That little snippet of folk wisdom may account for more electrical fires or severe shocks than all other reasons put together. I say may, since I am not a legal authority on the matter. But I know what I know and I've seen what I've seen.
At any rate.....
All over Hyde Park, and the South Side, and probably all over the city, there are back porches built years ago that have porcelain bare-bulb light socket fixtures on the exterior brick wall to illuminate the porch area. In most cases, the lights on the ground level exterior, near the basement doors and in the gangways and walkways on each side of the building are also these bare bulb porcelains, leftover from the 1950s or 60s. Most people do not realize that these are no longer legal, (if they ever were, technically) and they are often close to falling apart on the inside.
The Chicago electrical code mandates that fixtures that are mounted outdoors be watertight and weatherproof. These never were. Now, if you are an electrician you may make a nuanced (picky) argument that the code defines dry, moist, and wet locations differently, and "moist" locations are not subject to a 3-hour beating rain, and being on back porches that are shielded from the beating rain made them OK. This may be true in a narrow, technical sense, but that's like saying "I should be alive because the guy who ran me over by blowing through the intersection technically entered it on a yellow light, not a red light." Unfortunately, that nuanced technically legal observation doesn't make me any less dead. The "moist" vs "wet" location code interpretation here just doesn't hold water. (OK, I couldn't resist.) These fixtures are subject to seepage and soaking in any serous rainstorm, as the wind regularly blows the rain over them.
The problem is the porcelains have no seal between the fixture and the junction box. This means over the years, water drips down inside and corrodes the wires. As you can see in the middle picture of two of them, (after being taken down) the terminals where the wires attach have corroded to the point that they are barely identifiable as screw terminals to accept the wires at all. At this point, there is so much corrosion and oxidation on the wire and terminal that the electricity has to "fight" its way through the high-resistance corrosion rather than flow easily through copper-to-brass connections. That makes the fixtures heat up a lot, and also wastes a ton of electricity.
But the REAL HAZARD of these fixtures is that, since there is no effective seal for waterproofing between the "live" wires on the inside and the metal junction box and piping they are attached to, a good soaking can create a temporary secondary path for current to flow between the "live" connection, through the moisture, and into the metallic junction box. (and thus all the pipe connected to it) Under optimal conditions, that metal pipe is grounded, so when a live connection is accidentally created, it should trip the breaker or blow the fuse. Except for two problems. The first is this would create neither a high-fault current dead short, nor an amperage overload, which is what is required to trip a standard circuit breaker or fuse. Only GFCI protection would address this issue. and no one ever installs that level of protection for exterior piping! The second issue is that these pipes are also 50 years old, and they themselves are often corroded enough to lose their low-resistance path to ground. The rust on them acts as an insulator, so they don't provide an uninterrupted path to ground anymore. In addition, the ground path can easily be broken when pipes are pulled apart, couplings and connectors broken by falling ice in the winter, ect. or, as in here at this building, a very powerful grape vine trunk line literally "yanked apart" two pieces of conduit that had been previously coupled together. Once you lose that low-resistance grounding path, any "hot" or "live" wire that makes contact with the junction box will extend that "live" condition across the entire metallic system if it is not grounded properly. In the worst case, on a very wet day, someone standing on her back porch in a puddle of water and touching or leaning against the pipe running up the wall could potentially recieve a serious, if not fatal, shock.
BY THE WAY, EVEN THOUGH CHICAGO AND NATIONAL ELECTRICAL CODE ALLOW THE PIPE ITSELF TO BE CONSIDERED AS AN ADEQUATE GROUNDING SOURCE, WE ALWAYS PULL AN ADDITIONAL COPPER GROUNDING CONDUCTOR TO BOND ALL METAL PARTS TOGETHER WHEN WE RUN OUTSIDE EXPOSED PIPE. CODE DOESN'T DEMAND IT. BUT MY CONSCIENCE DOES. DOES YOUR ELECTRICIAN FOLLOW THIS PRACTICE?
So by replacing these unsafe porcelain fixtures with bonafide waterproof fixtures, we have eliminated this hazard!
The bottom two pictures show exposed porcelains at a job we are doing this summer. This is another pet peeve of mine. These are "flopping in the breeze" as the inspectors say. Code mandates these junction boxes (and thus their attached light fixtures) be securely and permanently mounted to the wall or surface of the structure. In this case, they were securely mounted originally, when the old porch was in place. The porch contractors obviously disassembled the old porch, left them hanging, and then rebuild the new porch around them. Porch contractors rarely use real electricians, licensed or otherwise, to deal with the issues of the power and lighting they impact when re-building a porch. They have neither the respect for craftsmanship nor safety that would require. And of course, the layman customer who does not know any better, is left holding the bag. There are some exceptions to this practice. But they are few.