Introduction
Grounding is a critical aspect of any industrial electrical system. Proper grounding provides a safe path for fault currents to flow, ensures electrical noise is safely dissipated, and provides personnel protection. However, many electrical engineers and technicians are unaware of some key grounding requirements outlined in the National Electrical Code (NEC) and by organizations like IEEE. In this article, I'll cover several important industrial grounding requirements and best practices you may not know about. Adhering to these guidelines will help ensure your facility's electrical system is safe, reliable, and code-compliant.
Equipment Grounding Conductor Sizing
The NEC provides specific rules for sizing equipment grounding conductors in industrial applications. Here are some key takeaways:
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For feeders and branch circuits over 600 volts, equipment grounding conductors must be sized according to NEC Table 250.122. This table outlines the required grounding conductor size as a percentage of the circuit conductor size.
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For feeders and circuits under 600 volts, equipment grounding conductors must be sized according to NEC Table 250.122 or be the same size as the circuit conductors. This means you can't just default to using a smaller gauge grounding wire.
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Raceway grounding conductors can be sized according to NEC Table 250.122 based on the enclosed conductors. Additional bonding conductors may be needed.
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Where parallel circuit conductors in multiple raceways are used, grounding conductor sizing is based on the total circular mil area (CMA) of all conductors.
Properly sizing grounding conductors is critical to ensure faults safely clear. Oversizing may be needed for large inductive loads. Consult NEC Article 250 for complete grounding conductor sizing requirements.
Grounding Electrode System
Industrial facilities require a grounding electrode system consisting of one or more grounding electrodes permanently connected to earth. This provides a low impedance path for faults and electrical noise. Here are some key grounding electrode requirements per NEC Article 250:
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Industrial installations generally require a grounding electrode system with multiple interconnected electrodes for maximum effectiveness. This includes a concrete-encased electrode, ground ring, ground rods, etc.
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Ground rings used as electrodes must be buried at least 2.5 feet below grade and be at least 20 feet from foundation footings. Use bare copper cable sized per NEC Table 250.66.
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Ground rods should be at least 8 feet long and spaced at least 6 feet apart. Connect rods with suitable grounding electrode conductor.
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A separate bonding jumper must connect interior metal water piping to the grounding electrode system. This bonds exterior metal water pipes via the water service entrance tube.
Properly installing multiple, redundant grounding electrodes ensures a low ground potential rise during faults. This is critical for personnel safety and protecting sensitive equipment.
Equipment Bonding and Grounding
In industrial settings, all exposed non-current carrying metal equipment must be bonded and grounded. This includes:
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Metal frames and enclosures of all electrical equipment. These must be bonded and grounded via equipment grounding conductors.
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Structural steel and metal building frames. These must be bonded directly to the grounding electrode system.
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Metal raceways and cable trays. These must have bonding jumpers at all discontinuities.
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Door frames, window frames, etc made of metal. These should all be bonded together and connected to the grounding system.
Proper bonding equalizes potentials and provides a low impedance path for faults. Use suitably sized bonding conductors per NEC Article 250 and common industry best practices.
Special Situations
Certain equipment requires special grounding consideration:
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Substations and switchgear require very low ground potential rise. A ground grid consisting of buried bare copper conductors is typically used.
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Large motors and variable frequency drives need star point grounding and often shielded power cables. Stray currents can otherwise damage bearings.
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Sensitive electronics like PLCs may need isolated grounding not connected to the main ground system. This avoids electrical noise issues.
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Instrumentation cabling requires shielded cables with shield grounded at one end only to prevent ground loops. Surge protection should also be provided.
Consult manufacturers' recommendations and design guides like IEEE 142 for grounding these types of systems. Specialized grounding methods are often needed.
Conclusion
Grounding and bonding are complex but critical aspects of industrial electrical installations. This article reviewed several key grounding requirements related to conductors, electrodes, equipment, and special situations that you may not have been aware of. Following NEC guidelines, IEEE standards, and manufacturers' recommendations for your specific equipment will ensure a code-compliant, safe grounding system. Consult with an electrical engineer if you have any questions. Proper grounding is well worth the effort and prevents countless issues down the road.