Last Wednesday, we lost a leg of power to the house. I called AEP, and had them come out and see what was going on with our power feed. It turned out, before I bought my house 10 years earlier, someone had “temporarily” hooked up the power to the house with electrical box wire clamps! So the main wire going to my house was just clamped to the wire that fed the meter! They somehow lasted 10+ years before the aluminum began to melt and finally failed. I was told by the AEP line man, it is quite common, and often they will even glow at night from the arcing and sparking electricity! It only took him 10 or so minutes to fix it correctly with led couplers and a crimping tool.
What do I mean a leg of power? For those that don’t know, I will summarize. In oder to have 240 volts A/C in our homes, you will have three wires, two that are 120 volts, and one common neutral. If you look at the power line coming to your house, it is actually a twist of those three wires, the common usually aluminum and un-covered. In the breaker box, the two 120 volt feeds run down the center, and fork off alternating. So each slot will alternate between which of the legs it hooks too. A breaker will clip or hook on to one of them in various ways, depending on the type you have. Then that will feed that circuit 120 volts. For a 240 volt breaker, it will be the size of two 120 volt breakers, and hook on to each 120 volt leg making a 240 volt total.
So what happens when you lose power to only one of the connections? Well some rather strange stuff can happen, as it did to us. You might think just half the circuits in the house will get power, but in-fact, some power will cross over on the 240 circuits. So we had lights blinking, strobing, and flickering on half the house. When the 240 volt breakers were turned off, anything on that leg turned off.
We were very lucky that it did not arc enough to start the roof or a near by tree on fire!
Read more about breaker boxes here:
http://www.bobvila.com/articles/wiring-a-breaker-box/ //intro by Bob Vila.
http://waterheatertimer.org/See-inside-main-breaker-box.html //Lots of great info and excellent diagrams to explain things better then I have.