Always know the voltage of the system you plan to install, and review the applicable Code sections.
It seemed like just another rainy day for a laborer working alone on a small residential job. A homeowner had hired him to install a large flagpole on the property. However, this simple task quickly took a turn for the worse. As the worker began to dig a hole, his shovel cut through the insulation of a cable and contacted an energized power line 19 in. below the ground. Although the worker survived, he suffered shock injuries and debilitating burns. Luckily, a groundskeeper working nearby witnessed the incident and immediately went for help.
After the accident, the victim's attorney hired my forensic engineering firm to investigate the situation and decide who was at fault. We knew what happened — the wet shovel had served as a path for electricity to flow from the energized conductor, through the shovel, through the laborer's body, and then back to ground via his wet shoes. What we needed to uncover was how it happened, so our investigation would center on the buried cable installation.
Unfortunately, the documents sent to us by the attorney did not identify the voltage of the conductor. Without the knowledge of the precise voltage, we initally advised our client of sections of the National Electrical Code (NEC) applicable to various voltage levels. In the event the power line did not exceed 600V, Sec. 300-5 of the Code was the controlling factor. Cables installed in a trench, per Sec. 300-5(a) of the Code must have a “cover” of earth that varies in depth with the voltage of the line. To meet the requirements of this rule for a power line rated at less than 600V, the installer must cover the line with at least 18 in. of earth from the top of the cable.
In addition, the NEC states that the location of specific power lines (service-laterals) not encased in concrete and buried 18 in. or deeper must be identified by a warning ribbon placed in the trench at least 12 in. above the underground installation. However, an exception to Sec. 300-5 states: “Where the enclosure or raceway is subject to physical damage, the conductors shall be installed in rigid metal conduit, intermediate metal conduit, or Schedule 80 rigid nonmetal conduit or equivalent.”
Our follow-up investigation revealed the cable's voltage actually exceeded 600V. We decided to check out the installation history. At the time the cable was placed in operation, the 1996 version of the NEC was in effect (with Art. 710 as the controlling factor). Table 710-4(b) outlines minimum cover requirements for underground cables rated 600V to 22kV. This requirement offers three separate burial criteria:
- Direct burial 30 in.
- Rigid nonmetallic conduit approved for direct burial 18 in.
- Rigid metal and intermediate metal conduit 6 in.
Although this rule offers various exceptions, none of them applied to this case. The line in question, which someone installed 19 in. below ground, violated the 30-in. minimum burial depth stated in Table 710-4 (b) by 11 in. The initial specification and schematic of the underground cable installation show the installer should have recognized this was a high-voltage line (over 600V). However, by installing the line at a depth of 19 in., he failed to follow the correct section of the Code.
In our report, we noted the installer had the opportunity to provide and install a warning ribbon 12 in. above the cable, as well as provide protection to the cable by placing it in pipe or Schedule 80 PVC. Unfortunately, he did neither.
During our investigation, we identified the contractor that installed the cable and passed this information along to the client. We also briefed the client on the pertinent sections of the 1996 and 1999 versions of the Code. Furthermore, we explained the contractor neither provided a warning ribbon nor encased the cable with appropriate protection.
Using our report, the attorneys resolved the issue through a settlement conference. Once they reviewed both sections of the Code, the respective parties recognized the installer also violated the mandated depth protection.
What's the most valuable lesson to learn from this case? Contractors should always make sure they know the voltage of the system they plan to install. The incident also illustrates the fact that a casual review of the Code can sometimes be misleading. Thorough examination of the Code is always recommended before beginning an installation project. In this case, a failure to recognize the system's voltage and subsequently applying the wrong section of the Code resulted in an error that caused serious injury to an innocent party.
Pritzker is a senior forensic engineer in Punta Gorda, Fla.