Housed in VaultsNetwork transformers, which are distribution transformers serving grid and spot networks, are large - sized three - phase units.According to ANSI C57.12.40 - 1982, network units are typically categorized as vault - type or subway - type:Vault - type transformers: Suitable for occasional submerged operation.Subway - type transformers: Suitable for frequent or continuous submerged operation.Network transformers are also employed in buildings, typically in the basement. In such case