For more than 100 years, Commonwealth Edison – commonly known as ComEd – has been powering the lives of customers across Northern Illinois, including those in Chicago, a city that has thousands of circuit miles of medium-voltage distribution cables installed in conduit and manhole systems.
Over the decades, ComEd’s underground cable splicers have experienced failures of distribution cable system components, including cables, joints and terminations, while employees were working in manholes and vaults. A large number of cable system failures occurred at cable joints in underground manholes. Some of these failures were due to degradation of the electrical connection inside these joints.
One of the hazards associated with a cable system failure is the risk of employee exposure to an electrical arc flash. This type of event can result in temperatures in excess of 35,000 degrees Fahrenheit, producing a blinding flash and causing aluminum and copper cabling components to instantly expand. If an employee is working adjacent to equipment affected by the blast, the heat generated can cause third-degree burns, and the pressure wave can damage hearing and throw the worker into the surrounding structure.
A Culture of Safety
Past experience at ComEd has demonstrated that thermal issues with joints are centered on mechanical connections, typically those that are crimped. Such mechanical connections are used in pre-manufactured joints.
According to OSHA 29 CFR 1910.269(t)(7)(i), “hot localized surface temperatures of cables or joints” are an abnormality that may be indicative of an impending fault. Unless the employer can demonstrate that the conditions could not lead to a fault, “the employer shall deenergize the cable with the abnormality before any employee may work in the manhole or vault, except when service-load conditions and a lack of feasible alternatives require that the cable remain energized.” However, “employees may enter the manhole or vault provided the employer protects them from the possible effects of a failure using shields or other devices that are capable of containing the adverse effects of a fault.”