Two compartments, one door
The cabinet is split down the middle. High voltage on one side, low voltage on the other, with a barrier between them. The doors are interlocked so the high voltage side cannot be opened without the low voltage side being opened first, and the whole thing is padlocked. On a dead front unit nothing energised is exposed even with both doors open.
The high voltage side
Six 200 amp loadbreak bushings on a loop feed unit, three in and three out, so the transformer sits in a loop and can be fed from either direction. A four position switch lets an operator isolate either source or the transformer itself without pulling elbows. Bay-o-net fuses in the oil protect the transformer, backed by current limiting fuses for high fault currents.
The low voltage side
Three spade terminals for the phases and one for the neutral, normally four hole NEMA pattern, sized for the large secondary conductors a building service needs. The neutral is bonded to the tank and to the grounding electrode here, which is what makes the transformer the origin of the separately derived system.
Behind both, the tank
The core, the windings and the mineral oil sit in a sealed tank that forms the back of the cabinet. Everything you interact with is a connection point into it. There is nothing serviceable inside for anyone but the manufacturer, which is why the oil is the diagnostic rather than an internal inspection.
Common questions
What is inside a pad mounted transformer
A sealed tank holding the core, the windings and insulating oil, with two connection compartments in front of it. The high voltage compartment has the loadbreak bushings, the switch and the fusing. The low voltage compartment has the spade terminals and the neutral bond.
Why does a pad mounted transformer have six bushings
Because it is loop feed. Three bushings take the incoming cable and three pass it on to the next unit, so the transformer sits in a loop and can be fed from either direction. A radial feed unit has only two or three.
Is it safe to open a pad mounted transformer
No, not for anyone who is not qualified and authorised. Dead front construction means no energised part is exposed, but the equipment inside is at distribution voltage and the cabinets are locked for that reason. Utilities treat them as their own equipment.