A General Transformer Pole
This is a 3 phase Delta-Star 11 Hours (Dy11) 20/0.4 kV 630 kVA oil Transformer. The 20 kV primary side's star point is Petersen compensated 3 wires system (half-insulated, in Europe isn't 4th ground whire than in America on white spools.. The 0,4 kV star point is the neutral wire, that is fix grounded (4 wires TN system: 3 phase and neutral). This transformer provides about 300-400 houses, 3-4 streets. In the houses are 3 or 5 wires TN system (phase-neutral-ground or 3 phases-neural-ground)

That transformer is on a T-arm. It comes from a ridge line cross a pole-switch. The switch is on either pole of the T-arm. On the transformer pole are lightning arrestors first on every phases, the second are 3 pipe type sand fuses. After it cames the transformer. After the transforer come the low voltage secudary distributor box with breaker and fuses (there parts miss in America). After the distributor box come the line contact onto radial type low voltage distributor grid.
New constructions: The primary side lightning arrestors are built into the fuse bracket's above side post Insulators and the primary side switch can be on the transformer pole. New does too the SF6 transformers. That ones use SF6 (sulphiur-hexa-fluroid gas) in placee of the oil. That gas is the best Insulator which we know: 400 kV/cm spark voltage. On there pictures is a new Siemens oil transformer.

Info: The transformer is Hungarian invention. Invertors: Karoly Zipernowsky, Miksa Deri, Otto Blathy. They invnted it at 1884 with the AC distributor systems. The ancestor transformer was a cape type one 60/60 V some 100 watts. Thesla's idea was the more phases distributor grids, but Thesla wanted invent the wireless power distributing. Before ther transformer the Hungarian inventors (Kando, Zipernowsky, Deri, Blathy) and the Croatian Tesla were in science war against Edison. Edison wanted DC distribution. After the Transformer Edison lost, the AC distribution won, but I respect Edison, he was great inventor too. Edison had right in one theme of DC: the longer than 1200 km trabmission lines, long earth or sea cables have to be DC line because of the reactances, radio wave emitting etc.