Electrical Equipment Vaults

The Canadian Electrical Code defines a vault as “an isolated enclosure either above or below grade with fire-resisting walls, ceilings and floors for the purpose of housing transformers and other electrical equipment.” This article discusses the CEC requirements for electrical equipment vaults designed to house flammable liquid-filled equipment.

Rule 26-012 of the CEC specifies that when installed indoors, transformers and other equipment containing more than 23 litres of flammable liquid in one tank or 69 litres in a group of tanks must be located in an electrical equipment vault.

Rules 26-350 to 26-356 provide some clarification and overall requirement for electrical equipment vaults as follows:

  • When used, the word “vault” means “electrical equipment vault.” In this article, I will use the word “vault.”
  • Vaults must not be used for storage and must be of sufficient size to meet CEC clearances from electrical equipment;
  • Vaults must be constructed in accordance with the requirements of the National Building Code of Canada; and
  • Adequate lighting and receptacles must be provided.

The National Building Code of Canada specifies the requirements for construction of vaults containing flammable liquid-filled equipment in buildings. Vaults must be constructed of solid masonry or concrete having a fire resistance rating of at least 3 hours. Fire resistance rating may be reduced to 2 hours if the vault is equipped with a fire extinguishing system.

When a building is required to be sprinklered, a vault within the building need not be sprinklered if the vault contains only electrical equipment, and a smoke detector in the vault actuates the building fire alarm system in the event of a fire in the vault.

Vaults must have reinforced concrete roofs or ceilings and floors minimum 150 mm thick. Floors at excavation level may be a minimum 100 mm thick. Walls, ceilings and floors must be anchored together so that no damage will result due to an explosion. The floor must be liquidtight and the electrical equipment surrounded by sills of sufficient height so as to contain all of the flammable liquid, with a minimum height of 100 mm. A floor drain must be installed to drain any liquid spillage to a closed sump of sufficient size to contain all of the flammable liquid.

Vaults must have ventilation systems to prevent the ambient temperature from exceeding 40°C. When natural ventilation is provided from outdoors, the total areas of wall inlet and outlet openings must be at least .002 square mm per kVA of the equipment contained in the vault. In the case of power transformers, the ventilation requirements may be based on the full load losses of the transformers. When electrical equipment is used exclusively for emergency backup and is not normally energized, ventilation of the equipment need not be considered for design of the ventilation system.

When a vault is ventilated by outdoor air, the bottom of the fresh air inlet must be at least 1000 mm above the floor level in the vault. Screens over ventilation openings must be fastened so that they cannot be removed from outside the vault.

When mechanical ventilation is provided, it must be separate from the building’s ventilation system and:

  • The temperature in the vault is thermostatically controlled;
  • The ventilation fan is located so that it can be serviced without any danger to servicing personnel;
  • A high temperature alarm is provided and controls to shut off the ventilation system in case of a fire in the vault; and
  • The fresh outside air inlet is filtered if necessary.

As with previous articles, you should always consult with the electrical inspection authority in your province or territory for a more precise interpretation of any of the above.

Leslie Stoch
Leslie Stoch, P. Eng, is principal of L. Stoch & Associates, providing electrical engineering and ISO 9000 quality systems consulting. Prior to that, he spent over 20 years with Ontario Hydro as an electrical inspection manager and engineer. Les holds a B. S. in electrical engineering from Concordia University in Montreal.