High-Resistance Neutral Grounding (Industrial Medium-Voltage Networks)

A neutral-grounding method for medium-voltage networks in which the star point of the supplying transformer is connected to earth via a resistor with a relatively high ohmic value (as opposed to direct or low-resistance grounding), sized such that the earth-fault current during a single earth fault is limited to a small, predetermined level (typically a few tens of amperes, instead of the many hundreds or thousands of amperes in a directly earthed network). This significantly limits equipment damage and the risk of a dangerous step voltage during an earth fault, and in many cases allows the plant to keep operating through a single earth fault (similar in purpose, though not identical in implementation, to the IT system in low-voltage installations) until the fault can be repaired on a planned basis, rather than having to shut down the entire network immediately and unplanned. The downside is that a persistent, undetected earth fault places additional stress on the insulation of the other phases, and sensitive earth-fault detection (often based on the residual current through the neutral resistor) is needed to signal and locate the fault in good time, even though it does not immediately lead to tripping.