Hint: Water played a part.
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I discovered this water-damaged GFCI receptacle when a customer called me to see why his Christmas lights had stopped working. The cover that was installed on this outdoor location GFCI receptacle was the type that is weatherproof only when the cover is closed. Unfortunately, the Christmas lights and some other cords were left plugged into this receptacle almost year-round. This meant that the cover was open, which allowed water to infiltrate the enclosure and eventually cause significant damage to the GFCI receptacle. Believe it or not, the circuit breaker for this receptacle never tripped. The conductors feeding this receptacle were still energized when I arrived. Of course, the receptacle had stopped working. It’s pretty scary to think what might have happened if the customer did not call when he did.
Section 406.9(B)(1) of the 2017 NEC requires weatherproof while in-use covers to be used for 15A and 20A, 125V and 250V receptacles installed in wet locations. The use of a “bubble cover,” as they are commonly called, may have prevented this serious water damage from happening. Listed weather-resistant (WR) type receptacles are also required to be used for wet locations.