- Electrical Safety During Excavations
11.0 ELECTRICAL SAFETY DURING EXCAVATIONS
provides information and guidance for detecting underground and embedded utilities. It focuses on the need for configuration control of these
utilities, as well as establishing and maintaining a procedure to identify these hazards prior to assigning work near them. Such a procedure
can be a significant element in the site’s Electrical Safety Program as well as a critical element of an ongoing Safety Through Damage Prevention
Program. Sites which have such procedures have demonstrated significant reductions in the numbers and serious nature of electrical injuries
and utilities damage sustained during excavations.
Occurrences involving unexpected contact with underground or embedded
excavations, concrete drilling and earth moving operations can occur at DOE facilities and
throughout private industry.
Inadvertent striking of underground utilities can result in electrical shock, injuries, explosions,
utility outages and death. These
pipes, wires or conduits are frequently missing from as-built or
other record drawings.
Large-scale decontamination and decommissioning
(D&D), environmental restoration, and new
construction projects performed at DOE facilities bring with them a significant risk of contact
underground or embedded utilities. It is therefore essential that effective policies and
procedures be implemented to control and
minimize this risk.
11.2 REGULATIONS, CODES AND REFERENCES
a. 29 CFR 1910.335, Personal Protective Equipment
b. 29 CFR 1926.416,
Protection of Employees
c. 29 CFR 1926.651, Specific Excavation Requirements
d. DOE/EH-0541, Issue number 96-06 (1996), Underground Utilities
Detection And Excavation
e. DOE Operating Experience Summary 2001-12, Backhoe Ruptured a Natural Gas Line During Excavation
f. DOE Operating
Experience Summary 2002-10, Temporary Electrical Power Line Severed By Trackhoe
g. DOE Operating Experience Summary 2003-09, Energized Power
Line Damaged During Excavation
h. DOE Operating Experience Summary 2003-18, Underground Electrical Cable Snagged And Cut
11.3 UTILITIES IDENTIFICATION
Before locating underground or embedded utilities, facility personnel should obtain and review
available information for the location area. Resources might be available drawings, sketches,
and site knowledge. Failing to thoroughly research
and review all available information, from
the original installation up to the present layout, before beginning an underground or embedded
utility detection survey can lead to possible hazards and problems for locators. Facilities often
utilize direct burial trenches to stack
utilities and locator equipment often identifies only the utility
closest to the surface.
Facility drawings and/or configuration control
methods often separate utilities by function,
making this initial research very important.
Relying on current locating technology,
alone, has resulted in many encounters with buried and
11.3.1 CONFIGURATION MANAGEMENT
a. Facility controlled
drawings identifying utilities locations.
- Voltage levels, burial depth, and elevation details for “stacked” utilities
information to include on these drawings.
b. As-built drawings/sketches are often
required to be submitted for all new utilities
c. Survey point locations are required at some DOE
- This provides very accurate locations for future reference.
- Some DOE facilities utilize
Global Positioning Satellite (GPS) equipment to mark
d. An permanent above ground, marking
program also provides excellent ongoing visual
safety awareness reminder.
11.3.2. EXCAVATION PERMIT
excavation permit ensures that the scope of the excavation is clearly defined. It also
ensures that the area to be excavated is reviewed
by the appropriate facility personnel, prior to
beginning the work. The use of this permit is recommended for operating facilities and older
facilities which do not have exceptional configuration management. If a requirement, the permit
also assures that consistent work authorization
A typical excavation permit contains the following, as a minimum :
a. Detailed scope of work to be performed
b. Accurate definition of boundaries for the proposed excavation.
c. Signoffs for appropriate subject matter expert reviews, if required.
d. Accurate excavation maps, with all known utilities identified and boundaries clearly marked.
e. Work authorization signoff
f. Controlled length of validation time period after which a revisit and remarking of the site
UTILITIES VERIFICATION AND MARKING
It is important to establish and maintain policies and procedures to assure accurate and
methods are used to verify and mark utilities to support various facility excavation
activities. Once the configuration control resources
have been exhausted, there are a number
of locator equipment methods which can be used to help identify utilities not previously
Once identified, all utilities must be marked at the field work location in order to
establish effective communication with the excavation
work group. Some facilities use survey
paints sprayed on the ground or on the concrete walls/slabs. Some facilities use whiskers, flags
or postings. Whatever the methods used, they must be effective. Most DOE facilities have
some form of utilities verification and marking
system. Here are a few examples of effective
methods used :
a. Field walk-downs of excavation site by approved subject matter experts.
b. Testing to validate identified utilities.
c. Testing to identify utilities not yet identified
− Ground penetrating
− Passive/active frequency method.
d. Survey paint markings
− must be durable
and timed to effectively coincide with the work group mobilization.
These markings should be made no
more than two or three days prior to the actual
excavation work. They should be verified immediately
prior to the beginning of the
e. Survey flags
− must be
durable and timed to effectively coincide with the work group mobilization.
These flags should be installed
no more than two or three days prior to the actual
excavation work. They should be verified immediately
prior to the beginning of the
f. Utilities color coding
− same Utilities & Transportation Commission (UTC) color coding as municipalities use.
− offsite subcontractors are
very familiar with this work practice.
11.4.1 FIELD LOCATION OF EXCAVATION BOUNDARIES
Accurate identification of excavation boundaries is absolutely necessary.
communicates those boundaries to all workers and supervisors. It also sets the excavation
limits, based on the research
and locator testing provided for that particular excavation.
a. Excavation maps can be used to identify exact excavation boundaries (Field
can easily be transferred from these maps)
b. Excavation maps can be attached to the excavation permits
c. Field excavation boundaries should be appropriately marked.
LOCATOR EQUIPMENT SELECTION AND LIMITATIONS
It is important to note that no locator technology should be relied upon as the sole source
identifying buried or embedded utilities. Configuration management is the most effective
of underground utility locators being used today apply a signal to the underground
system being located. This signal may either be of an
audio or radio frequency. An active
signal is applied to the underground utility by various methods with the signal being generated
the transmitter unit of the location system. Proper setup of the transmitter increases the
accuracy of the receiver unit. Proper use of the
receiver unit requires that the antennas be
moved in a straight path and not in a swinging motion. Accurate depth readings are gained
The latest electromagnetic pipe and cable locators feature microprocessor-controlled
transmitters and receivers capable
of detecting power lines, telephone cables, and metal piping
at depths up to 15 or 20 feet. These detection devices operate with multiple
frequencies, antenna configurations, and grounding capabilities.
DIRECT OR CONDUCTIVE METHOD
The most desired
and accurate method of applying a signal to the underground system is the
direct or conductive mode, where there is access to a contact point
on the utility to be located.
The direct connection is always the preferred and most accurate method of applying the signal
to the utility,
as the operator has the most control of the signal. When using the direct mode, it
must be understood that the utility line to be located
becomes part of the detection circuit. The
detection signal is applied to the utility using attachment leads. The signal is applied using
lead, and returns on the other lead. An accurate location begins with the proper set up of the
transmitter and the key to setting
up the transmitter is the grounding method used. A significant
percentage of all incorrect locates result from the failure to properly control
the signal applied to
the target utility. Personal that is not properly trained, cannot notice the difference in most
CLAMP OR COUPLER
A second method of applying a signal to a known utility is the inductive clamp or coupler, where
the signal is induced
to the utility by means of a jawed clamp placed around the utility access
point without the grounding system being disturbed. Inductive couplers and clamps all apply a
signal to the utility in basically
the same manner. The signal is induced onto the utility to be
located by an electromagnetic field created by the coupler and clamp. It is
a requirement that
the utility have grounds in place at both ends of the section to be located. Missing bonds across
of the utility will prevent this method from being used along the entire length
of the utility. Underground metal pipes and cables may not
be joined with conductive materials,
thus making them short separated pieces of a broadcast antenna. There may be multiple
together (ie., cathodic protection and common grounding), making their
individual resolution difficult. These and other problems create the
need for variety within the
electromagnetic method itself. Couplers and clamps should be positioned below the electrical
on CATV, electrical, and telephone cables. Systems that do not use earth
ground, such as railway signal cables, cannot be located with couplers
or clamps. Gas meters
with insulated couplings should not be bonded. They should be direct-connected.
INDUCTIVE OR INDIRECT
A third method is the inductive or indirect mode. Here the transmitter is placed on the surface of
the ground above the known utility.
The signal from the transmitter is induced onto the utility,
making the location possible. Inductive transmitter use should only be used
when access points
for the utilities are not available. This method can lead to serious tracing errors, especially if
are buried/embedded in the same area.
DETECTOR FREQUENCY AND POWER CONSIDERATIONS
As an operator attempts to detect the location
of underground/embedded utilities, the frequency
and power capabilities of the instrument being used must be understood. In many detectors,
the frequency increases, the available power decreases.
Frequency selection can affect depth of penetration, distance of travel,
and resolution issues, as well as other operating characteristics. Most locator manufacturers
and there will be an optimum frequency to use for a particular utility system.
These are some of the most frequently used :
(Extremely Low Frequency) operates at below 300 Hz. It is typically used for
power cables energized and carrying a load,
water pipes grounded to a power system
energized and carrying load, and deep, very conductive, long-length utilities.
b. VLF (Very Low Frequency) operates between 3kHz and 30 kHz. It is typically used for
very long, continuous conductors,
and deep conductive long length utilities.
c. LF (Low Frequency) operates between 30kHz and 300kHz. It is typically used for
shallow (8 feet or less) conductors of medium length.
d. HF (High Frequency) operates between 3MHz and 30MHz. It is typically used for
cables; shallow short conductors.
Not all utilities are metallic or of sufficient length in the ground. Therefore, resonant
methods will not always be the answer. The current family of surface
geophysical methods available for utility and other near-surface structure
detection is extensive. Other electromagnetic techniques such as measuring eddy
currents, differential heat, and thermal reflection are also available. Here are three types
presently in use :
Conductivity uses the VLF range, using Eddy Currents to measure
differences in ground conductivity. It has proven useful
in locating very deep or short
metallic utilities and air/gas-filled utilities.
b. Ground penetrating Radar uses
1-100 gigahertz microwaves measures reflections
due to dielectric differences in subsurface materials. It measures strength
amount of time necessary to bounce signal off different layers. This technology
soils. Utilities must be of sufficient size to be detected. The deeper
the utility, the larger it must be in order to
be detected. Highly different soil/utility
materials give the best results.
c. Thermal measures heat output. When
the amount of heat from the utility is different
from that of the surrounding soil, it might be detectable. Some utilities
own exothermic heat or retain heat longer than the surrounding soil.
appropriate locator technology and methods will greatly improve the chances
11.4.3 LOCATOR OPERATOR
Each piece of locating equipment is unique. It is very important that operators be trained to use
the equipment before applying
it in a field application that will determine safe boundaries for
excavation workers. Some of the ground penetrating radar equipment is very
operator interpretation of the characterization profiles generated from the locator equipment.
Operators unfamiliar with
the equipment specifications and operating instructions can make
interpretive mistakes which can lead to serious injury to the excavation
Proper selection of available techniques and the use and interpretation of data produced by this
equipment is essential to
the accurate and comprehensive detection of underground utilities. A
regular calibration and maintenance of locator equipment should be established.
11.4.4 FIELD MARKING OF IDENTIFIED UTILITIES
Paints/Surface Markings are used at many jobsites. Care should be taken to ensure the
markings are clearly present and identifiable at the time the excavation workers arrive at the
excavation site. Many cases of faint markings
and/or markings washed away by rain have been
documented at jobsites. The recent use of biodegradable survey paints has increased this
possibility. They are not as durable as former types of survey paints used. Lawn mowing
equipment can erase or diminish the effectiveness
of survey paints in outside areas.
Stakes or Flags are used to mark identified utilities at other jobsites. These must be durable,
well. They must be able to withstand the environments they are exposed to. Lawn mowing
equipment can erase all traces of survey flags in
very short order. Incidents resulting from
missing and even relocated stakes or flags have been documented.
Utilities Color Coding
is used at most work locations. The standard utilities color-coding is
well understood by most excavations subcontractors, and serves as
a very effective
communications tool for the facility owner.
Plastic Utilities Ribbon Tape is used in most facilities as well as commercial and industrial
applications. It is installed directly
above the utility, but below the surface of the ground so an
excavator will dig it up, and thereby indicate the presence of a utility, before
the utility is
Metallic Utilities Ribbon Tape is used in many facilities. It is buried at a specified depth above
utility. This ribbon can be detected and its route traced by a metal detector.
Traceable Ribbon Tape is used in many facilities. It is buried
at a specified depth above the
utility. This ribbon can be detected and its route traced by a metal detector or by a passive
burial, traceable ribbon tape has been known to lose it’s traceable effectiveness
after being buried for a period of time and also has been
known to create a false positive adding
to confusion that a utility or a abandoned pipe is there causing a miss-marked utility.
ribbon in bright colors with “Utility Buried Beneath,” buried 1½ to 2 feet above the
utility, provides adequate warning for the excavation
operator that a utility is near.
Tracer Wire is the preferred method of tracing non-metallic utilities. When installed correctly
combined with a proactive open trench policy and programmable Marker Balls is a highly
effective combination. Directly connecting to the
traceable wire using any locating instrument,
provides a highly accurate locate.
Marker Balls installed at the beginning, ends, tees
and turns, provides a highly cost effective
solution to finding exact locations of where a particular section of a utility is located. Add
programmable capability, and you can positively know your locating the exact utility your looking
for by the press of the button and
reading the preprogrammed data indicating size, material,
date installed etc.
11.5 UTILITIES DISPOSITION
during the planning stages should be evaluated to determine their function as
it relates to the facility. Every effort should be made to
de-energized and lock out power cables
which could be encountered by excavations workers. Telecommunications and
likewise, require consideration. Planned outages is the preferred
approach. NFPA 70E is the recognized standard for de-energizing equipment
as the first
priority. Don’t let schedule and/or convenience take precedent. Impact to the facility in the
event of damage to these cables/circuits
should be part of the evaluation.
11.6 WORK CONTROL DURING EXCAVATIONS
The facility’s safety program should provide clear, consistent
direction to the excavation
workers. It should address facility expectations for workers encountering unexpected utilities
excavation. The excavation permitting program, open trench policy, no-dig zone,
above ground utility identification program, hazardous energy
control, and submittal of as-built
drawing requirements prior to permit closure should be considered.
11.6.1 SAFETY EQUIPMENT AND
The requirements for safety equipment should be understood. Some concrete wall and slab
excavations require both mechanical
and electronic drill-stops in some facilities. Some facilities
require insulated footwear and/or gloves for certain types of excavations
or certain site areas.
These requirements are required to be effectively communicated to the excavation workers.
11.7 THE EXCAVATION PROCESS
Once the planning is complete, the excavation process is ready to begin. Work control
are in place, all hazards have been identified, hazards elimination/mitigation is
established and personal protection requirements have been
determined and are understood by
all involved excavation personnel.
11.7.1 EXCAVATIONS IN CONCRETE WALLS AND SLABS
in concrete walls and slabs should be included in facility procedures. Drilling holes,
coring, chipping, or cutting holes in these areas
is included. Most facilities require excavation
permits be issued for penetrations to a depth of 3 inches or greater in most concrete walls
slabs. Some facilities, due to their design, will vary this depth reference.
The concern is encountering structural rebar or, worse
yet, energized electrical power cables.
DOE sites have recorded many incidents of this type. The opportunity for this type of incident
seems to be higher when dealing with older facilities/buildings. Configuration management for
these commodities can be just as bad in newer
buildings. Embedded commodities are often
omitted from as-built drawing requirements. Most installation drawings are diagrammatic, or
field routed commodity driven. This policy can provide significant problems in the years
following installation. It might be suggested to
design and build future buildings, where electrical
conduits are only allowed to penetrate through, but not allowed to be encapsulated in
concrete structure. Where this is impossible to accomplish, a red powder like dye should be
spread above the encapsulated conduit
at the time of the concrete pour to warn future workers
of the hidden dangers below. A inside marker program combined with accurate as-built
drawings of embedded power cable conduits is investing in their electrical safety program.
One significant enhancement in the concrete
excavation safety program has been the increased
use of electronic drill-stops. Used across the DOE Complex and in commercial/industrial
facilities, this safety instrument will interrupt power and stop the drilling process at first
encounter with a grounded metal object. Information
on several of these types instruments can
be found at www.lorien.com and www.drillco.com.
The use of insulated rubber gloves is recommended
for all workers drilling or cutting into
concrete walls and slabs, as an additional safety measure. Very few facilities have
management of embedded power cables that would preclude their use.
11.7.2 MACHINE OR HAND DIGGING
Machine digging in earth requires
careful consideration and a carefully defined “No-Dig Zone”
must be utilized. The identified depth of utilities is not accurate in many cases.
operators, in general, do not appreciate the value of maintaining the integrity of energized power
cables. They rarely understand
the purpose of the power feeds, so digging them up is not very
important to them. They are usually in a hurry to dig the trench and go to
another job. A strong
safety program, with specific procedures, can make the difference between a successful
excavation and an incident
A significant number of facilities require identified utilities to be positively located by hand
digging to uncover them.
That provides additional assurance of their location and depth of
burial. Many times, both dimensions are significantly different from those
marked by locating
equipment. Special tools and personal protective equipment are often used to accomplish this.