There are hundreds of thousands of man-accessible vaults in North America, with potentially tens of thousands of utility worker entries into those vaults each year. And it’s likely that every worker who enters a vault appreciates the safety procedures that govern the work. The combination of high-voltage electrical cables and aging infrastructure can exponentially complicate even the most routine vault-related task. In addition, many utilities across North America continue to report electrical vault failures, some of which lead to violent explosions.
For the most part, utility owners have a good understanding of the risks of entering man-accessible vaults and conducting work inside of them. There are many stories and equally as many opinions as to the safety and stability of the underground electrical network. The intent of this article is to summarize some known conditions your employees may face during execution of work in underground vaults. Although explosions may constitute the bulk of catastrophic events, thorough consideration of all hazards should be included in risk analyses.
There is no uniform standard pertaining to vault configurations, but utilities regularly have an engineering standard. Man-accessible spaces can be as shallow as 8 feet deep with 340 cubic feet to approximately 30 feet deep with 3,000 cubic feet. Each vault is connected to others in the underground system through ducts and can have one to several high-voltage cable circuits passing through it. Some cables pass through directly while others contain splices, connections, transitions and some high-voltage switchgear or similar equipment. Most common cables are cross-linked polyethylene – often referred to as XLPE – or lead cable circuits. There is the potential for other systems to be present in spaces associated with lower voltages and communication cables. Some vaults have standard manhole lids while others have latch plates. The number of combinations is endless, but hazard exposure in these spaces is similar and can be categorized into exposure tables. When conducting your risk assessments, it is generally accepted in most jurisdictions to group your spaces by similar configurations of space and type. This will help organize information, reduce the volume of documentation and provide your field crews with clear data to safely perform their work.