What are the limitations of inverse time overcurrent relays in protecting against high impedance faults?

Inverse time overcurrent relays operate based on the principle that the tripping time is inversely proportional to the fault current magnitude. This means that for higher fault currents, the relay trips faster, and for lower fault currents, it trips slower. A high impedance fault is a fault where the fault current is limited by a high impedance, such as when a conductor falls to the ground through a tree or makes contact with a poorly grounded object. The primary limitation of inverse time overcurrent relays in protecting against high impedance faults is their inability to reliably detect these faults due to the low fault current magnitude. The fault current may be too low to exceed the relay's pickup setting, meaning the relay will not even initiate a trip. Even if the fault current does exceed the pickup setting, the inverse time characteristic means the relay will operate very slowly, potentially allowing the fault to persist for an extended period. This can lead to equipment damage, safety hazards, and difficulty in locating the fault. Furthermore, high impedance faults often produce arcing, which can be intermittent and unpredictable, making it difficult for the relay to accurately measure the fault current. Traditional overcurrent relays are designed to detect bolted faults, where the fault current is high and relatively constant. High impedance fault detection requires specialized techniques, such as voltage-based relays, arc flash detection, or sensitive ground fault protection, which are more sensitive to low-magnitude, arcing faults.