Transformers installed interior to a building require special consideration for their placement because of the potential hazard should a catastrophic failure occur. The objective of the following rules is the practical safeguarding of people and property.
Less-flammable liquid-insulated transformers. The specific NEC requirements for these transformers can be found in Sec. 450-23. Transformers qualifying for placement in this category must contain a liquid with a fire point not less than 300 [degrees] C. The flow chart in Fig. 1 (on page 75) describes an overview of the requirements for this section.
Nonflammable fluid-insulated transformers.The specific NEC requirements for these transformers can be found in Sec. 450-24. Transformers in this class must have a nonflammable dielectric fluid that does not have a flash or fire point and is not flammable in air. The flow chart in Fig. 2 describes the requirements for this section.
Askarel-insulated transformers. NEC requirements for these transformers can be found in Sec. 450-25. Transformers in this class are insulated with Askarel fluid. If the transformer is rated over 25 kVA, it must be provided with a pressure-relief vent. If installed in a poorly ventilated area, the transformer must be provided with a method to absorb any gases generated by arcing inside the case, or the pressure relief vent must be vented to the outside of the building.
As with less- and non flammable liquid-filled transformers, those filled with Askarel are not required to be installed in a vault unless rated greater than 35kV. The requirements in this section have remained unchanged for many years, since no new installations have been made in recent years.
Oil-insulated transformers. The specific NEC requirements for these transformers can be found in Sec. 450-26. In general, these transformers always must be installed in a vault. Exceptions to the vault requirements are as follows.
* If the total transformer capacity is 112 1/2kVA or less, the vault may be constructed of 4-in. reinforced concrete.
* If the nominal voltage is 600V or less, the vault may be omitted if arrangements are made to prevent a transformer oil fire from igniting other materials, and the total transformer capacity does not exceed 10kVA in a section of a building classified as combustible or 75kVA in a section of a building classified as fire-resistant construction.
* Electric furnace transformers can be installed without a vault where the total rating does not exceed 75kVA, and arrangements are made to prevent a transformer oil fire from igniting other materials. These transformers are very specific to applications involving electric furnaces, and since many precautions must be taken for the electric furnace itself, the vault requirement is relaxed.
* Transformers are permitted to be installed in a detached building that does not comply with the vault requirements 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.
* The vault may be omitted for transformers used in portable and mobile mining equipment with additional conditions specified.
(by Loyd, Richard E)
to be continue...