Underground vaults sometimes serve areas with underground distribution lines. These vaults are generally located in urban areas. They contain much of the equipment found on overhead utility poles, including power lines, transformers, regulators and switches.
Overheated or burning electric cables in an underground vault can produce highly reactive gases, such as hydrogen, acetylene and ethylene. The gases may contain elevated concentrations of carbon monoxide and may be toxic and combustible. This is why even a small fire in an underground vault can create an atmosphere immediately dangerous to life and health (IDLH).