I come across this alot, especially in the inner city. Seems that people who cannot afford an electrician just fix the problem themselves by going to a larger breaker/ twist-in fuse. It's a shame because this is one of the easiest ways to start an electrical fire. Like the saying goes, good electricians aren't expensive, they're priceless!
I don't think that this pertains to overload specifically, but simply something that happens whenever current flows; conductors move when current flows through them. Conductors in a circuit tend to push themselves apart. The force will be roughly proportional to the square of the current flow, so the effects of this force will be more noticeable the higher current is. I've seen large cables jump during short circuit conditions.
Jon, I agree, I once had the priveledge to wire a testing station for spot welding guns for automotive manufacturing. The leads to the guns were about eight feet long and made of 500mcm welding cable. the cables would just hang there, side by side, until you threw the juice to them and then they would jump straight up and over so that they were 180 degrees apart and as far away from eachother as possible. It was fun to watch.
I have a good sized air compressor which runs on 120 or 240. I am currently running it on 120. It is plugged into a receptacle which is wired with thhn in emt. Every time it starts I can hear what sounds like one or two of the thhns slapping against the inside of the emt, very distinctly. I think the thhns are solid. I have also heard thhns inside emt make a lot of buzzing when energizing into a short circuit.
We get to home on a service call, wife called with bedroom circuit will trip breaker, so we look in the bedroom, TV, Computer with printer, electric baseboard space heater, at this point she informs us that her husband has fixed this type of problem before, so we go to the basement, and look at the panel, every 15 and 20 amp breaker except the bedroom circuit, was replaced with 30Amp breaker, we advised her this was a dangerous condition, and explained proper circuit protection, at this point, she went to the phone, and called her electrical expert husband, she returned from the phone, to tell us to please leave my house, my husband said, he will call someone that knows what they are doing.
Yes they will hum, usually not for long. Then they'll burn. Another example of this that is legal and normal is a large diesel geny (250-500 or so) as it starts. Watch the battery cables. The huge draw that occurs when the diesel starts causes the cables to shake and hum/scream for just an instant.
LK: I wonder what qualifies her husband as an "electrical expert"? It's obviously not the same standard that qualifies real electricians.
All too often, I have seen overfusing and "overbreakering" reduced to merely a matter of economics and convenience. The 15 amp fuse blows (or breaker trips), so I will put in a 30. It costs the same and you don't have that annoying blowing.
Logic without reason... . It keeps the Fire Departments and the Funeral Homes operating.
[This message has been edited by mamills (edited 11-29-2005).]