Ask A Scientist© Environmental Earth Science Archive

name         Gene U.
status       other
age          40s

Question -   Why are there polarize and unpolarized plugs for
electrical devices, like drills, toasters, blenders, radios, etc for 120 volts?
What could happen if there were to be a problem, like a internal short circuit?
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Polarized plugs are used to connect the most exposed part of an appliance
to the ground wire so that if you are touching a ground (like a pipe,
bathtub, faucet, etc.) and the exposed part of an appliance (the case, the
threaded part of a light bulb socket, etc) you will not get an electrical
shock.  Many appliances, such as electrical drills, are now "doubly
insulated" so the probability of any exposed part of the appliance being
connected, by a short or other problem in the appliance, to either wire is
very small.  Such devices often use unpolarized plugs where the two prongs
of the plug are identical.  On a polarized plug, the ground prong is
larger.

Polarized plugs are a safety feature, but certainly not an infallible one.
"Ground fault" circuit breakers, which break the circuit for both wires
when there is an indication of a possibly dangerous problem provide much
greater safety.  They monitor the difference in the current in the "hot"
wire and the "ground" wire.  If the currents differ by as little as 0.005
amperes, they break the circuit in as little as 0.025 seconds.  This is
adequate protection to keep a person from getting a dangerous shock.  The
difference in currents could be caused by a person touching the hot wire
and a faucet, thereby sending some of the current directly to "ground"
instead of back through the "ground" wire.

Best, Dick Plano...
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