This guide mainly focuses on the application of protection relays for the protection of power transformers, with emphasis on the most frequent protection schemes and transformers. Principles are emphasized. Fixing procedures are only generally discussed in the material to follow. Refer to your relay specific instruction manuals. References provide a source for additional theory and application guidance. The engineer must balance the expense of applying a particular protection scheme against the consequences of depending on other protection or sacrificing the transformer. Allowing for long failure would increase the damage to the transformer and the possibility of rupture of the tank with the consequent oil fire and consequent safety risks of the personnel. There is no rule indicating which specific protection scheme is appropriate for a given transformer application. There is a tendency to associate the protection schemes to the MVA and the primary kV of a transformer.