With respect to power transformers, the following protections:

• Protects the transformer against sustained overcurrent above rated capacity, which would overheat the windings and insulation and shorten life.
• Provided by thermal overload relays or by oil/winding temperature indicators (WTI/OTI) operating on the temperature. The relay has an inverse time characteristic - the higher the overload, the faster it operates.
• Gives an alarm first and trips the breaker if the overload persists. Allows short-term overloads (e.g. motor starting) but trips on sustained overload.

• Protects against short circuits between phases, 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. 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, 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 vector sum of the three phase currents, zero under balanced conditions, non-zero on an earth fault).
• Alternatively a core-balance (zero-sequence) current transformer surrounds all three conductors and detects the imbalance due to earth leakage. The relay trips the breaker on an earth fault.

• Compares the current entering the 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 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.
• 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.