Transformers are protected by fuses or circuit-interrupting devices such as breakers or circuit switchers with relays detecting faults and providing trip signals to the circuit-interrupting devices. Transformers 5 MVA and below are almost always. Core idea: Transformer protection detects electrical, thermal, pressure, gas, and insulation-related problems before a transformer failure damages equipment or spreads through the power system. Relay protection of transformers. The following sections in this article provide more detail on the individual protection methods. Note that combined differential and REF, overfluxing, tank-earth and oil/gas protection are described in the next part of this article.