A differential relay can measure the vector difference between two similar quantities, which can be either currents or voltages, depending on the specific application and the component being protected.

For example, in a differential protection scheme for a transformer, the differential relay would typically measure the vector difference between the currents entering and leaving the transformer, as this provides a reliable indication of whether there is an internal fault in the transformer.

However, in other applications, such as the protection of busbars, a differential relay may measure the vector difference between the voltages at different points in the system, as this provides a reliable indication of whether there is an internal fault in the busbar.