Increasing concerns over fire safety in and around buildings can result in questions about transformer installations, specifically for liquid-filled transformers. Also, installation requirements per the National Electrical Code (NEC) and the National Electrical Safety Code (NESC) as well as insurer and listing requirements can influence your decision as to the type of transformer specified. The following will answer these questions and help you make informed decisions.
A variety of requirements may be applied to a transformer installation, but each one must be considered for the application. We'll focus our attention on the following:
* The NEC (1993 and 1996 editions), which is generally enforced by local authorities responsible for building and/or electrical codes;
* Listing requirements, which are based on third-party evaluation of the transformer or components and are specific to the product and manufacturer;
* Insurer requirements, since the failure of a transformer can result in property damage, loss of life, and loss of revenue (due to downtime) for the business affected by the failure; and
* Local amendments, which the local code enforcing body may have adopted, additional requirements above and beyond the basic installation Code.
Transformer installation requirements in the 1993 NEC
In order to evaluate the specifics of NEC requirements on transformer installations, you should review the requirements for each transformer type. To show the differing requirements related to various transformer types, we'll cover the specifics for less-flammable liquid-, non flammable liquid-, Askarel-, and oil-filled transformers.
Vaults. In determining the type and location of a transformer in a design, the requirement to have a transformer vault plays a key role in the cost and safety of an indoor installation. The expected construction costs and space impacts of a vault must be considered before selecting the particular transformer to use. So that you can understand the magnitude of the vault issue, the following requirements for transformer vaults are summarized from Part C of Article 450 in the NEC.
Walls, roofs, and floors shall be constructed of materials providing a minimum fire resistance rating of 3 hrs. Typical of this rating is 6-in.-thick reinforced concrete.
Floors in contact with the earth shall be 4-in.-thick concrete minimum. If located with space below, the floor shall have structural strength for the expected load and have a fire resistance rating of 3 hrs.
Doorways leading to the interior of a building from the vault shall have a 3-hr construction. A door sill or curb must be provided that will confine the oil from the largest transformer within the vault. Minimum sill height is 4 in.
Doors shall be equipped with locks and accessible only to qualified personnel. Doors shall swing out and be equipped with panic bars, pressure plates, or other devices that are normally latched.
Ventilation (per specifics in Sec. 450-45): The vault must be ventilated to remove the heat generated by the transformers during operation so as not to create a temperature rise beyond the transformer rating.
Drainage: Vaults containing more than 100kVA of transformer capacity shall be provided with a drain or other means that will carry off any accumulation of oil or water in the vault unless local conditions make this impracticable. The floor shall be pitched to the drain where provided.
Note: If the transformer is protected with an automatic sprinkler, water spray, carbon dioxide, or halon, the entire vault construction is permitted to be of a 1-hr rating.
As you can see, there's a substantial investment in construction costs and physical space when a transformer vault is needed. However, by utilizing the following additional safeguards, a vault may not be necessary.
(by Loyd, Richard E)
to be continue...