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Page 48

| withstand the pressure of watertightness tests. Between the cover and the tank, there is a gasket to make the seal watertight; in larger sizes, it may be necessary to space closely a relatively large number of bolts around the periphery of the cover and cover plates. |

| One method of connecting to the transformer is by bringing a lead-covered cable into the transformer, thus making a wiped joint between a bushing of tinned brass and the cable sheath where it passes through the bushing. A barrier is inserted in the connector or sleeve connecting the internal and external cables to prevent the transformer oil being siphoned into the cable. In another type, the wiped joint to the cable sheath is made at the outer end of a wiping bushing, and the inner end is attached to the tank by a pipe-union type or bolted-flange type joint with a gasket provided to insure watertightness. In some instances, to simplify the process, the connections are made in a chamber attached to the transformer case into which the terminals of the windings are brought by means of water and oil-tight bushings, which sometimes may be of the porcelain type. In special instances, where such transformers supply low-voltage networks, these chambers may also contain high-voltage grounding or disconnecting switches. |

| In the supply to large loads by means of low-voltage networks (Fig. 3-7), the transformers may also be equipped with a network protector, a relay-activated secondary breaker. This breaker or protector may be installed in a separate tank or may be in a compartment attached to the transformer tank. |

| For economy reasons, line-type transformers are occasionally used in manholes under sidewalks or streets where there is little possibility of their being submerged. |

| The ventilation of transformer manholes and vaults is of special importance. Not only should the temperature of the transformer be kept within safe limits even under extreme operating conditions, but the design must consider the possibility of the formation of explosive mixtures of oil vapor or gasses and air in the event of a transformer failure. This is achieved by insuring sufficient vent area outlet to the volume of the manhole or vault; the smaller the ratio of the volume of air in the manhole or vault to the total net ventilating area (openings), the less will be the pressure developed in the manhole or vault, and the lighter the construction necessary to withstand the force of the possible explosion. |

| In some instances, the transformers have been installed in vaults located on the upper floors in tall buildings or to supply specialized equipment. The transformers may be of the subway type or the line type depending on the application and overall economy. |

| In suburban residential or rural areas, transformers may be buried directly in the ground, or installed in commercially manufactured enclosures of steel or concrete which may be partially or completely located below ground level; in some instances these installations are made in sections of concrete pipe or other makeshift enclosures. The transformers installed may be of the |

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