use accurate and less-confusing
The word "ground" means: