For AC
and DC electrical systems, the flow of electrons is always from a
negatively charged source to a positively charged source. However, for the
controlled flow of electrons to occur, a complete circuit is required.
Generally speaking, electrical current follows the path of least
resistance. Because metals such as copper provide little resistance, they
are frequently used as conductors for electrical current. Conversely,
materials such as glass, rubber, and plastic provide more resistance.
Therefore, they do not make good electrical conductors. Instead, these
materials are frequently used as insulators. They are used on conductors to
prevent shock, fires, and short circuits.
Electrical power is
usually delivered to a pole-mounted transformer. The transformer reduces
the high voltages, used in the transmission, to the 120 or 240 volts used
by typical consumer electrical appliances.
Figure shows
a familiar object, electricity as supplied through wall outlets in the US
(other nations have different wall outlet configurations).. The top two
connectors supply power. The round connector on the bottom protects people
and equipment from shocks and short circuits. This connector is called the
safety ground connection. In electrical equipment where this is used, the
safety ground wire is connected to any exposed metal part of the equipment.
The motherboards and computing circuits in computing equipment are electrically
connected to the chassis. This also connects them to the safety grounding
wire, which is used to dissipate static electricity.
The purpose of
connecting the safety ground to exposed metal parts of the computing
equipment is to prevent such metal parts from becoming energized with a
hazardous voltage resulting from a wiring fault inside the device.
An accidental
connection between the hot wire and the chassis is an example of a
wiring fault that could occur in a network device. If such a fault were to
occur, the safety ground wire connected to the device would serve as a low
resistance path to the earth ground. The safety ground connection provides
a lower resistance path than your body.
When
properly installed, the low resistance path, provided by the safety ground
wire, offers sufficiently low resistance and current carrying capacity to
prevent the build up of hazardously high voltages. The circuit links directly to
the hot connection to the earth.
Whenever an electrical
current is passed through this path into the ground, it causes protective
devices such as circuit breakers and Ground Fault Circuit
Interrupters (GFCIs) to activate. By interrupting the circuit,
circuit breakers and GFCIs stop the flow of electrons, and reduce the
hazard of electrical shock. The circuit breakers
protect you and your house wiring. Further protection - often in the form
of surge suppressors and Uninterrupted Power Supplies (UPS) - are
required to protect computing and networking equipment. ![](./86_files/image004.gif)
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