Pad mount is the most common type of underground transformer in the distribution system.

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Pad-mount transformers are designed specifically for underground service, sitting on a concrete pad at grade with a weatherproof, tamper-resistant enclosure. This setup provides a compact, accessible, and safe way to step down medium-voltage distribution to low voltage for customers while keeping the equipment protected from moisture and tampering. The at-grade pad-mount design also simplifies maintenance and fault isolation since technicians can reach the unit without digging deeper vaults or climbing structures, and it fits well into urban or suburban underground networks where visible equipment is minimized. While vaults or other underground enclosures exist for particular space, security, or layout constraints, pad-mount units are the standard choice for most underground distribution configurations, which is why the statement is true.