Grounding
The term
"ground" refers to a conductive body, usually the earth.
"Grounding" a tool or electrical system means intentionally creating a
low-resistance path to the earth. When properly done, current from
a short or from lightning follows this path, thus preventing the buildup
of voltages that would otherwise result in electrical shock, injury and
even death.
There are
two kinds of grounds; both are required by the OSHA construction standard:
- System
or Service Ground: In this type of ground, a wire called
"the neutral conductor" is grounded at the transformer, and again
at the service entrance to the building. This is primarily designed
to protect machines, tools, and insulation against damage.
- Equipment
Ground: This is intended to offer enhanced protection to
the workers themselves. If a malfunction causes the metal frame
of a tool to become energized, the equipment ground provides another
path for the current to flow through the tool to the ground.
There is
one disadvantage to grounding: a break in the grounding system may occur
without the user's knowledge. Using a ground-fault circuit interrupter
(GFCI) is one way of overcoming grounding deficiencies.
Summary
of Grounding Requirements
- Ground
all electrical systems [for exceptions, see 29 CFR 1926.404(f)(1)(v)].
- The path
to ground from circuits, equipment, and enclosures must be permanent
and continuous.
- Ground
all supports and enclosures for conductors [for exceptions, see
29 CFR 1926.404(f)(7)(i)].
- Ground
all metal enclosures for service equipment.
- Ground
all exposed, non-current-carrying metal parts of fixed equipment [for exceptions, see
29 CFR 1926.404(f)(7)(iii)].
- Ground
exposed, non-current-carrying metal parts of tools and equipment
connected by cord and plug [for exceptions, see
29 CFR 1926.404(f)(7)(iv)].
- Ground the metal parts of the following non-electrical equipment:
- Frames and tracks of electrically operated cranes.
- Frames of non-electrically driven elevator cars to which electric conductors
are attached.
- Hand-operated metal shifting ropes or cables of electric elevators.
- Metal partitions, grill work, and similar metal enclosures around equipment
of over 1kV between conductors.
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Methods
of Grounding Equipment
- Ground
all fixed equipment with an equipment grounding conductor that
is in the same raceway, cable, or cord, or that runs with or encloses
the circuit conductors (except for DC circuits only).
- Conductors
used for grounding fixed or moveable equipment, including bonding conductors
for assuring electrical continuity, must be able to safely carry
any fault current that may be imposed on them.
- Electrodes
must be free from nonconductive coatings, such as paint or enamel,
and if practicable, must be embedded below permanent moisture level.
- Single
electrodes which have a resistance to ground greater than 25 ohms must
be augmented by one additional electrode installed no closer
than 6 feet to the first electrode.
- For grounding
of high voltage systems and circuits (1000 volts and
over), refer to 29 CFR 1926.404(f)(11).
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