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DOE Handbook
Electrical Safety
- Electrical Safety During Excavations -
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11.0 ELECTRICAL SAFETY DURING EXCAVATIONS
This section 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.
11.1 GENERAL
Occurrences involving unexpected contact with underground or embedded utilities during
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 with
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
i. DOE-STD-1073, Configuration
Management
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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
embedded utilities.
11.3.1 CONFIGURATION MANAGEMENT
a. Facility controlled drawings identifying utilities locations.
- Voltage levels, burial depth, and
elevation details for “stacked” utilities are useful
information to include on these drawings.
b.
As-built drawings/sketches are often required to be submitted for all new utilities
installations.
c. Survey point locations are required at some DOE facilities.
- This provides very accurate locations for
future reference.
- Some DOE facilities utilize Global Positioning Satellite (GPS) equipment to mark
utility locations.
d. An permanent above ground, marking program also provides excellent ongoing visual
safety awareness reminder.
11.3.2. EXCAVATION PERMIT
The 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 is maintained.
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
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f. Controlled length of validation time period after which a revisit and remarking of the site
becomes necessary.
11.4 UTILITIES VERIFICATION AND MARKING
It is important to establish and maintain
policies and procedures to assure accurate and
effective 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
identified. 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 radar method.
− 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
excavation work.
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
excavation work.
f.
Utilities color coding
− same Utilities & Transportation Commission (UTC) color coding as municipalities
use.
− offsite subcontractors are very familiar with this work practice.
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11.4.1 FIELD LOCATION OF EXCAVATION BOUNDARIES
Accurate identification of excavation boundaries
is absolutely necessary. It accurately
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 markings
can easily be
transferred from these maps)
b. Excavation maps can be attached to the excavation permits for additional
communication assurance.
c. Field excavation boundaries should be appropriately marked.
11.4.2 LOCATOR EQUIPMENT SELECTION AND LIMITATIONS
It is important to note that no locator technology
should be relied upon as the sole source for
identifying buried or embedded utilities. Configuration management
is the most effective
identification method.
The majority 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
from 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
from experience.
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 discrete broadband
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 one
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
cases.
INDUCTIVE 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
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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
insulated sections 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
utilities bonded 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
grounding point 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
other utilities 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, as
the frequency increases, the available power decreases.
Frequency selection can affect depth of
penetration, distance of travel, resonance efficiency,
and resolution issues, as well as other operating
characteristics. Most locator manufacturers
preset frequencies, and there will be an optimum frequency to use
for a particular utility system.
These are some of the most frequently used :
a. ELF (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
electromagnetic methods will not always be the answer. The current family of surface
geophysical methods
available for utility and other near-surface structure or hazard
detection is extensive. Other electromagnetic
techniques such as measuring eddy
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currents, differential heat, and thermal reflection are also available. Here are three types
presently in use :
a. Terrain 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 and
amount of time necessary to bounce signal off
different layers. This technology
requires resistant 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 produce their
own exothermic heat or retain heat longer than the surrounding soil.
Choosing the appropriate locator technology and methods will greatly improve the chances
of success.
11.4.3 LOCATOR OPERATOR TRAINING
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 dependent on
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 workers.
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,
as 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.
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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
damaged.
Metallic
Utilities 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.
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
sweep. Direct 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.
Nontraceable 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
and 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 the
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
Utilities identified
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
signal/monitoring circuits, 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
during the 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 PROCEDURES
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.
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11.7 THE EXCAVATION PROCESS
Once the planning is complete, the excavation process is ready to
begin. Work control
procedures 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
Excavations 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 or
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 the
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
configuration 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. Machine
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 or accident.
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.
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