Why is the NGR continuously monitored in an HRG system?

What does the neutral grounding resistor do?

In high resistance grounding (HRG), the supply neutral is connected to earth through a high-resistance neutral grounding resistor. The neutral grounding resistor, or NGR, limits the current during an earth fault. Some industrial installations use this approach to reduce damage and unplanned downtime. Current limitation depends on the entire system configuration as well as the resistance value. A single resistor or fault current value therefore cannot be prescribed for every installation.

NGR and NGRM: the resistor and its monitoring device

NGR (Neutral Grounding Resistor) is the resistor connected between neutral and earth. NGRM (Neutral Grounding Resistor Monitor) is the device that monitors its condition and the associated current and voltage. The neutral grounding resistance monitored by an NGRM differs from the system insulation resistance monitored by an IMD and from earth electrode resistance.

• HRG (High Resistance Grounding): earthing through a high resistance. NGRM500 and NGRM700 are intended for this application scope.
• LRG (Low Resistance Grounding): earthing through a low resistance. NGRM550 and NGRM750 are intended for this application scope.
• The NGR limits earth fault current; the NGRM monitors the resistor and related electrical quantities. Alarm and tripping are assessed together with suitable external circuit-breaking and control equipment.

Why is resistor integrity monitored separately?

An interruption in the NGR path can impair the expected fault detection behaviour. Excessively low resistance also changes the current-limiting function. Bender NGRM700 monitors NGR resistance, current through this path and neutral-to-earth voltage. With a suitable arrangement, resistance monitoring can continue while the installation is de-energised. Transferring measured values to a monitoring system, alongside event history and alarms, supports maintenance assessment.

Plan alarm, fault location and tripping together

Depending on system design, NGRM700 can be connected to an earth fault alarm or tripping arrangement; RCMS solutions can support separate assessment of outgoing circuits. Overall earthing information and investigation of the faulty feeder then complement one another. Since healthy phases may rise in voltage relative to earth after the first earth fault, the risk of a second fault must not be ignored. HRG is not presented as a solution for limiting phase-to-phase arc energy. Particularly in mobile equipment and mining, monitoring, disconnection and continuity requirements depend on the application and local rules; continued operation after the first fault must not be assumed under all conditions.