David E. Baker and Rusty Lee
University of Minnesota Extension Service
Electricity,
one of the most versatile and widely used power sources, is
used extensively on almost every farm. Farmers are so familiar
with electricity that they may take electrical safety for
granted. When this happens, accidents often follow. The National
Safety Council reports more than 800 electrical fatalities
annually. On average, more than 40 of these deaths are directly
related to farming operations.
How
much electricity is fatal? People can feel electrical currents
at levels as low as approximately 1 milliamp (mA), which produces
a slight tingling sensation. Increasing current levels above
the 5 mA "let go" threshold can cause loss of muscular control,
irregular heart rhythm and, finally, cardiac arrest. Five
mA is only a small fraction of the current needed to power
a 60-watt bulb, which draws about 1/2 amp, or 500 mA.
Electrical
shock occurs when a person touches an electrically charged
object at the same time they are touching another surface
capable of conducting electricity, such as the ground. A current
then passes between the points of contact. The shock effects
depend on the amperage, duration of contact and resistance
of the pathway through the body. For example, damp skin is
less resistant to current flow and permits greater shock effects.
For this reason, you should work in a dry environment while
handling electrical equipment.
The
seriousness of a shock depends on the path the current takes
through the body. For example, a small current passing through
the heart is much more critical than a current passing between
two fingers of the same hand. Testing for live current with
one hand instead of two reduces the risk of a dangerous shock
by making current less likely to flow through the heart.
Portable
power tools are one of the most frequent causes of death by
electrocution on farms. Farm workers typically expose themselves
to more electrical hazards than do workers in other industries.
They frequently work alone, often operating electrical equipment
in damp and dusty places with no one nearby to help in case
of an accident. Farm work thus requires extra caution in using
power tools. A few precautions:
- Never
use any electrical hand tool that does not carry the Underwriters
Laboratories Seal of Approval. This seal indicates the tool
has undergone extensive testing and has been found to be
safe when properly maintained.
- Older
tools with a two-wire design should be immediately upgraded
to a three-wire system by qualified personnel. If your tool
cannot be upgraded, discard it. Most power tools in use
today have a three-wire system. This third wire serves as
an emergency ground in the event of an internal short or
ground fault.
- Make
sure the three-wire system is maintained through all adapters
and extension cords. Adapters not properly connected to
receptacle boxes make the ground fault wire ineffective.
- Never
disconnect or carry power tools by their cords. This causes
the cord's insulation to rapidly deteriorate. Inspect tool
and extension cords routinely for fraying and other signs
of deterioration. Repair or replace immediately.
- Consider
using a ground fault circuit interrupter (GFI). GFIs do
not replace traditional protection provided by current isolation,
insulation and grounding, but they are a backup if insulation
or grounding fails due to age, abuse or mechanical breakdown.
The GFI detects low levels of leaking current and cuts off
power quickly at leakages greater than 5 mA.
Too often, careless handling of farm equipment around electric
power lines and utility poles leads to tragedy. (See Figure
1) Accidents can be prevented by insisting that hired hands
and family members learn to survey work areas before using farm
equipment. Make each worker determine that equipment will not
come within 10 feet of power lines or power support equipment.
Electrocutions may occur even if actual contact is not made
with the line. The greater the line voltage, the further electricity is able to "jump" to a ground conductor.
Recognize
that power lines often follow property lines. As workers reach
the end of the field and turn the equipment, there is a good
chance power lines will be nearby. Power lines also are often
near grain and equipment storage facilities. Be sure that
paths from equipment storage areas to fields and from the
fields to grain storage areas are safe routes. If you have
any doubts that your equipment will clear a line, assume that
it will not and take measures to avoid possible contact.
Crop
storage equipment such as balers and stackers can be extended
in height to exceed electric code clearances for power lines.
When storing hay or baled straw, take precautions to be sure
the stacking equipment will not come into contact with power
lines.
Portable
grain augers are a leading cause of farm electrocutions. Lower
augers when moving them from one bin to another. The operating
height of an auger usually is greater than the height of power
lines. If workers hand-push one into a line, they will be
instantly electrocuted.
Voltages
greater than 1v disturb cattle, decrease dairy cow milk production
and lower the average daily gain in beef cattle. If animals
are exposed long enough to voltages greater than 25v, the voltage
can be fatal.
Stray
voltage on farms may stem from several sources. The voltage
may occur predictably throughout the day, or it may occur
randomly. These factors make controlling stray voltage a major
problem.
On-farm
stray voltages have been traced to the following sources:
- Ground
faults on the farm.
- Voltage
gradients across the ground or floor due to wires faulted
in the earth.
- Electric
fence wires shorting directly to equipment or energizing
pipes and equipment.
Electrical
motors and equipment can "short out" or ground fault to their
outside casings. The severity can vary from a fault with no
visible effect to a complete short circuit. When short circuits
occur, all conductive materials, such as stainless steel milk
lines and pipe fencing, become energized. Wood also can conduct
electricity when it is wet. To correct the problem, faulty
equipment should be repaired or replaced immediately by someone
with proper training. The three-wire electrical supply line
helps reduce the problem when the fault first occurs.
Voltage
gradients through the earth or across a floor occur when an
underground wire faults to earth. This often happens when
the insulation of underground wires not rated for direct burial
becomes damaged. Typically, the wires have not been buried
at the proper depth. Underground wires buried many years ago
are suspect, particularly if voltage gradients are detected
in the earth. To correct the problem, the lines must be replaced
with ones rated for direct burial.
Electric
fence wires running through buildings can cause problems when
building electrical wire is used. The maximum insulation rating
of building electrical wire is 600 volts, which is not adequate
insulation for the high voltage pulse output of a fence charger.
When this type of stray voltage is suspected, look for breakdowns
in the insulation that could be causing a fault to metal equipment
inside a barn. To prevent this problem, use insulated wire
rated beyond the maximum voltage output of the fence charger.
MU publications
with further information on electrical safety include G01020,
Lightning Protection for Missouri Farms and Homes, and G01406,
Preventing Shocks to Cows in Milking Parlors.
Disclaimer
and Reproduction Information: Information in NASD does not represent
NIOSH policy. Information included in NASD appears by permission
of the author and/or copyright holder. More
NASD Review: 04/2002
This
document is
G1934
,
a series of the University of Extension, University of Missouri-Columbia.
If you have special needs as addressed by the Americans with
Disabilities Act and need this publication in an alternative
format, write ADA Officer, Extension and Agricultural Information,
1-98 Agriculture Building, Columbia, MO 65211, or call (314)
882-8237. Reasonable efforts will be made to accommodate your
special needs. Publication date: October 1993.
Partial
funding for this publication was provided by the University
of Missouri-Columbia/National Institute for Occupational Safety
and Health Cooperative Agricultural Promotions Agreement U05/CCU706084-01.
David
Baker and Rusty Lee, Dept. of Agricultural Engineering, University
Extension, University of Missouri-Columbia, Columbia, MO 65211.
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