Outdoor or weatherproof transformers for ordinary distributing purposes, and for potentials up to about 2500 volts, have long been in general service. With the exception of arc lamps and series incandescent lamps, transformers have been about the only apparatus in high-tension service not protected by buildings. Distributing circuits operating at 6600 volts are now quite common. There are also a few 10,000-volt, and a small number of 15,000-volt, distributing systems using outdoor transformers.

Thus far, about 50 kilovolt-amperes has been the limiting capacity found in outdoor service, but there are a few exceptions. Outdoor transformers of 100 kilovolt-amperes capacity or greater, for any voltage, have been almost unknown. Recently, however, large transformers have been taken up; a few are in service, and most of them but for a short time.

The outdoor problem may be divided into two parts:
1. The production of satisfactory outdoor apparatus.
2. The application of outdoor apparatus.

The design and construction will be discussed, and the application will be considered.

Construction. Transformers for outdoor service may be built for any requirements that the ordinary indoor type of oil-insulated unit will satisfy. As to capacity, the limit of approximately 500 kilovolt-amperes will apply to the self-cooled type, depending somewhat on voltage and frequency. As with the

self-cooled indoor transformers, the case is the principal problem. It is difficult to obtain the radiating surface required for cooling very large transformers, and still retain a simple mechanical construction. Oil-insulated, water-cooled, outdoor transformers can be built for any capacity irrespective of voltage and frequency.

The first problem apparent in the development of out

door transformers concerned the terminals: how to make them reliable in all kinds of weather and service. The next problem was to weatherproof the case satisfactorily. This much of the problem has been worked out, and now outdoor transformers up to 500 kilovolt-amperes capacity have been built, and units for potentials up to 60,000 volts are in service.

The downward projecting lead which issues from an over

hanging pocket near the top of the transformer case is a quite satisfactory construction for moderate potentials. 10,000 volts or more can better be carried by upward projecting terminals, this arrangement being particularly attractive for convenience in wiring. About the same practice for placing outlet terminals serves for both indoor and outdoor transformers, so far as the general arrangement and convenience are concerned. The essential requirements of outdoor terminals are that they.

FIG. 2-This construction for outdoor service has been used for 6600-volt 25-cycle, three-phase transformers from 75 to 350 kilovolt-amperes.

retain their insulation characteristics and that they do not deteriorate by exposure to the elements. Outdoor terminals are larger and require much more room than those for indoor service. Fig. 11 illustrates a self-cooled, 300-kilovolt-ampere, 33,000volt, outdoor unit. The case is of corrugated sheet-iron with welded vertical seams. The bottom and top are cast on the corrugated shell. The resulting construction is strong, oiltight, and is not subject to damage by the elements. The cover,

also of cast iron, has a considerable overhang or eave to guard the joint between it and the top of the case, and is fitted with the outlet terminals and a small inspection door. All joints are protected by an overhang and are designed for making tight with gaskets.

Outdoor water-cooled units employ the same general construction as the self-cooled type, except that the cases will usually be of boiler iron, and cooling coils with connections must be provided.

Weatherproofing outdoor transformer cases is doubtless best done by making the joints vacuum-tight. Eliminating leaks means that nothing can enter, and therefore a clean, dry unit would remain in that condition. Moisture is the only enemy to be feared, either in the form of rain, snow, or humidity. The "breathing" of wet air is the most important source of trouble because it is the hardest to eliminate. As a transformer heats, part of the air in the top of the case will escape if an outlet exists and new air will return as the initial temperature is resumed. Under certain conditions moisture which may have entered with the new air will condense. The amount of moisture which will accumulate in this way, in a short time is quite surprising. Well-made gasket joints, with deep. overhanging eaves and carefully sealed-in outlets, give good results.

Application. Whether outdoor apparatus is really desirable, involves a great many points even after it is proved satisfactory in the individual piece. Some of the more important considerations are: location and climate; cost of building and ground for indoor station; cost of corresponding ground for outdoor station; capacity of station; high-tension and low-tension voltages; number of high-tension and low-tension circuits; method of operation and control; method of cooling; attendance and supervision; instruments and their housing; and the cost of indoor versus outdoor apparatus.

A 20-kilovolt-ampere, 2200- to 220-volt transformer immediately suggests a pole installation. But if the figures are multiplied by ten, a 200-kilovolt-ampere, 22,000- to 2200-volt transformer suggests indoor service. If these figures are multiplied by three, a 600-kilovolt-ampere, 66,000- to 6600-volt transformer certainly has always demanded housing.

The large clearance required for exposed high-tension wiring and disconnecting switches; the expensive construction de