Mitigating the Effects of Explosions in Underground Electrical Vaults

Abstract

An explosion software package is used to assess the effectiveness of several simple and inexpensive safety devices that can minimize the dangers of explosions in underground electrical vaults. The software is capable of determining the forces on a manhole cover as high-pressure air and gases are expelled from the vault. This information is used to determine the feasibility of several devices designed to mitigate the effect of the explosion. The potential designs focus on modifications to the vault and manhole cover that limit the motion of the cover and reduce the severity of the explosive forces. The devices that are examined include bolts that fasten the cover to the vault, vented and lightweight manhole covers and covers that are attached to the vault by tethers. No single safety device will completely eliminate all of the dangers associated with an explosion. Rigidly attaching the manhole cover to the vault with bolts is not recommended, because a reasonable number of bolts are often not sufficient to withstand the high pressures that result when the cover is held down and the vault is unable to vent. If the bolts finally fail, the vault will vent at a high pressure and subject the manhole cover to potentially dangerous forces. A lightweight, vented cover restrained by elastic webbing, on the other hand, will permit gases to vent at a lower pressure and greatly reduce the hazards posed by the explosion. The use of a rigid tether such as a steel cable or chain is not recommended due to the excessive forces that the manhole cover will exert on the tether and the attachment points.