What happens when one phase of a Delta connected primary feeder is grounded?

When one phase of a Delta connected primary feeder is grounded, the correct response is that nothing noticeable happens to the system under normal conditions. In a Delta configuration, the three phases are interconnected in such a manner that the grounding of one phase typically does not result in immediate, significant issues for the overall system.

The Delta connection allows the other two phases to continue to operate normally. This is because there are no neutral points in a Delta system, so the ungrounded phases can still carry the load effectively. The grounding might cause a slight imbalance, but it does not usually lead to noticeable overloads, complete system failures, or shutdowns, especially if the grounding fault is temporary and promptly addressed.

In contrast, if the question referred to a grounded wye system, grounding one phase would indeed create a more pronounced and detrimental effect, such as serious imbalances and increased risk of equipment damage. However, due to the inherent properties of Delta configurations, the grounding of one phase does not have the same severity of consequences.