It’s obviously safer to de-energize a circuit before working on the equipment supplied by that circuit. A classic example is opening the supply breaker to a 120V receptacle before replacing the receptacle.
But let’s look at another 120V circuit to see how this is problematic. Consider a three-way switch. You would open the breaker to disconnect the wires, but you need it closed for you to determine which wire is the hot wire unless that wire is already marked.
Conducting thermography on de-energized circuits is also safer than conducting it on live ones, but you’ll have to be really quick with that camera to get anything meaningful once the relevant circuits are de-energized. Realistically, you must do this sort of work hot.
The examples of thermography and hot wire identification fall under the rubric of “infeasibility” [NEC Sec. 110.4(B)]. But disconnecting those three-way switch wires does not.
With thermography, it seems like a time-saver to tighten any hot connections with an insulated tool and watch the image to see if it gets less hot. Aside from the unnecessary exposure to energized equipment, this practice has a technical flaw. Simply tightening a connection usually makes it worse, not better.
This is because of the mechanical physics involved in making a tight connection. The fastener gets stretched by the application of X amount of torque; the tension resulting from the stretching is what makes the connection tight. But it’s a one-shot deal. Once you stretch a fastener to its elastic limit, it’s permanently deformed. That means you can’t “retorque” it. You have to replace the bolt and the locking hardware (e.g., locknut). It’s unsafe to do this with an energized circuit, and you can’t measure the resistance or conductance across the connection when you’re done.
It’s also worth noting that just because a connection looks hot in the thermographic image, that doesn’t mean it’s bad. It means it’s suspect. Unless you want to go through the time and hassle of replacing all connection hardware that is merely suspect, you should measure across the de-energized connection to determine its conductance.
So, while the infeasibility rule of Sec. 110.4(B) applies to working while de-energized, it also has a flip side. It is often infeasible to “work hot” — even if you think you can do so safely.
About the Author
Mark Lamendola
Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.
Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.
Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.