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Are your disconnects or safety switches rated for grounding when using the concentric knockouts, or are bonding bushings needed?

Here is a response we have on file from the product line addressing this issue.

Let me give you the standard answer for the occasional question of whether our knockouts in our device enclosures are suitable to be used with self grounding hub fixtures without the need for additional conduit grounding straps, washers or bushings.  Keep in mind that some customers are not happy with this answer and would like to see us test for this issue, however they were comparing different products with different UL standards and our schedule has not yet permitted such testing and evaluation.

            Our safety switches comply with the requirements of UL98, the Standard for Safety of Enclosed and Dead-Front Switches, and have undergone the necessary tests to obtain UL's approval to display the UL mark on said devices.  UL98 refers to UL50, the Standard for Safety of Enclosures for Electrical Equipment, for enclosure construction requirements which our safety switches must also meet.  It is, however, the responsibility of the installer to install these devices in accordance with all local and national codes.

            We understand concerns of a possible break out of a larger knockout when rigid metal conduit is secured within a smaller knockout opening and the NEC addresses this issue in the 1999 NEC, Article 250-96.  The installer is to remove the inner knockouts without disturbing the integrity of the larger ones.  All webs should stay intact on knockouts that are to remain.  Our knockouts are designed and tested to accomplish this, however the ability of the installer ultimately determines the final results.

            Frequently tangential knockouts are acceptable in the field where concentric knockouts are not, however this does not correspond with any UL or NEC requirement.  To our knowledge, no specific standard or local installation requirement exists that discriminates between these two knockout designs.

            Our devices comply with the UL standard requirements for grounding provisions.  Typically, at least a #4-14 AWG wire size lug (lay in type) is factory installed for grounding our safety switch enclosures.  Grounding kits are also available for many catalog numbers.  On any metallic enclosure, unless otherwise restricted by local codes, a self grounding hub fixture should be acceptable for conduit grounding regardless of size of the knockout in which it is placed, provided the integrity of the larger knockouts have not been compromised during installation.  This should hold true for both tangential and concentric knockouts in metallic enclosures.  Only when nonmetallic enclosures are used is it normally required to have bond straps, washers or bushings on rigid metal conduit.

            In conclusion, field inspectors typically have the final say in accepting or rejecting installations based on their own interpretation of the local and national code requirements;  therefore it is suggested to discuss this issue with the field inspector before or as early as possible during installation of electrical equipment to determine if separate bonding needs are required for your specific applications.