Burn, baby burn! Who was around in the late 70s and is hearing the Tramps in their mind right now? For the younger folks out there, that was the refrain from an awesomely bad disco song. Unfortunately, it applies to electrical work as well some times.
This GFCI outlet has seen better days. This is what can happen when it is installed either without a truly effective seal, or installed in such a location that it is subject to way too much moisture no matter what type of cover is on it. In this case, it was a little bit of both in my opinion.
On this service call, the exterior lights were out all over the place, and the owner called me out to try to fix the lights before someone fell down the stairs. It was in a late 1990s 3-flat. The owner first told me about the problem, and then showed me a circuit breaker in the public panel that would not remain re-set. (BTW, that is usually the sign of a dead short. You reset it, and instantaneously it pops again. And again. And again.) What the breaker is trying to tell you is that there is a dead short in the line, and it refuses to supply electricity until you "clear the short." Even though homeowners who experience this often think the problem is in the breaker, (The darn thing won't stay re-set!) it is not. The breaker is doing exactly what it is supposed to do. It is automatically disconnecting the power source from the shorted premises wiring. The continual tripping every time you reset is the symptom, not the cause, of the problem. The cause is...well, you can pretty much see what the cause is.
At any rate, as it turned out, the reason the circuit breaker tripped every time was there was a dead short between the hot and neutral sides of the outlet, created in my view, by the moisture that seeped in, soaked the whole outlet, and created a path for current flow. The moisture, which was a moderately low resistance path (high enough that it did not immediately trip the breaker) But allowed current to flow for a good long time as the plastic casing melted down in the outlet. Eventually, enough of the copper came into direct contact as to create an extremely low resistance shorted path and then tripped the circuit breaker. And every time the breaker was re-set, it created another momentary surge of electricity to melt it down a bit more.
It looks to me like there were 2 contributing faults here. The first is that the outlet cover was not effectively sealed. This cover is loosely hinged with no spring or mechanism to insure pressure remains to keep it tightly closed. The two plastic pieces probably did not fit together tightly enough to keep all the water out. That is par for the course these days. Most 'in-use covers" required by code today do not hermetically seal the surfaces. They rely on the installer to locate the outlet where water will drain away, not directly into the outlet.
Second, in my humble opinion, the location of this outlet is not very advisable . I'm not sure if you can tell from the pictures, but this outlet is recessed into the side brick wall that encloses a set of concrete stairs. There is only one course of brick between the "floor" of the stairs and the outlet elevation. (It faces the inside of the stairs) In Chicago, it is pretty safe to say that this elevation will be completely encased in snow for a good part of the winter. Anyone who remembers the blizzard we had last winter will realize that. People shovel their stairs, of course. But they are probably not as diligent and precise as it would take to be sure no snow gets packed up against this cover. In rental property, where a maintenance man has to keep the steps clean (as well as the steps of all his other buildings, all of which are screaming for him to be here NOW!) it is even less likely. He may have even grazed it with a shovel, jarring it open. Based on my memory of an impassible, shut-down Lake Shore Drive last winter, with 3-5 foot drifts burying hundreds of cars, I would bet my bottom dollar that this outlet had a huge amount of snow packed up tight against it, and as it slowly melted, the GFCI receptacle was given a bath. Long story short. Think about where you locate exterior outlets.
The ironic thing was that this short had no effect on the exterior lights that were off. I discovered it by accident, really, in that I traced the wiring and piping from the shorted breaker the owner identified to me. Of course, the breaker was not labeled. As it turned out, there was a photocell up on the roof that was pretty well hidden, which we located a couple days later, when the lights still did not come on after I eliminated this outlet. The photocell was bad.