In discussing the requirements of various codes on the installation of transformers using less-flammable liquids, you'll encounter a requirement to comply with the listing of the liquid. Due to this requirement, it's important that you understand the various listing agency requirements for these units. In our discussion here, the use of the term "listing" or "listed" follows the basic definition from the National Electrical Code (NEC). This definition reads as follows:
Listed: Equipment or materials included in a list published by an organization acceptable to the authority having jurisdiction and concerned with product evaluation, that maintains periodic inspection of production of listed equipment or materials, and whose listing states either that the equipment or materials meets appropriate designated standards or has been tested and found suitable for use in a specified manner.
Less-flammable liquids are currently listed with Underwriters Laboratories (UL) and FM.
Factory Mutual. FM recommends certain practical and effective mechanical and electrical protection schemes for particular types of transformers. Fire protection is a base level of protection that FM requires for transformer installations that expose buildings or other property to potential damage. However, FM also gives consideration to omitting fire protection if the transformer installed is a less-flammable type with proper electrical and mechanical protection and if it is not installed as a network transformer.
Protection systems should be determined by an engineering study that considers the criticality of the supplied loads and the level of fire exposure that may be presented by a transformer failure. Various schemes of electrical protection can be employed from normal overcurrent protection to the inclusion of differential relays, ground relays, etc. FM's guidelines give varying degrees of overcurrent protection required based upon the transformer kVA and the supply configuration.
Per FM, all indoor transformers should be installed at least 3 ft from building walls, and containment systems should be provided for the transformer liquid in case of tank rupture. The containment area should be capable of containing the liquid from the largest transformer located within the space.
For transformers that are FM approved, the following elements are required in order to install the transformer without additional fire protection:
* Tank design strength to prevent tank rupture under low energy fault conditions;
* A pressure relief device to relieve pressure if a low current fault occurs until it can be cleared by the electrical protection;
* Electrical protection in the form of a ground fault relay, sudden pressure relay, or other device of equivalent reliability to clear sustained low current faults;
* A liquid with a firepoint greater than 300 [degrees] C; and
* Electrical protection to clear high current faults. This protection is based on the liquid volume of the transformer and is intended to electrically isolate the transformer [TABULAR DATA FOR TABLE 2 OMITTED] rapidly enough to prevent pressure increase to greater than half the tank burst pressure.
If the transformer is used on a network system, FM also requires that the transformer be installed in a room with a 3-hr construction or a room with 1-hr protection and automatic sprinkler protection.
For transformers installed outdoors, you must give some additional considerations to their location based on the insulating liquid used. In general, FM requires that they meet the above items outlined for indoor transformers and, in addition, the building should be protected by separation, fire barriers, or a water spray system.
The separation distances are shown in Table 1 (on page 80). These values show the varying degrees of separation needed between a transformer and an adjacent building. The distances increase depending on the fire hazard introduced by the volume of liquid in the transformer and the liquid type.
If the separation shown in Table 1 cannot be maintained, a fire barrier should be provided to protect the building from exposure to the fire. Fabricated barriers should be constructed of concrete block or reinforced concrete construction with a 2-hr fire rating. This barrier should extend beyond the transformer by the horizontal or vertical distances shown in Table 1.
If a building wall is used as the fire barrier, the exposed wall should be fire-resistive or noncombustible construction for transformers with less than 500-gal fluid capacity. For transformers with more than 500-gal capacity, the building wall should be of 2-hr fire resistance. In all cases, the wall should extend the horizontal and vertical distances specified in Table 1 from the transformer.
A water spray system may be provided for additional protection provided it has a discharge density of .20 gal/min over the exposed surface.
UL listing requirements
Significant revisions in the Factory Mutual Transformer Loss Prevention document in October, 1994 resulted in FM adopting a protection scheme similar to the original UL requirements. FM dropped requirements based on heat release rate and incorporated protection against tank rupture. Now, both UL and FM use mechanical and electrical protection combined with the good fire-resistance properties of less-flammable fluids to prevent transformer tank rupture, explosion, and fire. There have been no reported eventful failures involving less-flammable units with these requirements. Similar protective devices (with the addition of low current fault protection) are also used by FM for its new FMRC-Approved Transformer standard.
Presently, UL has classified only two liquids in the less-flammable category: a high molecular weight hydrocarbon (HMWH) and a silicone. Common to both classifications are the following additional use restrictions:
* They are applicable to 3-phase transformers only;
* The transformer tank must be able to withstand an internal pressure of 12 psig;
* The transformer must be equipped with a pressure relief valve with a capacity based on the transformer kVA rating;
* The transformer primary side over-current protection must be selected to meet specific energy let-through ([I.sup.2]t) specified in the use restrictions; and
In mid-1995, a revision in both UL less-flammable fluid classifications banned the use of immersed expulsion fuses that vented during operation, unless the classified fluids are tested effectively and the installation includes primarily current limiting fusing. A representative classification marking is shown in Table 2 for a specific silicone brand. In December of 1995, UL tested and reversed the internal expulsion fuse ban for an HMWH fluid classification. The resulting UL marking is shown in Table 3.
The requirement noted above takes into account the fact that many transformer designs have the primary fusing in the transformer case, under the same insulating fluid as the transformer itself. The UL-classified HMWH fluid allows the additional option of current limiting fusing (which may be used alone or in combination with under-oil expulsion fuses inside the transformer tank), or the option of external expulsion fusing without current limiting fusing, provided the let-through current/time is within the maximum allowable limit
(by Loyd, Richard E)