What is an open delta?

An open delta refers to a type of three-phase transformer connection that is created using only two transformers instead of the usual three. This configuration allows for a three-phase delta connection to be established with less equipment while still providing three-phase power. The open delta is useful in situations where one transformer in a traditional delta configuration fails or when the full capacity of three transformers is not required.

In this arrangement, while the system can supply three-phase loads, it will have some limitations when compared to a full delta configuration, notably in terms of the capacity and the voltage balance across the phases. It is generally used for lighter loads or in temporary installations.

In contrast, the other options depict configurations that either require the full complement of three transformers, refer to a single-phase system, or describe a four-wire service, which does not pertain to the concept of an open delta.