8. Resistant fault detection

8.1 Notion of sensitivity

The sensitivity of a protection scheme is its ability to detect the most resistant faults possible, and is expressed in ohms. In fact, for MV networks, this notion only makes sense in the case of earth faults and when the neutral is not directly earthed. Indeed, in the case of polyphase or direct-to-earth neutral faults, given the low value of the short-circuit impedances involved, the fault resistance, i.e. the arc resistance and the direct impedances upstream and of the faulted line, is low. Fault currents are high, and the fault is easy to detect simply by measuring current intensity.

In the case of earth faults occurring on networks with low-impedance, compensated or isolated neutral, calculations are based on the network's equivalent...