Drilling at construction site hits two high voltage lines causing power loss

Incident Investigation

Drilling at construction site hits two high voltage lines causing power loss

October 19, 2018


Reference Number:

II-762488-2018 (9225)

Incident overview

Test monitoring holes were being drilled prior to building construction. During drilling, 2 high voltage lines were hit causing the breakers ahead of them to trip and power to the building to be lost.

Regulated industry sector

Electrical - High voltage electrical system (greater than 750V)



Investigation conclusions

Site, system and components

Underground power lines distribute electrical power throughout the grid. As-built drawings keep a record of where these lines are located. There are also electronic and radar methods of locating these lines.

Failure scenario(s)

The 12.5 kv underground line was not located by the company marking the drill locations.

Facts and evidence

Statements from building and other investigating personnel.
A subcontractor who was hired to drill monitoring holes contacted the underground 12.5 KV conductor.
The employer's representative (Project Manager) who was coordinating the project on behalf of the building owner produced drawings which identified approximate locations of the electrical service, however, the depths and the exact location were not known.
It was also reported that this employer hired an outside contractor to conduct a survey on the locations but could not detect the locations with their equipment.
It was also determined that the monitoring well bore holes did not have to be drilled where contact with the underground electrical utility occurred but could have been drilled in other locations well away from the service.

Causes and contributing factors

It is highly likely that lack of communication between identifying the location of the conductors and the actual drilling of the holes.was the contributing factor.


  • Injury
    • Qty injuries: 0
    • Injury description: N/A
    • Injury rating: None


  • Damage description: 2 underground high voltage lines were damaged during drilling of monitoring wells in preparation for new construction.
  • Damage rating: Moderate
  • Incident rating: Moderate
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