The NGR is a customer-engineered neutral grounding resistor designed to protect industrial medium-voltage power distribution systems from the destructive effects of ground faults. In a solidly grounded three-phase system, a single line-to-ground fault can drive fault current to between 10,000 and 20,000 amperes, causing severe and often irreversible damage to transformers, generators, motors, cabling, and associated switchgear. Connecting a RARA NGR between the system neutral and earth ground limits this fault current to a safe, controlled level — typically 50 A to 500 A depending on the system specification — while preserving sufficient current flow to operate fault-clearing relays and protective devices reliably.
Each NGR is custom-designed to the voltage class, current rating, and time rating required by the specific installation. Voltage coverage spans from 440/√3 V low-voltage systems through 3.3 kV, 3.45 V, 6.6 kV, 6.9 kV, 22 kV, and 22.9 kV medium-voltage classes, with enclosure voltage ratings available up to 72 kV for high-voltage substation environments. Time ratings of 10 seconds and 30 seconds define the thermal duty cycle of the resistor element during a fault event, and all units are designed and tested in accordance with ANSI/IEEE Std 32-1972 to confirm compliance with industry requirements.
The resistive elements are fabricated from AISI-304 stainless steel in strip, edge wound, or wire wound form depending on the power level of the unit. Element dimensions range from 0.6 mm to 2.5 mm thickness and 4 mm to 100 mm width. Contact joints between element sections are spot-welded or argon-welded to eliminate resistance variation at connection points, and advanced cutting and trimming processes ensure that the delivered resistance falls within the ±10% tolerance. The elements are fully insulated from ground within the enclosure for personnel safety. Enclosures are constructed with corrosion resistance suitable for indoor and outdoor deployment, with IP ratings up to IP54 and NEMA ratings up to NEMA 3S available for exposed or harsh-environment installations.
Units are assembled in several stages to suit shipping requirements, and RARA provides full project documentation including dimensional drawings and test records. To configure a unit, the specifying engineer supplies the line-to-neutral voltage, rated fault current, time rating, resistance value, indoor or outdoor designation, and ambient environmental conditions.
