I was recently asked to provide information about the challenges and opportunities found when working on direct-buried underground distribution (UD) systems. In light of that request, I’ll address those topics in this installment of “Voice of Experience.”
My first opportunity to work on UD systems was as a truck driver operating a trencher in the late 1960s. UD systems were fairly new at the time; lineworkers were learning new techniques, using different types of tools to terminate cables and installing switchable elbows. In that day, some elbows were non-load-break. Back then the work was all about proper use of tools, identifying equipment and following the minimum rules. There were no OSHA regulations. We learned many techniques and work practices the old-fashioned way: through the school of hard knocks.
The challenges that workers faced back then are much the same as they are today, with two exceptions: The industry has more experience installing and operating UD systems, and equipment is now much more technically sophisticated and reliable. For many years, maintenance of UD systems was nonexistent. The common approach was to dig a ditch and put cable in the ground, and industry workers believed everything would last forever. That belief was short-lived; within a few years, external concentric neutrals began oxidizing, and radial and loop-fed systems suddenly became single-conductor, earthen-ground return systems. Driven ground rods at transformers split coil for secondary voltages. There was no neutral conductor for return currents or fault current flow.
Oxidation of copper concentric neutrals was first noticed because of open neutrals on services, and it caused damage to customers’ appliances and equipment. All 120-volt equipment suddenly was exposed to 240 volts because the neutral had been opened. Utilities replaced or repaired many televisions and small appliances. The loss of the neutral also introduced workers to stray voltages and return current flows on plumbing, telephone messengers, fences and other conductive paths.
The absence of the neutral on UD primary cable permitted higher voltages on telecommunications cables and other utilities. Employees other than those of electric utilities were now challenged with new hazards they had never seen before. Also, with neutrals missing in electric circuits, the primary conductors were subjected to maximum numbers of fault currents to blow a fuse or trip a circuit relay because ground relays could not realize the faults. Ground relays were normally set at a lower value to operate the system faster, thus reducing the electrical stress on the system.
At that point, the challenge to utility companies was to start replacing external concentric neutral primary and secondary conductors with insulated weatherproof cables to stop neutral failures – very expensive and time-consuming projects. Thousands of miles of primary and secondary cables had been installed by this time. Many companies started major reconductoring and replacement after weatherproof primary cable and insulated neutral service cables were introduced around 1978.
From the early days of UD systems until now, many utility companies changed engineering and design practices to have all UD cables installed in conduit. This practice increases protection of cables from exposed direct-buried cables affected by rocks and debris. It also is a quicker and less expensive conductor replacement when there is a cable failure, if the conduit stays intact and undamaged. Proper installation of cable in conduit is critical to ensure reliability in the future. Some low-lying and wet-location conduit systems flood. Manholes and slice pits fill with water, which presents additional issues affecting the safety and reliability of the electrical system. This submersible equipment presents unique challenges, such as corrosion and difficulty identifying parts of switch gear for employees performing switching on the system. There is a history of failures by employees to properly identify switches and cables that operate UD equipment, which has contributed to accidents, flashes and even fatalities. Tagging cables and identifying switch numbers are the first steps to properly identifying and isolating cables and equipment. Verification of absence of voltage with a properly rated voltage detector is required by regulations and industry. The failure to do so can be deadly.
I have had to investigate numerous accidents involving switching errors that resulted in injuries. In many cases, employees failed to follow all the required steps to identify and isolate cables and equipment, including failure to follow the last step: verifying the absence of voltage. Tagging is the proper procedure to mark cables and equipment for identification. Misidentification is often due to improperly marked cables. Cable tags are installed by the contractor or utility employee who installs the cable and terminates cables at equipment. Many times, cables are marked with colored tape at installation, verified by either a phone conversation or with an ohm meter/buzzer to ensure proper identification for cable tagging. Errors occur either then or later, when cables are spliced or damaged and repaired, and by not repeating the cable verification procedure. I have always taught students in my classes that the tag only means that someone wrote on it or that the marker worked that day. An employee using the tag only for identification without checking for absence of voltage is not only failing to comply with regulations, but he or she also is engaging in hazardous behavior at the very least.
Secondary cables are no different. Employees are required to properly identify and isolate service cables by similar methods; verifying the absence of voltage should be the one of the last steps. Multiple services in the same direct-buried trench can be damaged, and isolating the correctly identified service may not de-energize the service in question. About seven years ago I became familiar with a situation involving a near miss that could have resulted in a secondary contact if the employee had failed to check for absence of voltage after the service was disconnected at the transformer. Multiple services in the trench were damaged, resulting in voltage conducted between the services that remained energized. Three of the four services were damaged due to horizontal direct boring by another telecommunications contractor, and only one hot leg of one service was completely open. The other damaged services energized the isolated service after it was disconnected. There was a fault, but not enough fault current to blow the UD padmount fuse.
Paralleling of service cables also can be hazardous if the cables are improperly marked. Open delta-banked transformers with three-phase, four-wire services can be hazardous if transformers are not correctly installed and service cables aren’t correctly tagged.
Many hazards that are visible on overhead systems are unseen on UD systems. Correct procedures must be used to ensure the safety of electric utility lineworkers.
About the Author: Danny Raines, CUSP, safety consultant, distribution and transmission, retired from Georgia Power after 40 years of service and opened Raines Utility Safety Solutions LLC, providing compliance training, risk assessments and safety observation programs. He is also an affiliate instructor at Georgia Tech Research Center OSHA Outreach in Atlanta. For more information, visit www.electricutilitysafety.com.