Thermodynamic and mechanical analysis of short circuit events in an underground vault

Abstract

The occurrence of an electric arc in an underground vault can be a very catastrophic event. The sudden release of a large amount of electric energy associated with the arc can cause the air pressure in the vault to reach levels that can damage the vault and the event can result in personal injury. This paper describes a computer model that is capable of calculating the internal pressure and temperature in the vault throughout an arcing event. The pressure is then used to predict the forces on the manhole cover and ultimately the motion of the cover if it experiences a pressure sufficient to dislodge it from its seat. With the forces known, the program can be used to evaluate modifications to the vault design in an attempt to improve system safety. The program results are compared with experimental measurements conducted for arcing events that were carried out inside rigid test chambers. Results are generated for a series of arc strengths and vault geometries that can be expected to exist in an actual underground vault that contains distribution cables. The generated results are then used to suggest mitigations schemes that would lessen the dangers of the event.