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An overcurrent protection scheme is one of the simplest protection schemes used in power systems. It is based on the observation that under fault conditions, current flow towards the fault tends to increase dramatically. An overcurrent protection scheme therefore works by tripping circuit breakers when the current on any phase exceeds a prescribed threshold in an attempt to isolate the fault from the system.

Overcurrent protection is considered a non-unit protection scheme meaning that, when used in a transmission system, no remote information is needed to calculate when to trip. In fact, the decision can be made by the relay based solely on the readings of the local current transformers. This means that there are no well-defined boundaries of protection.

If all relays in a system were to initiate a trip immediately upon current flow exceeding a prescribed threshold circuit breakers further back from those nearest to the fault may trip if the fault current is great enough. To overcome this issue, a delay that is proportional to how many multiples of the nominal current (the maximum current expected during normal operation) is used. The benefits of this can be seen in the second figure shown. If all relays were to initiate trips immediately following their prescribed threshold being exceeded all feeders would lose power in the case of the fault shown. However by using a delay porportional to the multiple of the nominal current, the fault will always be isolated from the system while the healthy feeders remain active. The process of reconciling settings to ensure protection schemes trip in the correct order and fast enough to protect plant is known as grading.

The complexity of grading and the improved range of alternatives mean overcurrent protection is falling out of favour for use in transmission systems although it is still frequently used as a backup protection often if the primary protection mode (e.g. digital differential protection) of a relay should fail. Overcurrent protection however remains widely used for distribution feeders in combination with earth fault protection. A similar form of protection called [[overload] protection is often used for motor protection.

Reference
R. C. Mason, The Art and Science of Protective Relaying, John Wiley and Sons, June 1956.