This is a bank that provides three phase power with two transformers. It usually consists of a larger transformer paired with a smaller one (50kVA and 25kVA for example). The 50kVA would be used for lighting and other single phase loads, which is why it is bigger. It can also be used as a temporary fix for a Wye-Delta bank, but the kVA capacity is about 58% of the original capacity. The one important thing to remember if you do this is to take the floating primary neutral of the Wye-Delta and attach it to the neutral for an Open bank. Wye-Delta and Open Wye-Open Delta both have 120V phase to ground on two phases, 208V phase to ground on the third phase, and 240V phase to phase. They can also be used as a power bank (strictly three phase load). They will have 240V or 480V phase to phase and one of the phases could be grounded, called corner grounding. There are advantages and disadvantages to corner grounding.