Hazards of Electricity in Construction
Hazards of Electricity in Construction
Hazards of Electricity in Construction
in Construction
Overview of the hazards of electricity and
how to avoid them
December, 2010
This presentation will cover the following:
How electricity works
Grounding hazards
Additional information
How Electricity Works
Operating an electrical switch is like turning on a water
faucet. Behind the faucet (or switch) there is a source of
water (or electricity), a way to transport it, and pressure
to make it flow. The faucet's water source is a reservoir
or pumping station. A pump provides enough pressure
for the water to travel through the pipes.
TRANSFORMER
Metallic parts of electric tools and machines can become energized if there is a break in
the insulation of their wiring. A low-resistance wire between the metallic case of the
tool/machine and the ground – an equipment grounding conductor – provides a path for
the unwanted current to pass directly to the ground. This greatly reduces the amount of
current passing through the body of the person in contact with the tool or machine.
Properly installed, the grounding conductor provides protection from electric shock.
How Electrical Current Affects the Human Body
Three primary factors affect the severity of the shock a person receives
when he or she is a part of an electrical circuit:
Amount of current flowing through the body (measured in amperes).
Path of the current through the body.
Length of time the body is in the circuit.
Other factors that may affect the severity of the shock are:
The voltage of the current.
The presence of moisture in the environment.
The phase of the heart cycle when the shock occurs.
The general health of the person prior to the shock.
Effects can range from a barely perceptible tingle to severe burns and
immediate cardiac arrest.
Wet conditions are common during low-voltage electrocutions. Under dry conditions, human
skin is very resistant. Wet skin dramatically drops the body's resistance.
How Electrical Shocks Occur
This table demonstrates the effects o a a 60-cycle, hand-to-foot shock of one
second duration:
Current level
Probable effect on human body
(in milliamps)
Perception level. Slight tingling sensation. Still dangerous under
1 mA
certain conditions.
Slight shock felt; not painful but disturbing. Average individual can let
5 mA go. However, strong involuntary reactions to shocks in this range may
lead to injuries.
Painful shock, muscular control is lost. This is called the freezing
6-30 mA
current or "let-go" range.
Extreme pain, respiratory arrest, severe muscular contractions.
50-150 mA
Individual cannot let go. Death is possible.
Ventricular fibrillation (the rhythmic pumping action of the heart
1000-4300 mA ceases.) Muscular contraction and nerve damage occur. Death is
most likely.
10,000 mA Cardiac arrest, severe burns and probable death.
Electrical Burns
Shock-related injuries include burns, internal injuries, and injuries due to
involuntary muscle contractions..
The most common shock-related injury is a burn. Burns suffered in
electrical incidents may be one or more of the following three types:
Electrical Burns cause tissue damage, and are the Photo Examples
result of heat generated by the flow of electric
current through the body. Electrical burns are one of Warning:
the most serious injuries you can receive and should these images
are of a graphic
be given immediate attention.
nature.
High temperatures near the body produced by an
electric arc or explosion cause Arc or Flash Burns.
They should also be attended to promptly.
Thermal Contact Burns occur when skin comes in
contact with overheated electric equipment, or when
clothing is ignited in an electrical incident.
Electrical Shock Internal Injuries
Excessive electricity flowing through the human body can
cause serious damage to internal organs.
Also, injury or death may result when violent muscle contractions cause
workers to fall from ladders and scaffolds or inadvertently strike other objects.
Contact with Power Lines
Overhead and buried power lines at a
construction site are especially hazardous
because they carry extremely high voltage. Fatal
electrocution is the main risk, but burns and falls
from elevation are also hazards. Using tools and
equipment that can contact power lines increases
the risk.
Examples of equipment that can contact power
Overhead power lines are lines:
un-insulated and can carry
tens of thousands of volts, Backhoes
making them extremely Concrete pumpers
dangerous to employees
who work in their vicinity. Cranes
Long-handled cement finishing floats
Metal building materials
Metal ladders
Raised dump truck beds
Scaffolds
Contact with Power Lines continued…
Source: OSHA
Both the operator and the victim failed to notice that the boom had contacted a
2,400-volt overhead power line. When the victim reached down to connect the
choker to the hoist cable, he suddenly collapsed.
Co-workers provided CPR, but were unable to revive the victim. Only after a
rescue squad arrived about 4 minutes later did anyone realize that the crane
was in contact with a power line -- all those present had assumed that the victim
had suffered a heart attack.
Two incidents of cranes contacting power lines
Identify overhead power lines and mark safe routes where cranes must
repeatedly travel.
Operate the crane at a slower-than-normal speed in the vicinity of power
lines.
When working around overhead power lines, de-energize and ground them,
or take other protective measures such as guarding or insulating the lines.
If the power lines are not de-energized, operate cranes in the area ONLY if a
safe minimum clearance is maintained.
Address Power Line Risks (continued)
• If maintaining safe clearance by visual means is
difficult, designate a person to observe the
clearance and to give immediate warning when
the crane approaches the limits of safe clearance.
The GFCI will not protect you from line contact hazards (i.e. a person
holding two "hot" wires, a hot and a neutral wire in each hand, or contacting
an overhead power line). However, it protects against the most common
form of electrical shock hazard, the ground-fault. It also protects against
fires, overheating, and destruction of wire insulation.
For construction applications, there are several types of GFCIs available,
with some variations:
The Receptacle Type incorporates a GFCI device within one or more receptacle
outlets. Such devices are becoming popular because of their low cost.
Portable Type GFCIs come in several styles, all designed for easy transport.
Some are designed to plug into existing non-GFCI outlets, or connect with a
cord and plug arrangement. The portable type also incorporates a no-voltage
release device that will disconnect power to the outlets if any supply conductor
is open. Units approved for outdoor use will be in enclosures suitable for the
environment. If exposed to rain, they must be listed as waterproof.
The Cord-Connected Type of GFCI is an attachment plug
incorporating the GFCI module. It protects the cord and any
equipment attached to the cord. The attachment plug has a non-
standard appearance with test and reset buttons. Like the portable
type, it incorporates a no-voltage release device that will disconnect
power to the load if any supply conductor is open.
Because GFCIs are so complex, they require testing on a regular basis. Test permanently wired devices
monthly, and portable-type GFCIs before each use. All GFCIs have a built-in test circuit, with test and
reset buttons, that triggers an artificial ground-fault to verify protection. Ground-fault protection, such as
GFCIs provide, is required by DOSH in addition to (not as a substitute for) general grounding
requirements.
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 from
follows this path, thus preventing the buildup of voltages that would
otherwise result in electrical shock, injury and even death.
Improper grounding
Grounding
There are two kinds of grounds; both are required by the
DOSH construction standard:
Am I In Danger?
Two workers were using a 110-volt auger to install tie-down rods for a manufactured home.
The auger has a one-quarter horsepower motor encased in a metal housing with two
handles. One handle has a deadman's switch. Electricity to the auger was supplied by a
series of 50-foot extension cords running to an adjacent property. Since the outlet at the
adjacent property had no socket for a ground prong, the extension cords were plugged into
the outlet using an adapter, but the ground wire of the adapter was not grounded. Two of
the extension cords had no ground prongs, and some of them were repaired with electrical
tape. The workers had removed their shirts and were sweating. One worker, holding the
deadman's switch, received a shock from a ground fault in the auger and was knocked
back from the machine. The auger then fell across the other worker, the 24-year-old victim.
The first worker knocked the auger off the victim, but saw that the electric cord was
wrapped around the victim's thigh. He yelled for his co-workers to disconnect the power,
which they did. The workers administered CPR to the victim, but to no avail.
Death Due to Missing Or
Discontinuous Path to Ground
Short In Power Saw/Ungrounded Temporary Power Supply
A 22-year-old carpenter was working at the construction site of large apartment complex,
using a portable electric saw to construct the wooden framework of a laundry building.
Electricity to operate portable power tools was supplied by a temporary service pole 50 feet
away. The pole had not been inspected by the city and was not in compliance with code
requirements (it was not grounded). The victim used two extension cords to supply power: a
home-made cord plugged into an ungrounded receptacle on the pole, and a UL-approved
cord extending from the homemade cord to the saw. The accident site was wet; also,
humidity was high and the victim was sweating. Reportedly, he was shocked throughout the
morning, and he had replaced one of the extension cords in an effort to eliminate the
shocks. The source of the shocks -- the saw -- was not replaced. As the victim climbed
down a makeshift ladder, he shifted the saw from his right hand to his left, and was shocked.
This caused him to fall from the ladder and land in a puddle of water, still holding the saw.
Apparently, his hand contracted and he was "locked" to the saw. A co-worker disconnected
the power cord to the saw, but it was too late and the carpenter died. to
Power Tools
Because power tools are so common in construction, workers are
constantly exposed to a variety of hazards. The very tool that makes their
job easy and efficient may become the cause of a tragic accident.
Grounding
Extension cords must be 3-wire type so they may
be grounded, and to permit grounding of any
tools or equipment connected to them.
Flexible Cords continued
Wet conditions
When a cord connector is wet, electric current can leak to the equipment
grounding conductor, and to humans who pick up that connector if they
provide a path to ground. Such leakage can occur not just on the face of the
connector, but at any wetted portion. Limit exposure of connectors and tools
to excessive moisture by using watertight or sealable connectors.
Deaths Causes Due to Improper Use
of Extension and Flexible Cords
Flexible Cord Not 3-Wire, Hard Service Variety
A worker received a fatal shock when he was cutting
drywall with a metal casing router. The router's 3-
wire power cord was spliced to a 2-wire cord and
plug set which was not rated for hard service. A fault
occurred, and with no grounding and no GFCI
protection, the worker was electrocuted.
No Strain Relief
A worker was operating a ¾" electric chisel when an
electrical fault occurred in the casing of the tool,
causing him to be fatally electrocuted. A safety
inspection revealed that the tool's original power
cord had been replaced with a flat cord, which was
not designated for hard service, and that strain relief (Photo courtesy of Elcosh Images)
was not provided at the point where the cord Potential electrical shock hazard. Damaged electrical
entered the tool. Additionally, the ground prong was cord, potentially overloaded connection. No GFCI,
missing and there was no GFCI protection. potential trip hazards. Electric cord sets not designed
for commercial application.
Equipment Not Used In Manner Prescribed
If electrical equipment is used in ways for which it is not
designed, you can no longer depend on safety features built
in by the manufacturer. This may damage your equipment
and cause employee injuries.
eLCOSH – Electrocution