In article <372b9$4e67eabc$5419acc3$ me.nl>,
Skybuck Flying <> wrote:
>It does raise the interesting question, why PC's need ground and everything
>else does not.
Your assumption ("everything else does not") is false.
Many other sorts of device do (for legal and safety reasons) require a
ground connection. Here in the U.S., most major appliances do require
three-wire grounded power cords.
Here's the short version, as it applies to power-mains circuitry in
the United States. Details may differ for mains connection rules in
other countries.
An appliance generally requires a safety-ground connection, if it has
a metal chassis or other electrically-conductive exterior, which can
be touched by a human. This chassis is required to be connected to a
safety ground - one which returns to the ground connection at the
power distribution panel, and which does not carry any portion of the
normal load-return current.
This safety grounding of the chassis is intended to protect humans
against shock, if an electrical fault develops inside the device (e.g.
a loose wire) that could create an electrical connection between the
power wiring and the chassis.
Without a safety ground to the chassis, this sort of fault will result
in a "hot" chassis. If a person were to touch the chassis, at the
same time that they were touching a grounded object (or standing in
water, or etc.), their body would complete the circuit and AC power
would flow through their body. The amount of current needed to kill a
person is *far* too small to blow a fuse, and thus the person could die.
Grounding the chassis prevents this. If a "weak" connection between
the power wiring and the chassis occurs (e.g. low-level leakage), the
safety ground will hold the chassis very near to the ground voltage
level, reducing the risk of electrical shock by quite a lot. If a
real short-circuit to the chassis occurs, the grounding of the chassis
will either cause the fuse to blow, or will result in a loud
SPLUT-BANG! which will vaporize the shorting wire, or both.
Appliance do not require a safety ground if they're designed in a way
which prevents an internal fault from creating a hot chassis. One way
is often referred to as a "double-insulated" design... the wires
themselves are insulated, and the device's outer shell consists only
of nonconductive (insulated) materials.
In the U.S., you can also do away with the need for a safety ground
(in some cases) by using a "ground-fault interruptor" - a device which
detects the fact that "current is flowing where it should not" and
disconnects the power before injury can occur.
Desktop and server PCs usually require safety grounds because they are
not double-insulated... they have exposed metal on their backplanes
(and often the case is metal) and their internal power supplies have
metal cases which are connected directly to the outer case.
Laptop PCs can sometimes operate without a safety ground, because they
have nonconductive (plastic) cases, and because the power entering the
PC is at a low enough voltage that it does not create a significant
shock hazard.
--
Dave Platt <> AE6EO
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