TRANSFORMERS AND TRANSFORMER VAULTS 70-453

and rated over 35,000 volts shall be installed in a vault. Such transformers installed indoors shall be furnished with a liquid confinement area and a pressure relief vent. The transformers shall be furnished with a means for absorbing any gases generated by arcing inside the tank, or the pressure relief vent shall be connected to a chimney or flue that will carry such gases to an environmentally safe area. (FPN): Safety will be increased if fire hazard analyses are performed for such transformer installations. For the purposes of this section, a nonflammable dielectric fluid is one that does not have a flash point or fire point, and is not flammable in air. 450-25. Askarel-Insulated Transformers Installed Indoors. Askarelinsulated transformers installed indoors and rated over 25 kVA shall be furnished with a pressure-relief vent. Where installed in a poorly ventilated place, they shall be furnished with a means for absorbing any gases generated by arcing inside the case, or the pressure-relief vent shall be connected to a chimney or flue that will carry such gases outside the building. Askarel-insulated transformers rated over 35,000 volts shall be installed in a vault. 450-26. Oil-Insulated Transformers Installed Indoors. Oil-insulated transformers installed indoors shall be installed in a vault constructed as specified in Part C of this article. Exception No. 1: Where the total capacity does not exceed 112½ kVA, the vault specified in Part C of this article shall be permitted to be constructed of reinforced concrete not less than 4 inches (102 mm) thick. Exception No. 2: Where the nominal voltage does not exceed 600, a vault shall not be required if suitable arrangements are made to prevent a transformer oil fire from igniting other materials, and the total capacity in one location does not exceed 10 kVA in a section of the building classified as combustible, or 75 kVA where the surrounding structure is classified as fire-resistant construction. Exception No. 3: Electric furnace transformers having a total rating not exceeding 75 kVA shall be permitted to be installed without a vault in a building or room of fire-resistant construction, provided suitable arrangements are made to prevent a transformer oil fire from spreading to other combustible material. Exception No. 4: Transformers shall be permitted to be installed in a detached building that does not comply with Part C of this article if neither the building nor its contents presents a fire hazard to any other building or property, and if the building is used only in supplying electric service and the interior is accessible only to qualified persons. Exception No. 5: Oil-insulated transformers shall be permitted to be used without a vault in portable and mobile surface mining equipment (such as electric excavators) if each of the following conditions is met: a. Provision is made for draining leaking fluid to the ground. b. Safe egress is provided for personnel. c. A minimum ¹/₄-inch (6.35-mm) steel barrier is provided for personnel protection. 70-454 NATIONAL ELECTRICAL CODE 450-27. Oil-Insulated Transformers Installed Outdoors. Combustible material, combustible buildings, and parts of buildings, fire escapes, and door and window openings shall be safeguarded from fires originating in oil-insulated transformers installed on roofs, attached to, or adjacent to a building or combustible material. Space separations, fire-resistant barriers, automatic water spray systems, and enclosures that confine the oil of a ruptured transformer tank are recognized safeguards. One or more of these safeguards shall be applied according to the degree of hazard involved in cases where the transformer installation presents a fire hazard. Oil enclosures shall be permitted to consist of fire-resistant dikes, curbed areas or basins, or trenches filled with coarse, crushed stone. Oil enclosures shall be provided with trapped drains where the exposure and the quantity of oil involved are such that removal of oil is important. (FPN): For additional information on transformers installed on poles or structures or underground, see National Electrical Safety Code, ANSI C2-1990. 450-28. Modification of Transformers. When modifications are made to a transformer in an existing installation that change the type of the transformer with respect to Part B of this article, such transformer shall be marked to show the type of insulating liquid installed, and the modified transformer installation shall comply with the applicable requirements for that type of transformer. C. Transformer Vaults 450-41. Location. Vaults shall be located where they can be ventilated to the outside air without using flues or ducts wherever such an arrangement is practicable. 450-42. Walls, Roof, and Floor. The walls and roofs of vaults shall be constructed of materials that have adequate structural strength for the conditions with a minimum fire resistance of 3 hours. The floors of vaults in contact with the earth shall be of concrete not less than 4 inches (102 mm) thick, but where the vault is constructed with a vacant space or other stories below it, the floor shall have adequate structural strength for the load imposed thereon and a minimum fire resistance of 3 hours. For the purposes of this section, studs and wall board construction shall not be acceptable. (FPN No. 1): For additional information, see Method for Fire Tests of Building Construction and Materials, ANSI/ASTM E119-83, and Methods of Fire Tests of Building Construction and Materials, NFPA 251-1990. (FPN No. 2): Six-inch (152-mm) thick reinforced concrete is a typical 3-hour construction. Exception: Where transformers are protected with automatic sprinkler, water spray, carbon dioxide, or halon, construction of 1-hour rating shall be permitted. 450-43. Doorways. Vault doorways shall be protected as follows: (a) Type of Door. Each doorway leading into a vault from the building interior shall be provided with a tight-fitting door having a minimum fire