Grounding Resistors


        Neutral Grounding Resistors Overview

    Powerohm Type NR Neutral Grounding Resistors are used in industrial power systems for resistance grounding of wye connected generators and transformers. A neutral grounding resistor limits the fault current to a value which is sufficient enough to operate protective relays, yet prevent unwanted fault damage.

    Powerohm Neutral Grounding Resistors feature low coefficient of resistance stainless steel elements. The neutral grounding resistors shall be designed and factory-tested to IEEE Standard 32. In addition, the neutral grounding resistors shall be designed to meet all applicable specifications of ANSI, NEC, NFPA, NEMA, and OSHA. Unless otherwise specified, the neutral grounding resistors shall be designed to conform to Seismic Zone 2 requirements of the Uniform Building Code. A copy of the certified test report will accompany each resistor.


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        Advantages

    The purpose of resistance grounding is to provide protection of a transformer/generator by solving the problem of transient over-voltages thereby reducing equipment damage. It accomplishes this by allowing the magnitude of fault current to be predetermined by Ohms law I=V/R where I=fault current, V=line to neutral voltage, R= Ohmic value of resistor. In addition, limiting fault current to predetermined maximum values permits the designer to selectively co-ordinate the operation of protective devices, which minimizes system disruption and allows quick location of the fault.

    Neutral grounding resistors will save money by reducing magnitude of transient over-voltages, thereby reducing equipment damage, simplifying ground fault location, improving system and equipment fault protection, reducing maintenance time and expense, creating improved safety for personnel, improving lightning protection and reducing fault frequency.

    When the neutral of a system is not grounded destructive overvoltages appear from line to ground during normal switching of circuits. It has been shown that these overvoltages cause aging and insulation failure at various locations throughout the system. As a result, a line to ground fault can cause considerable damage, interruption of service and difficulty in fault location.

    Advantages of Neutral Grounding Resistors Include:

    1. Reduced overvoltages.
    2. Reduced equipment damage.
    3. Isolation of faulted circuit.
    4. Increased protection of transformers, generators and auxiliary equipment.
    5. Reduced operation/maintenance expense.
    6. Increase safety.
    7. Improved reliability.
    8. Reduced frequency of faults.
    9. Quicker fault location.
    10. Increased lightning protection.


        Neutral Grounding Resistor Options

    Enclosure Types: Nema 1/IP20 (Vented), Nema 3R/IP23 (Louvered).

    Finishes: Mill-Galvanized (Standard), Powder Coat (ANSI-61, ANSI-49, ANSI-70), Hot Dipped Galvanized, Aluminum, Stainless Steel (304), Stainless Steel (316), Custom Colors.

    Bushings: None, One, Two, Top Mounted, Side Mounted and Enclosed (Cable Box Mounted).

    Elevating Stands: Two, Three, Four, Five, Six, Seven and Eight Feet. Custom Heights Also Available.

    Current Transformers: Window Type, Bar Type, Indoor and Outdoor Rated.

    Potential Transformers: Unfused and Fused.

    Disconnect switches: Load, No Load, Vaccum, Oil Filled, Manual, Electric and Auxilary Contacts.

    Junction Boxes: Nema 4 and 4X. House Terminal Blocks, Shorting Blocks and Other Auxilary Equipment.

    Seismic Qualification: Zone 4 Pre-Approved up to 44”W x 54”Dx 54”H Enclosure. Custom Certifications, Calculations, PE Stamped Drawings and Shake Table Tests Available Upon Request.

    Neutral Grounding Monitors: Startco Monitors, Current and Voltage Sensing Devices and Self Testing Devices.

    Third Party Approvals: CSA, ETL and UL.



        Neutral Grounding Resistor Specifications

    Neutral grounding resistors are rated in line-to-neutral voltage (system voltage divided by 1.732), initial fault current and maximum time on. Powerohm neutral grounding resistors are designed to dissipate the required amount of energy and not exceed the temperature limitations of IEEE Standard 32. As defined in this publication, the time and temperature ratings for neutral grounding resistors are as follows:

    Short time: Short time ratings are 10 and 60 seconds. Since short time rated resistors can only withstand rated current for short periods of time, they are usually used with fault clearing relays. The short time temperature rise for the resistive element is 760°C.

    Extended time: A time on rating greater than ten minutes which permits temperature rise of resistive elements to become constant, but limited to an average not more than 90 days per year. The extended temperature rise for the resistive element is 610°C.

    Continuous: Capable of withstanding rated current for an indefinite period of time. The continuous temperature rise for the resistive element is 385°C.

    Resistor Assembly: The resistor coils consist of a stainless steel elements wound around a ceramic core supported on a through-rod. Glazed insulators are attached to each end of the coils and fastened to a heavy gage, corrosion resistant frame. The unit is designed to permit the expansion of supporting rods when submitted to high operating temperatures. Resistor elements are joined by stainless steel connectors, which are welded in place, to form a positive electrical path.

    Safety Enclosure: Our resistor assemblies are available with grounded safety enclosures to protect personnel and wildlife from harm. Screened and louvered enclosures are available in a variety of finishes including painted, powder coated, mill galvanized, hot-dipped galvanized, aluminum and stainless steel.

    All units are factory tested in accordance with IEEE Standard 32, specifically Sections 10.1.4 and 10.3.2. The procedure includes a resistance measurement test to verify that the tolerance is within + 10%, and an applied potential test. A copy of the test report is included with each shipped unit.

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Powerohm Resistors, Hubbell ICD / 5713 13th Street / Katy, Texas 77493