If you live in a three-story walk-up like we have all over the neighborhood, chances are you have a back porch with a gutter up top and downspouts that bring the water down from the gutters to the ground, and into the sewer. It is very important to keep these gutters and downspouts open to allow the water to drain down off your roof. In the winter, the accumulated snow at the roof line can melt into water and then freeze again into ice numerous times as it gets above and then below 32 degrees outside. When the snow melts into water, it's vital that a path in the gutters and downspouts remain open for that water to flow down and away from the roof. If the gutter does not drain properly, the water can fill up and overflow, causing theses very sharp and dangerous icicles. But that is not all. If the gutter becomes a cake of ice, the water have no place to go and will accumulate at the roof-line. Then, very likely, it will begin to leak into the ceiling of the third floor unit, and down the wall as well. Severe plaster damage will result, as well damage to personal items in the apartment.
So it is a very good idea to have de-icing cables installed in the gutters and downspouts of your building. Chicago code calls for them to be GFCI protected, which is a very good idea when you think about it. Typically, we will set them up with the on/off toggle switches in the basement and the gfci protectors right next to them. It is VITAL that these GFCI protectors be monitored. It's not a "set-it-and-forget-it" operation. The GFCIs can trip based on very small amounts of stray moisture, and when that moisture condition evaporates, they are ready to go again. But someone has to be monitoring them and re-set them. They are fairly delicate products, and you have to keep an eye on them. If the GFCI trips repeatedly, have your electrician inspect the cables and replace them if necessary. Remember, these are de-icing cables, not snow melting cables. They are not massively powerful flame throwers that will Nuke away your snow and ice. They are only meant to keep a trickle path open, just a very small clearing in the snow and ice to keep open a path for the water to flow down and not overflow the gutters. If you let the gutters fill with solid blocks of ice, the de-icing cables won't melt that away. At that point, you will have to wait for mother nature to take her course and hope for the best.
And one last thing, your gutters have to be pitched properly. In other words, the gutter has to slant slightly towards the downspout. I have been on roofs where the deicing cables had melted a path for the water to flow down, but since the gutter was not pitched correctly, the water did not flow anywhere. The whole system depends on gravity.