NFPA 70E requires that all sources of energy be controlled “in such a way as to minimize employee exposure to electrical hazards” [120.2(C)]. On the surface, this would seem to mean if you’re working on a motor then you just need to open and lock the local disconnect for that motor. For a motor replacement task, this would be adequate since the motor terminals are the only ones the worker would be exposed to.
But what if the job is more extensive? What if it involves simulating the process for troubleshooting purposes? You’ve probably got a motor drive. Is it connected ahead of that disconnect or on the other side? How many disconnects are there? You’ve got a control cabinet with all kinds of energized terminals, many at 120V. Maybe 480V runs through there. And your tech is going to be poking around in that cabinet to figure out what’s causing the undesirable behavior with the equipment.
The answer to this puzzle is more complex. You engineer out exposure by several methods. For example:
- Group shock hazard terminals (120V and above) in one section of the cabinet, and guard them with a removable barrier.
- Add barriers between terminal strips to prevent inadvertent contact with one while working on the other.
- Install measurement ports rather than force electricians to connect directly to terminals. Even if you touch a port, you won’t get shocked (because the part that could shock you is recessed).
- Select lower voltage controls, equipment, and systems to begin with, provided that doing so won’t create performance problems in that particular application and environment.
- Install monitors to reduce the need for direct measurement.
- Use non-contact (e.g., inductive) measuring devices where practical.
- Have very clear troubleshooting procedures (written for sure, video if possible) to reduce or eliminate unnecessary poking around.
Another aspect is the method of locking out the equipment. How does that method expose an employee to danger and what can you do to engineer out that danger? This question is why systems for racking out breakers have evolved from those where the employee is right in the blast path to some systems that permit remote racking.
Related to that is the question of how to minimize exposure when using measuring devices on energized equipment. It’s why we now have DMMs that have remote displays. You lock out the equipment, connect the DMM, move away from the area of exposure, and energize the equipment again. That way, the equipment is de-energized when the employee would otherwise be exposed to electrical danger and then energized when the employee out remote enough from the point of measurement that there is no danger.
