Multiple-circuit transmission lines combining different voltage levels in one tower present extra challenges when setting a protection philosophy, as faults between voltage levels are possible. This paper presents a detailed theoretical analysis of such combined faults, including the development of a formula for estimating the magnitude of the short-circuit current. It is demonstrated that if the faulted phase from the higher voltage level leads the faulted phase from the lower voltage level, a distance relay at the higher voltage level sees the fault in the forward direction, whereas a distance relay at the lower voltage level sees the fault in the reverse direction. The opposite happens if the lower voltage level leads the higher voltage level. It is also demonstrated that the magnitude of fault currents of combined faults is normally slightly larger than of equivalent single-phase-to-ground fault at the higher voltage level.