Most engineers, electricians, and equipment manufacturers know that conductive (metal) electrical cabinets are required to be properly bonded to earth through a grounding conductor. Article 250.4 of the National Electric Code (NEC) details the requirements for the grounding and bonding of electrical enclosures in grounded (Article 250.4(A)) and ungrounded (Article 250.4(B)) systems. While grounded and ungrounded electrical systems are different, the requirement for conductive materials that are likely to become energized (i.e. enclosures) are the same; they shall be installed in a manner that creates a low-impedance path for ground-fault currents.
In practice, how does one properly bond a conductive (metal) enclosure to ground, especially since these components are treated to resist corrosion?
Article 250.12 of the NEC requires that all non-conductive coatings around threads and other contact surfaces shall be removed as to create good electrical continuity. For enclosures constructed from aluminum or stainless steel, the process is straightforward as no corrosion resistance treatment is typically applied to the enclosures due to inherent corrosion resistant properties of the material. To bond the enclosure to ground, simply install an equipment grounding terminal on the inside of the enclosure and connect the grounding conductor.
For enclosures constructed from steel, the process of properly bonding the enclosure to the grounding conductor is not as simple as placing a grounding terminal inside of the enclosure and securing it with a bolt and a nut. Because paint or lacquers are used to prevent corrosion, they interfer with low-impedance connections and must be removed. Proper removal of paint or lacquers or other corrosive protective coatings is not achieved by using “star washers” to “dig” into the paint. While “star washers” can pierce the paint and lacquers, these devices are inconsistent in ensuring a low-impedance connection for ground-fault currents. The only method for ensuring a low-impedance connection for ground-faults is to physically remove the paint or lacquer around the equipment ground lug terminal. This can be achieved through sanding, masking or other methods.
To ensure proper bonding and grounding of the electrical equipment, make sure that the main grounding terminal and all other equipment bonding locations are properly cleaned prior to securing the ground lug to the enclosure.
As a rule of thumb, the minimum amount of paint or lacquer to be removed should be twice the radius of the bolt or PEM stud used to connect the ground lug to the enclosure. Equipment manufacturers, in some cases, have removed paint or lacquer around the equipment ground connection up to the size of the equipment grounding terminal (see above). While removing a large portion of paint or lacquer to ensure a proper bond between the enclosure and the grounding conductor is acceptable, one needs to ensure that corrosion resistance of the steel enclosure is maintained.
The actual area of paint or lacquer that is removed around the enclosure grounding connection depends on the type of grounding connection, specification requirements, testing requirements, and specific manufacturing processes. In all cases, enclosures using paint or lacquers for corrosion resistance must be cleaned prior to connecting the grounding terminal to the enclosure.
The actual area of paint or lacquer that is removed around the enclosure grounding connection depends on the type of grounding connection, specification requirements, testing requirements, and specific manufacturing processes. In all cases, enclosures using paint or lacquers for corrosion resistance must be cleaned prior to connecting the grounding terminal to the enclosure.
