About Neutral Grounding Resistors
Neutral Ground Resistors, sometimes called Neutral Earthing Resistors (NERs) are commonly used as part of a Low Resistance Grounding System in industrial distribution systems. The NGR will limit fault currents that flow through the neutral star point of a transformer or generator, to a safe reasonable predetermined value during a fault event.
Our NGRs are made using a Heavy-Duty Resistor Grid from low temperature coefficient Fe-Cr-Al Stainless Steel alloy, that is compliant with the applicable new standard IEEE-C57.32, 2015 “Requirements, Terminology and Test Procedure for Neutral Grounding Devices”. These resistor are designed to permit thermal expansion under rated duty with spot-welded stainless-steel jumpers between resistor banks. Ceramic spacers and ANSI Standoffs electrically isolate the resistive elements.
Neutral Earthing Resistors can be supplied with outdoor or indoor enclosures with appropriate ingress protection levels. In the case of an outdoor enclosure, material can be mild steel finished in powder coat or Hot Dip Galvanized. Hot Dip Galvanized enclosures are built according to ISO 1461:1999. This ensures a high degree of protection.
against corrosion, fabricated from min 14 gauge mild steel (12 gauge for feet &side supports). Our metal parts are galvanized after all sheet metal processing operations (drilling, cutting etc.) are completed and the unit is pre-assembled for quality check and disassembled after approval. This two-stage approval before galvanizing is a key requirement to prevent any uncoated part, which may adversely affect the corrosion performance during service. Other enclosure options are unpainted 14-gauge AISI 304 (EN 1.4301) stainless steel to provide the best outdoor protection available.
Because of the heat generated by the resistive elements, slots and louvers are included in the enclosure to provide adequate ventilation for rapid cooling. Eyebolts for lifting and easy handling are provided along with Optional Window Type Current Transformers and Terminal Box for CT secondary (Optional). All stainless steel hardware is standard for added durability.
This safe level of fault current allows for selective and fast fault detection by means of simple over-current relay, to avoid network shutdown and equipment damage. In addition this flow of fault current can activate protection devices to locate and clear the fault.
NGRs dissipate and absorb an enormous amount of energy for the duration of the fault event without exceeding temperature limitations as defined in IEEE Standard 32-1972, typically 760C. Units are tested to all requirements prescribed by NEMA, ANSI, and IEEE Standard C57.321972 2015.
Parts of an NGRBonded fin heat sinks allow a smaller package size or lower thermal resistance than conventional aluminum extrusions in forced air applications. We offer signal fin, and corrugated fin options for our bonded fin heat sinks. Some versions of our corrugated fin stock have an off set providing greater turbulence resulting in larger heat transfer. It is normally possible to dissipate two to three times more heat than conventional aluminum extrusions with our bonded fin designs. In applications where space and high thermal density is a problem, copper bonded versions are available. Copper has a thermal conductivity twice that of aluminum. The fin density and fin height on both can be used in countless combinations. We can provide you with the best possible value, based on your unique application.
NGRs provide the following benefits:
- Reduces stress and the amount of damage to your power system equipment.
- Rapid isolation and clearing of the original fault.
- Electrical distribution network security is improved.
- For generators it limits the amount available of Line to Neutral fault current to below the design limits set by the NEMA MG1 generators design standard.
- Decrease in unplanned shutdowns.
- Prolong the life of transformers or other connected distribution equipment..
Why use a Neutral Grounding Resistor?
Typically, N G Rs are used in conjunction with generators or motors. With both of these devices, ground fault currents can be extremely high and if not controlled can result in damage to transformers, generators, motors, wiring and associated equipment. Moreover, neutral grounding resistor for generators will avoid damaging the stator core. Stator cores consist of laminations made from silica-steel, cold-rolled or grain-oriented steel. If damage were to occur to the Stator core it could result in the complete replacement of the motor/generator or a costly off-site repair. If just the winding were to be damaged, this also is costly but not catastrophic and can be completed by a local rewinding house. In each of these cases a Neutral Earthing Resistor is used to prevent damage from occurring in the first place.To limit the damage to the core, generator and motor manufacturers allow for a limited ground fault current, which is usually provided by the manufacturer in terms of Core Damage Curves. By inserting a Neutral Ground Resistor (NGRs) between the system neutral and ground, it lowers the prospective ground-fault current to a predetermined value; note below our standard 10s values of 100 to 1000A. By controlling the magnitude of the ground fault currents, it will reduce the stress and amount of damage to your power system equipment. By connecting an N G R to the neutral of a generator, it will limit the amount of the generator available Line to Neutral fault current to below the design limits set by the NEMA MG1 generators design standard. Another advantage of a Neutral Earthing Resistor system is that ground fault current can flow, permitting it to be detected and measured. Simple over-current relays allow a selective and fast fault detection, that can be used on feeders to provide careful coordination and the ability to quickly pinpoint and isolate the fault. Depending on the application, the resistor can be designed for either a high or low current value, or continuous or a predetermined duration. The resistor’s designed maximum current allows relays and protection settings to be effortlessly determined with the goal of avoiding damage to equipment, providing a safe operating environment and continuity of supply.
There are several methods of Neutral Grounding
We offer High and Low resistance grounding resistorsMedium and high voltage electrical networks use Low Resistance Grounding because of the high capital equipment used in these applications and the high cost of network interruption. NGRs are sized to permit 100A to 2500A of fault current, with typical time durations of 10s. This current permits protective devices to operate but does not allow major equipment damage. We provide NGRs with standard Line-to-Line voltages of: 2400, 4160, 4800, 6900, 7200, 8320, 12000, 12470, 13200 and 13800. Custom designs up to 111kV are possible.
Continuous process industries will typically use High Resistance Grounding because these commercial systems require continuous operation, even after a fault occurs. Our NGRs will reduce the current to a low value, 10 Amps or less. This occurs, so circuit breakers are not tripped and allow for Protection Systems to activate during this event, quickly locating and clearing the fault or providing for an orderly shutdown the system so no damage occurs.
Standard Low Resistance - Neutral Ground ResistorStandard NGRs can be operated in a 120°F maximum ambient temperature, 0°F minimum ambient temperature and an altitude of 1000 m. For altitudes above 1000 m, special designs are available to accommodate the dielectric strength of insulation parts corrected according to the given service altitude for up to and including 6000 m as per IEEE C37.20.2 NGRs are designed and factory tested to IEEE Standard C57.32-2015 and are designed to meet all applicable specifications of ANSI, NEC, NFPA, NEMA, and OSHA.. These conform to Seismic Zone 2 requirements of the Uniform Building Code unless otherwise specified.
Standard Enclosed NGRs Electrical Ratings
|System Voltage (L-L)||L-N Voltage||Current Ratings*|
|System Voltage (L-L)||L-N Voltage||Current Ratings*|
Standard Mechanical Ratings**
|System Voltage (L-L)||Width inches||Depth inches||Height inches||Weight lbs.|
Standard NGR design detailsOur NGRs are enclosed in outdoor NEMA/3R/IP23 enclosure, allowing for bottom entry and safety exit. They are fully assembled with options for a Neutral Current Transformer installed in an enclosure, with secondary leads wired to the terminal blocks for field interconnection to a ground fault relay.
Here are some parameters to consider when choosing and sizing an NGR
- Rated Line-to-Line voltage or Line-to-Neutral voltage.
- Time duration of the NGR will carry the maximum current.
- Short time rating: typically, 10 seconds or 60 seconds.
- Continuous rating: normally 5% to 10 % of full load current.
- Maximum current when the resistor is cold.
- Insulation specified based on line voltage.
- The maximum short time temperature rise for the resistive element is 760°C, in accordance with IEEE Standard 32-1972 (Requirements, Terminology and Test Procedure for Neutral Grounding Devices).
- Ingress protection IP rating for outdoor or indoor enclosures. Standard enclosures are manufactured having IP10, IP13 or IP23.
- Enclosure material can be mild steel, finished in powder coat or hot-dip galvanized, or Stainless-steel.
- Auxiliary items: Voltage transformers, off-load isolators, current transformers, or other options.