Thank you for your question regarding locknuts and metal conduit. There are numerous articles within the National Electric Code that relate to this topic: NEC Article 250, 314, 320, 358, 300, 342, 348, 344 and many more. The one that we are of course primarily concerned with is 250, the chapter on grounding.
The locknut provides two key factors, mechanical support for the conduit and box system, and is critical to ensuring the metal conduit is grounded. Both are requirements within the National Electric Code (NEC). That said, your local inspector will be a good source of information if you feel you have a special exception in this case for excluding the standard locknut. It is important to note that the NEC has been increasing its tie-down requirements for conduit and cables over the years, and as such inspectors are becoming far more critical on this topic. You may find that not only do you need the lock-nut, but you also need to provide edge protection in the form of a plastic insert at the coupling to ensure that wire insulation isn’t worn off over the years as the metal flexes and expands due to temperature changes (steel and copper expand/contract at different rates causing rubbing).
At a minimum you will need to ensure that you have an effective ground path for the metal conduit, or else your OCPD (circuit breaker) may not function during a fault, particularly if your electrical system has Edison circuits or questionable grounding to the outlets.
We hope you find this information useful, if you should have any further questions, please do not hesitate to contact us.
The Engineering Team at E&S Grounding Solutions