An overhead transformer that needs an external protective device (cutout switch) for protection is called what?

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Protection approach for overhead transformers hinges on whether the transformer relies on an external protective device or has built-in protection. When a transformer is described as conventional, it means the protection is provided by an external device, such as a cutout switch, connected in series with the transformer on the line side. If a fault occurs, this external device interrupts the current and isolates the transformer from the system. That reliance on a separate switch to protect the transformer is exactly what the description calls for.

In other setups, protection is handled differently. A self-protected transformer includes protection mechanisms built into the unit itself so it can trip or isolate without needing a separate external switch. Other terms describe different configurations or protection schemes that don’t match the idea of requiring an external cutout for protection, which is why they aren’t the correct fit for this description.