Through fault occurs outside of the protective zone. In other words, the occurrence of external faults outside of the equipment protective zone is called through fault.
A transformer has a unit protection or differential relay for protection from the internal fault inside of the transformer. The unit protection relay must not trip in the case of external faults outside of the equipment. The zone, unit, or differential relay remains inoperative for all the external faults. All these external faults are through fault.
Through fault is nothing but a fault that occurs outside of the protective zone or in simple words the external fault is called through fault. Two kinds of through fault is considered as through fault. One is an External short circuit and the other one is an overload condition.
External short circuit: The short circuit occurs between two phases or more phases for several hundred meters. The line fault reflects in the power generator. In, from the generator's point of view, this short circuit fault is called a through fault.
Overloading: Overloading means the power sources deliver excessive current.