Commonly misunderstood/misapplied terms, part 10.

  • Special permission. This does not mean the corporate office says it's OK so you can do it. This kind of permission comes from the authority having jurisdiction (AHJ, see Art. 80 for definitions and details), and must be written. Before you implement any special permission design or construction technique, also obtain the permission of your insurer. The insurer may not agree with the AHJ and, in the event of a conflict, you’ll want to keep your insurance policy in effect.
  • Structure. The NEC often refers to "a building or structure." However, the NEC defines a structure as "anything built or constructed" and defines a building as a special type of structure. All buildings are structures, but not all structures are buildings.
  • Surge arrester. This isn't a generic term for "any device that mitigates electrical spikes." It's a specific class of such devices (see Art. 280). Its mode of operation is it discharges or bypasses surge current and stays capable of doing this repeatedly. The spark gap arrester on the utility pole is an example.
  • Surge protective device (SPD). These are covered by Art. 285. The SPD mode of operation is it limits transient voltages by diverting or limiting surge current. An MOV is an example.