Guardian Of The Ground

Grounding electrode conductors are essential in the grounding and bonding scheme for services and separately derived systems. They generally must be sized according to Table 250.66 of the National Electrical Code (NEC) and are required to be installed in a continuous length or otherwise spliced in accordance with any alternative in 250.64(C). They must also be protected in accordance with 250.64(B) where subject to physical damage.

In addition to concerns about physical damage, magnetic fields can affect grounding electrode conductors. Section 250.64(E) includes requirements to address such protection. If a grounding electrode conductor is installed in a ferrous metal raceway, the raceway must be electrically continuous from the point of attachment to the cabinet or equipment to the grounding electrode and must be securely fastened to the ground clamp or fitting. Ferrous metal raceways contain iron or steel content, and examples include rigid metal conduit (RMC), intermediate metal conduit (IMC) and electrical metallic tubing (EMT). These conduits and tubing have a magnetic property that reacts to rising and falling magnetic fields present in alternating current (AC) systems.

Varying amounts of current can be present in a grounding electrode conductor during normal operation. During a ground-fault event, the current in a grounding electrode conductor can fluctuate and even be relatively high for the duration of the event.

Ferrous metal raceways must be bonded to the contained grounding electrode conductor to reduce the effects of magnetic fields that are present while the system is energized and in use. The grounding electrode conductor for an AC system or service is an AC-carrying conductor with the current flowing in one direction. This current can rise and fall significantly depending on events such as ground faults, short circuits or line surges. As the current rises and falls, the magnetic field of the contained conductor typically gets larger and smaller accordingly. This means the stresses on the contained grounding electrode conductor increase and decrease as the current goes up or down.

Because the ferrous metal raceway is enclosing this single conductor, there is an inductive reactance between the ferrous metal raceway and the contained grounding electrode conductor. This inductive reactance is one component of impedance and actually impedes current in the contained grounding electrode conductor. The magnetic field and the capacitance results in a coupling effect between the current in the conductor and the surrounding ferrous metal raceway. In actuality, the majority of the current would be present in the ferrous metal raceway rather than the contained grounding electrode conductor.

The magnetic field’s strength increases in proportion to the amount of current in the conductor. In many cases, the magnetic lines of force in the conductor are induced into the conduit enclosing the grounding electrode conductor; they can even surpass the saturation point of the steel raceway. At the point where the grounding electrode conductor exits the conduit, the magnetic lines of force generated by the fault current in the conductor will try to be induced on the end of the conduit, creating a saturation point that exceeds the conduit’s capacity. The steel conduit, in this instance, acts like a steel core of a coil to concentrate the magnetic lines of force. This condition is often referred to as the “choke effect” because it is actually the restriction of a grounding electrode conductor from performing its function. Because of this, specific bonding requirements are necessary for ferrous metal raceways that contain grounding electrode conductors. This is not a concern for grounding electrode conductors that are installed in PVC conduit or other nonferrous metal raceways such as aluminum or brass conduit. Sometimes the type of construction will not permit PVC conduit.

Section 250.64(E) requires ferrous metal enclosures for grounding electrode conductors that are not physically continuous from cabinets or equipment to the grounding electrode, such as sleeves or short lengths of conduit used for physical protection, to be made electrically continuous by bonding each end of the raceway to the contained grounding electrode conductor. This action puts the contained grounding electrode conductor in parallel with the enclosing ferrous metal raceway so the two work together when the current in grounding electrode conductors rises and falls in response to various events occurring on the system. The current will actually divide over both paths, but due to the skin effect, the majority will be present in the surrounding ferrous metal raceway.

The methods required for bonding each end of the raceway are provided in 250.92(B)(2) through (B)(4). These methods apply to all intervening ferrous raceways, boxes and enclosures containing the grounding electrode conductor. If a bonding jumper is used to accomplish this bonding to intervening metal raceways and enclosures, the size of the bonding jumper must not be smaller than the required contained grounding electrode conductor as provided in 250.64(E). Several manufacturers produce grounding and bonding fittings that are specifically designed and listed for this purpose.

About the Author

Michael Johnston

Executive Director of Standards, NECA
Michael Johnston is NECA’s executive director of standards and safety. He is chair of the NEC Technical Correlating Committee. He served as a principal representative on NEC CMP-5 representing IAEI for the 2002, 2005, and 2008 cycles and is currently...

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