Minimizing the Choke Effect: Understanding ferrous metal raceway bonding

Recently, I was asked to explain the reason for the ferrous metal raceway bonding requirements in 250.64(E). Although the National Electrical Code never uses the term “choke effect,” many in the business understand its meaning, and the Code rules that it applies when an AC grounding electrode conductor is installed in a ferrous metal raceway. Here is a closer look at why the bonding requirements exist for these installations.

Grounding electrode conductors must be protected if subject to physical damage. In addition to concerns about physical damage from external causes, grounding electrode conductors can be affected by magnetic fields. This is a not a concern for grounding electrode conductors that are not installed in ferrous (magnetic) metal raceways or enclosures, such as installations in aluminum conduit or rigid nonmetallic conduit. However, if a grounding electrode conductor is installed in a ferrous metal raceway or enclosure, it 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 or enclosures are those with iron or steel content such as rigid metal conduit, intermediate metal conduit or electrical metallic tubing. These ferrous conduits and tubing have a magnetic property that reacts to rising and falling magnetic fields present in AC systems. The amount of current in a grounding electrode conductor can be relatively high for the duration of a ground-fault event.

The strength of the magnetic field will also increase in direct proportion to the amount of current in the conductor. In many cases, the magnetic lines of force in the conductor are induced into the unbonded steel 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 is the conduit, which is where 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 is acting like a steel core of a coil to concentrate the magnetic lines of force. This creates what the industry refers to as a choke effect. Specific bonding requirements are necessary for ferrous metal raceways that contain grounding electrode conductors.

Ferrous metal raceways and enclosures are 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 alternating current carrying conductor flowing only in one direction. There are varying amounts of current in a grounding electrode conductor during normal operation. This current can also 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 can get larger and smaller accordingly. This means stresses on the contained grounding electrode conductor will increase and decrease as the current rises and falls.

Because the ferrous metal raceway or enclosure surrounds this conductor, there is an inductive reactance between the ferrous metal raceway or enclosure and the contained grounding electrode conductor. This inductive reactance is one component of impedance, and it disrupts current in the 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 or enclosure. In actuality, the majority of the current would be present in the ferrous raceway or enclosure rather than the contained grounding electrode conductor, which is related to the skin effect. It is often referred to as the choke effect because this condition is the restriction of a grounding electrode conductor from performing its function.

The NEC requires ferrous metal raceways or enclosures for grounding electrode conductors that are not physically continuous from cabinets or equipment to the grounding electrode 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 or enclosure 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 methods required for bonding each end of the raceway or enclosure are provided in 250.92(B)(2) through (B)(4). There are similar rules in the NEC that address this issue when a single grounding electrode conductor is installed in a ferrous metal enclosure, such as conduit or tubing. For example, see 242.32 that references Section 250.64(E).

About the Author

Michael Johnston

Executive Director of Standards and Safety, NECA

Michael Johnston is NECA’s executive director of standards and safety. He is chair of the NEC Correlating Committee; chair of the NFPA Electrical Section; and a member of the IBEW, NFPA Education Section and the UL Electrical Council. Reach him at mj...

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