Network Transformer is large three-phase unit. These transformers have a three-phase, primary-side switch that can open, close, or short the primary-side connection to ground. The standard secondary voltages are 216/125 V and 480Y/277V. Network transformers are often housed in vaults. Vaults are underground room accessed through manholes that house transformers and other equipments. Network transformers are also used in buildings, usually in the basement. Network transformers may also be made out of standard single-phase distributing transformers, but caution is warranted if the units have very low leakage impedances.