Solution Summary: Using Cable-Locating Device
Using cable locating devices is a work practice that can help to minimize the risk involved in construction operations which can expose workers to inadvertent contact with underground cables. They are hand-held equipment that uses radio frequency signals to locate cables accurately. These devices have an audible response as well as a visual response to help verify the locate and to help give an estimated depth.
Appropriate cable-locating devices must be used in conjunction with plans and other safety practices to pinpoint the location of underground cables as accurately as possible in the work zone. In the following, safe practices are provided to obtain as much information as possible about underground cables before any excavation and ground penetration work begins. There are three main elements of safe practices for ground work: informative plans, cable locating devices, safe digging practices.
1- Informative Plans
Informative plans that provide comprehensive information about all underground utilities, including locations of cables in the area, are necessary before starting excavation. Updated materials and as-built drawings should be passed out to site supervisors as soon as possible. Although attention must be paid to the evidence of underground cables on the plans, the absence of such indicators does not necessarily mean that the cables do not exist. To determine the actual position of cables, other techniques and tools must be used, including cable-locating devices, new Geospatial Augmented Reality technology, or a hand-digging technique.
IMPORTANT: Plans are rarely drawn to the right scale, and should not be relied upon alone; other practices must be used to obtain accurate distance measurements. Moreover, since these maps are low-scale, they may not give a reasonable indication of actual distance. Snake-like cables are presented as straight lines on maps, and the routes for old cables often have not been recorded and are not available in the plans. Service cables usually are not shown in plans (e.g., low-voltage cables, lampposts, traffic lights, telephone kiosks, etc.). Furthermore, these plans do not give information about the depth of the cables, which may be found in shallow depths.
2- Cable Locating Devices
Appropriate cable-locating devices must be used in conjunction with plans and other safety practices to pinpoint the location of underground cables as accurately as possible in the work zone. Even when the cable-locating devices cannot detect cables, lines still may be present underground. It should be noted that these locators cannot detect plastic pipes.
Hum Detector: A hum detector is a cable-locating device that operates as a radio frequency detector set on power mode. It works as a receiving tool for the electromagnetic field radiated by live line cables (cables with current flowing through them). This device will easily detect medium- to high-voltage currents but cannot detect unoccupied premises or street lighting cables during the daytime when there is little or no current flowing through them. Some high-voltage cables may not be detected because they radiate only a small electromagnetic field.
IMPORTANT: The absence of current in the cable does not necessarily mean it is not dangerous, and the risk of injury still exists if the cable is damaged. Since some of the underground cables are filled with oil, damaging cables can ignite the oil.
Figure 1. (a) Armada Pro900 digital cable locator (Source: http://armadatech.com); (b) Subsite Electronics 830R/T utility locating system (Source: http://subsite.com); (c) Ridgid SR-24 line locator with Bluetooth® and GPS (Source: https://www.ridgid.com); (d) The Subsite Electronics 830R/T utility locating system in use (Source: http://subsite.com)
Radio Frequency Detector: The radio frequency detector is a cable-locating device that receives low-frequency radio signals that may be absorbed and re-emitted through other cables and metallic pipes. The results from using this instrument may vary depending on locality, length of the cable below the surface, distance from the termination and geographical orientation.
While hum detectors (e.g. cable-locating devices set on power mode) are the easiest devices to use, they do not respond to unloaded and low-voltage current cables. Furthermore, they may fail to detect lightly loaded low-voltage cables (such as those used for street lighting) and well-balanced high-voltage cables. A locator with a radio frequency detection mode may detect these cables and therefore should be used as an additional backup device with a hum detector.
Ground-Penetrating Radar: This device not only detects live line cables, but also may detect any anomalies in the ground. It cannot determine the nature of the anomaly under the ground; it should be used in conjunction with plans and other informative documents to find out about these anomalies.
Figure 2. (a) Amprobe AT-3500 underground cable locator (Source: http://www.amprobe.com); (b) Subsite Electronics 2450GR ground penetrating radar (Source: http://subsite.com)
Underground Fault Finders: Fault finders can locate faults, shorts or any ‘events’ disrupting or damaging the cable.
Figure 3. (a) Armada Pro400 handheld graphical TDR fault finder (Source: http://armadatech.com); (b) Armada GFL3000 ground fault locator (Source: http://armadatech.com)
3- Safe Digging Practices
Most of the time, there will be no certain evidence or permanent surface markers to indicate the presence of cable under the earth. Even if no evidence appears on a plan or is detected with cable-locating devices, there is still the risk of existing underground cables that can be located one meter under the surface or just below the surface. So even shallow excavation may run the risk of hitting live lines. Occasionally, cables are encased in steel or plastic and covered with tile or bricks above them. However, the ways that lines are laid may be different. These factors highlight the importance of using safe digging practices. Handheld tools and mechanical excavators should not be used in the vicinity of cables; instead insulated hand tools (spades, shovels, etc.) must be used. If any cable has been exposed, all appropriate precautions must be taken to prevent damage and injuries. Any damage to electricity cables must be reported immediately for de-energizing and other safety actions.
These three elements complement each other and all should be used to ensure minimal risk in the work zone. Using appropriate PPE also must be considered when working on the ground.
Before starting any operation on the subsurface, it is necessary to look at the informative plans, scan the area with cable detectors, and look for any sign that lines are present underground in order to prevent accidental contact with underground cables. Enough time must be spent to mark the locations of underground cables before starting excavation or digging. After locating cables, insulated tools must be used to work in the ground. Manual excavation is better, but there still is a risk of hitting underground lines unexpectedly. These precaution practices prevent hazards and ultimately eliminate the risk of slowing down the project.
Contact with electric current is a major cause of injury and death among construction workers (Janicak 2008). In 2012, the Census of Fatal Occupational Injuries (CFOI) data produced by the Bureau of Labor Statistics (BLS) indicated that contact with electric current was the fourth leading cause of work related deaths—after falls, transportation incidents, and contact with objects and equipment (BLS 2012). Electricity can cause electric burns, electrocution, shock, arc flash/blast, fires, and explosions. Electrical hazards may happen due to failure to detect the accurate location of cables before construction site excavation, jack hammering, well drilling, landscaping, trenching for piping and fence installation – all of which can lead to accidental contact with power cables, resulting in electrocution, shock or burns (Figure 1). The risk of working near cables must be assessed and controlled.
Figure 4. Examples of causes of electrical hazards
The risks of coming in contact with underground lines are the same as those associated with overhead power lines. Underground lines lie at various depths below the surface that are not easy to determine. One of the main dangers in digging and excavation activities is damage to underground cables, which can cause explosive effects of arcing current and associated fire or flames. This will happen if a sharp object penetrates the conductor insulation and the cable is severely crushed, potentially causing severe fatal burns, electrocution, and direct electric shock. In addition to the risk of injury, hitting underground cables can result in excessive repair costs. Furthermore, since some of the underground cables are filled with oil, damaging cables can ignite the oil.
How Risks are Reduced:
The risks of coming in contact with underground lines can be addressed by using cable-locating devices. These devices use radio frequency technology to receive the electromagnetic field radiated by live line cables in order to pinpoint the location of underground cables. It should be noted that the cable-locating devices must be used in conjunction with informative plans and other safe digging practices to locate underground cables as accurately as possible in the work zone.
Effects on Productivity:
Even hitting and making contact with less dangerous utilities can cause various risks and damages, which can result in huge costs and reduced productivity. Using cable-locating devices can increase productivity by minimizing the risk of hitting underground cables during excavation activities.
- Permanent structures for buildings should not be erected over underground cables, since they prevent future access to these utilities and create additional risks for construction workers. Therefore, if the structure must be erected there, the cables must be relocated before construction begins.
- Some other solutions and precautions to take during work with underground power lines:
- Designate a person to oversee all excavation procedures and fully understand electrical hazards at the jobsite.
- Use a written excavation procedure to keep everyone on the same page.
- Require surveys of the area before any excavating or trenching and develop an interference map of the jobsite.
- When using subcontractors, ensure that all plans and requirements are updated by them.
- Here are some guidelines for looking for evidence of underground cable before excavation:
- Review all available informative plans of the original site drawings, which mention all hazardous areas. Be aware that they might not be updated.
- Perform a walk-through of the site surrounding the excavation area and look for visible conduits and raceways that could cross into the construction zone.
- Look for newly disturbed ground, such as manholes, new asphalt, other features not mentioned on the site prints, or disturbed soil, which may be signs of potential unmarked hazards.
- Here are some guidelines about how to prevent injuries when underground cable locations are identified:
- De-energize the lines (if possible) when working around them to reduce the risk of electric shock.
- Hand excavate near the marked locations; machinery is harder to control and can accidentally damage the line when working around it.
- Use appropriate PPE when working with any cables.
- Use double-insulated electric tools.
- Use fiberglass-insulated shovels and picks when excavating by hand.
Behzad Esmaeili, Ph.D. - University of Nebraska- Lincoln
Sogand Hasanzadeh - University of Nebraska- Lincoln
Pouya Gholizadeh - University of Nebraska- Lincoln
Erik Bruening - University of Nebraska- Lincoln
Brett Farquhar - University of Nebraska- Lincoln
To obtain information, visit http://www.amprobe.com/amprobe/usen/home/ or contact 1-877-267-7623
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To obtain information, visit https://www.ridgid.com or contact 1-800-474-3443
Pipehorn Locating Technology
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Schonstedt Instrument Company
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Leica Geosystems Inc.
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Armada Technologies, LLC
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