Hardinge Bridgeport EZ Vision, Installation, Maintenance, And Parts Manual

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PROTECTIVE GROUND
- DANGER -
PROTECTIVE GROUND IS REQUIRED. It minimizes the exposure to personal
shocks in the event of circuit shorts or other malfunctions. Failure to ensure
protective ground may create electrical shock hazard, causing serious per-
sonal injury or death.
Protective Ground (Chassis or Safety Ground) establishes a low impedance path from the
equipment enclosure and other mechanical parts of the system to earth ground. Protective Ground
assures that all conductive parts of the enclosure are safe. If any circuit inadvertently touches the
chassis, the voltage will be reduced to zero and the enclosure will be safe to touch.
Since safety requirements vary with locality, consult local codes that will take precedence over the
following guidelines.
The four methods listed below are commonly used to establish earth ground:
CONTINUOUS METAL WATER PIPE
When properly buried, length is installed below the permanent moisture level, impedance is
typically 3 ohms.
COPPER STAKE (¾ IN. DIA, 6-8 FT. LG)
When properly driven into the ground below permanent moisture level, the impedance of this type
is typically below 5 ohms.
FOURTH WIRE GROUND
Although a copper ground stake is preferred, an alternative is a fourth wire for ground included
with the power wires from the enclosure ground stud to the power company service panel ground
bus. (See Note below.)
OTHER METAL ELECTRODES (WELL CASTINGS OR THE LIKE)
Must be well chosen, since they generally exceed an impedance of 5 ohms but are well below 25
ohms.
- NOTE -
The resistance of the earth ground connection is measured from the enclosure
ground stud, through the ground wire, to the earth ground connection; then through
the earth ground to another earth ground connection (at least 20 feet away), and
through its associated wire to another ground stud. The resistance should not ex-
ceed 5 ohms. The wire used between the ground stud and the earth ground should
be AWG10 or larger and should be braided cable to minimize resistance at high
frequencies (cable resistance less than 0.075 ohm). The ground wire should be in-
spected for mechanical abuse periodically.
2-10 M-477A