With respect to power transformers, the following protections:

• Protects the transformer against sustained overcurrent above its rated capacity, which would cause overheating of windings and insulation and shorten life.
• Usually provided by thermal overload relays or by an oil/winding temperature indicator (WTI/OTI) which operates on the temperature of the oil or windings. The relay has an inverse time characteristic - the higher the overload, the faster it operates.
• The overload relay gives an alarm first and trips the circuit breaker if the overload persists. It allows short-term overloads (e.g. during motor starting) but trips on sustained overload.

• Protects against short circuits between phases (phase-to-phase faults), which produce very high fault currents.
• Provided by overcurrent relays (IDMT - inverse definite minimum time) connected to current transformers on the primary and/or secondary. These have an inverse time-current characteristic so that larger fault currents trip faster.
• The relay operates the circuit breaker to isolate the transformer quickly, limiting damage. Instantaneous elements may be added for very high fault currents.

• Protects against faults between a winding and earth (ground), which may not be detected by phase overcurrent relays if the fault current is small.
• Provided by earth fault relays connected in the residual circuit of the current transformers (the residual current is the vector sum of the three phase currents, which is zero under balanced conditions but non-zero on an earth fault).
• Alternatively, a core-balance (zero-sequence) current transformer surrounds all three conductors and detects any imbalance due to earth leakage. The relay trips the breaker on an earth fault.

• Compares the current entering the transformer primary with the current leaving the secondary, using current transformers on both sides.
• Under normal and through-fault conditions the currents are balanced (allowing for the turns ratio and vector group) and the relay does not operate. On an internal fault (winding-to-winding or winding-to-earth inside the transformer), the currents are unbalanced and the relay operates to trip the breaker.
• This gives fast, sensitive protection for internal faults and is the main protection for large power transformers. It is biased to prevent operation on through-faults and magnetising inrush current.

• Detects the direction of power flow and operates only when the fault current flows in a particular direction.
• Used where a fault could be fed from more than one source, e.g. in parallel feeders or ring systems. The directional relay compares the phase of the current with the voltage (or with a reference) to determine the direction of the fault.
• It ensures that only the circuit breaker on the faulted side operates, isolating the fault while leaving healthy sections in service. It is used for busbar and feeder protection where discrimination by time alone is not sufficient.