Howard
J. Doss and William McLeod
Michigan State University Extension
Agricultural
electrical systems have their own special requirements to
reduce the risk of fires, injuries, and to ensure that electric
motors and other equipment operate properly.
Some
of the special requirements needed for an agricultural setting
include increased height of overhead lines to accommodate
tall farm equipment such as combines and grain augers, special
insulated wire to withstand damp environments inside farm
buildings, and ground-fault circuit interrupters (GFCI) to
ensure operator safety in the event of an extension cord or
tool coming in contact with water.
Grain
augers, bale elevators and other tall equipment touching overhead
electric lines has claimed the lives of many Michigan farmers
over the past several decades. Generally, the accident has resulted
from the operator failing to lower an auger or elevator before
moving it in an area where overhead lines were located. While
increased line height may have prevented these accidents, lowering
the auger would have eliminated any risk of it touching electrical
lines.
It should
also be noted that there are several grain augers manufactured
that can be raised and lowered by a tractor:s hydraulic unit,
reducing the time and effort required to :crank: an auger
up or down by hand.
Placement
of Overhead Lines
There are
two sets of guidelines to consult for placement of overhead
power lines for grain-handling systems, the National Electrical
Safety Code (NESC) and the National Electric Code (NEC). NESC
standards apply primarily to electrical supply equipment wires
ahead of the transformer location that serves the farm. NEC
standards address electrical wiring from the meter to the load
site.
NESC
standards for portable systems (moveable augers) requires
that the height of the overhead line must be at least 18 feet
above the highest port of the grain bin. This line height
must be maintained on the loading side of the bin for an additional
18 feet plus the height of a bin. For example, the overhead
line for a 20-foot tall bin must be at least 38 feet above
the ground, and the post on the loading side must be a minimum
of 38 feet from the bin:
- 20'
Bin height
- +18'
NESC standard
- 38'
Overhead line height and distance from the bin on loading
side (see Figure 1)
Outside
this 38-foot horizontal offset, the line may be sloped down
to its regular height at a 1:1.5 ration (67 percent slope).
NESC
standards for fixed grain-handling systems require that the
overhead line serving the transformer must be a minimum of
12.5 feet above the roof of any solid structure that is not
easily accessible to people. In situations where the roof
of a structure is easily accessible, such as a grain bin,
the height must be 18 feet above the highest port of the bin.
A 7.5-foot horizontal clearance must be maintained for the
height of the overhead line.
NESC
also requires that any overhead lines over a roadway or path
where cultivation equipment travels must be a minimum of 18.5
feet above the ground. This ensures that equipment such as
a combine or tractor will not come into contact with the line.
Tillage or harvesting equipment that contacts an overhead
line can be as hazardous as touching the line with a grain
auger.
NEC
standards, which apply to electrical lines from the meter
to the distribution points, require an 8-foot minimum clearance
above rooflines that are easily accessible to people (grain
bin roofs). NEC standards also require a 3-foot horizontal
clearance between equipment and insulated conductors and an
18-foot clearance above roadways for the transportation of
forest, orchard, grazing or cultivation equipment.
There
are numerous variables for the electrification of barns and
other farm facilities that cannot be addressed in any single
publication. This section will provide recommendations as to
the type of wire and conduit to be used. For additional information
contact your local electrical inspector, electrician or electrical
supplier.
Non-metallic
Sheathed Cable
Table
1. Current Capacity of Certain Cables* |
Cable
type
| Conductor
size (AWG No.)
| Maximum
Allowable Amps (Fuse size)
|
(copper) |
(aluminum) |
NMC,
UF |
#14 |
#12 |
15 |
#12 |
#10 |
20 |
#10 |
#8 |
30 |
*
- There are many exceptions to this table; therefore,
check with a qualified electrician before purchasing cable
for a branch circuit. |
Because
agricultural structures, particularly animal facilities, are
frequently damp due to inadequate environmental controls, "Type
NMC" or "UF" cable is recommended for most situations. These
types of cables, because there is no paper installation, are
able to withstand damp environments without becoming corroded.
The
gauge of the cable needed for a specific circuit branch varies
with the length of run and the demand of the power equipment
to be connected. Table 1 is presented to give general guidelines
only, check with a qualified electrician before purchasing
cable to connect a branch circuit.
Protecting
Electric Cable
Encasing
electric cable inside conduit provides extra protection from
livestock and gnawing rodents.
There
are two basic types of conduit, metal and PVC (plastic). PVC
is preferred inside agricultural structures because it is
not corroded by moisture and is generally less expensive than
metal conduit. Both metal and PVC conduit can be sealed where
it joins receptacles, junction boxes, etc., reducing moisture
concerns.
All
electric cable in an agricultural structure, whether it be
encased in conduit or not, should be placed in open areas
for frequent inspection and maintenance. Where possible, keep
electrical switches, wires, etc., out or the reach of livestock.
A
ground-fault circuit interrupter (GFCI) is a circuit breaker
designed to prevent serious shock to people or animals under
certain conditions. It can reduce the risk of shock when using
electrical tools or appliances in damp or wet areas.
The
GFCI works on the principle that the two wires supplying a
single-phase electrical load must carry the same number of
amperes (current) when the circuit is operating properly.
If a ground-fault occurs, either to the grounding wire or
through a person or animal, some of the current will take
an alternate route back to the system:s grounding electrode.
One of the wires will then carry less current than the other
wire. When this occurs the GFCI will break the circuit, stopping
the flow of electricity in a fraction of a second to reduce
the electric shock hazard.
Ground-fault
circuit interrupters come in several styles. They are commonly
used as a receptacle outlet, part of an extension cord, or
can be installed in the main electrical panel to replace an
existing circuit breaker. When installed as a circuit breaker,
the GFCI offers shock protection to an entire electrical branch.
Ground-fault
circuit interrupters are available for 120-V circuits with
one hot wire and a neutral. A GFCI will work on older two-wire
electrical systems that have no ground wire. A 120-V, single-pole
GFCI fits into the same size space as a standard single-pole
breaker. There are also GFCIs for 240-V circuits using two
hot wires.
All
equipment plugged into a GFCI protected receptacle, including
any two-prong (two-wire) electrical plug, will have ground-fault
protection.
A portable
GFCI is recommended for persons using power tools in damp
or wet locations. The portable GFCI is plugged into an outlet
and the power tool is plugged into the GFCI.
Certain
conditions can result in "nuisance tripping" of a GFCI protected
circuit or receptacle.
Nuisance
tripping can be reduced by avoiding:
- Circuits
longer than 100 ft.
- Older
non-double insulated power tools that contain faulty electrical
insulation.
- Fluorescent
or other types of electric-discharge lighting fixtures.
- Extension
cords with cuts or splices where moisture has entered the
cut or splice creating a "leak" (ground fault) or path for
the current to flow outside of the wire to the ground.
- Permanently
installed electric motors.
Installing
a GFCI to prevent electrical shock from farm equipment seems
like a good idea, but nuisance tripping may become a serious
problem. The loss of a ventilation system in certain livestock
facilities can be fatal to animals if the GFCI tripped. Stock
waterers may freeze in northern climates if the GFCI trips.
Carefully consider the effects of loss of power to an agricultural
circuit before installing GFCI protection. The most effective
shock prevention system for agricultural equipment and circuits
is a good equipment grounding conductor run with the circuit
wires and connected to all metal agricultural equipment.
Damaged
or improperly used cords can result in electrical shocks or
start fires. Avoid using extension cords in farming operations.
Follow these precautions if you must use an extension cord:
- Do
not use in wet areas.
- Do
not try to repair a damaged extension cord or splice two
wires together. Replace the cord.
- Keep
cords away from sharp objects, heat, oil, and solvents that
can damage insulation.
- Check
an extension cord before each use for nicks and cuts. Replace
the cord if the insulation is damaged or worn.
- Use
an extension cord with correct size wiring (gauge) for the
intended use. Do not overload an extension cord or use a
"household" type extension cord to operate heavy-duty machinery.
Overloading may cause excessive heating that may result
in a fire.
Table
2. Hard Service Extension Cords |
Cord
Length
| Conductor
AWG No.
|
Up
to 35' |
#14 |
36'
to 75' |
#12 |
76'
to 100' |
#10 |
#16
cord should be primarily for service cords on appliances
and equipment where the maximum load does not exceed
10 amperes or 1150 watts.
#18 cord (PO-1 or SF-1) will safely carry a maximum
load of 7 amperes or 800 watts. |
- Use
a grounded wire (three-prong with a "safety grounding" wire)
for tools and machines having a grounded plug or use a portable
GFCI (ground-fault circuit interrupter).
- Be
sure the package for the cord indicates the maximum current
and/or wattage rating of the cord.
- Route
the cord to protect it from machinery and animals. Also,
people should not be able to trip over or accidentally damage
the cord.
- Two
extension cords plugged together for additional length will
reduce the amperage rating and increase the risk of an electrical
hazard.
- Extension
cords deteriorate; do not use them in place of permanent
electrical installations.
- When
purchasing extension cords, make sure the cord has a listing
mark or certification of a recognized independent testing
laboratory. For agricultural use, extension cords with a
strong outer coating should be purchased. Extension cords
labeled "Type S" (Hard Service Cord) have the strongest
outer covering available and should be purchased for work
on the farm. Do not be confused with other "S" ratings such
as "Type SJ" (the J stands for Junior Hard Service Cord),
which should not be used out-of-doors.
Extension
cords are sold in various cable sizes, with the smaller number
being the larger wire size (a number 10 wire is larger than
a number 14).
Table
2 presents some general guidelines for cord length of extension
cords.
- Guide
for Selecting Electric Cords and Cables, Truman C. Surbrook,
Agricultural Engineering Department, Michigan State University.
AEIS 236.
- Clearance
of Electric Supply Conductors to Grain-Handling Equipment,
R.J. Gustafson, Agricultural Engineering Department, Ohio
State University.
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
Michigan
State University, Agricultural Engineering Department.
Howard
J. Doss, Extension Agricultural Safety Specialist; and William
McLeod, Agricultural Engineering, Michigan State University
Extension, East Lansing, Michigan 48824. 5/93. Funded by the
National Institute of Occupational Safety and Health - #UO5/CC-4506052-02.
This information is for educational purposes only. Reference
to commercial products or trade names does not imply endorsement
by the MSU Extension or bias against those not mentioned.
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