Underground transformers are classified as:

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Underground transformers can be installed in two main ways: on a concrete pad at ground level (pad-mounted) or buried underground (subsurface). Pad-mounted units sit on a pad and are accessible from the surface, often in a controlled outdoor enclosure. Subsurface units are placed below ground, in vaults or buried in trenches, with connections run through ducts. Because both installation styles exist for underground distribution, the best way to classify them is as pad-mounted or subsurface. The other options omit one of these common configurations and don’t cover all practical setups.