When is an additional grounding connection required at a transformer outside the building?

An additional grounding connection at a transformer located outside a building is typically required when the transformer supplies the service. This is because the transformer, when it provides power to a building, needs to ensure that any fault current has a clear path to ground to enhance safety and effectively operate protective devices.

Grounding in these situations helps to prevent overvoltages during transient, fault, or lightning events by providing a path for excess current to safely dissipate into the earth. Additionally, it ensures that the system remains stable and reduces the risk of electrical shock to personnel.

While other scenarios may involve additional grounding requirements based on specific conditions, the essential need is dictated by the role the transformer plays in supplying power to the service. This establishes a critical link for the safety and operational integrity of the electrical system in the building.