ECN Forum Forums Technical Discussion Electrical Theory and Applications Rattling conduit

The AAAAHNKMPH of an overloading panel is quite a memorable sound...

I wired up some resistance welders that when the welder made a spot weld the cables to the part you held to weld would jump around a bit.

Just noticed this post. Just wondering. I've heard this sound in conduits( rattling sound of the wires), and the breaker not trip. Is there any way to troubleshoot this problem or fix it. Is it just the overload situation that does this, and it's not enough to trip the breaker?
Maybe to small wires for the load? I've mostly heard it on circuits that have motor start up's on them. Just curious. Need to know how to explain it. Thanks ....Steve

The official way to avoid this is to triplex the conductors (twist them together) prior to pulling them into the conduit. That is not always practical, but it does solve the problem. If they are already installed and you must solve this problem, adding a soft starter or other reduced voltage starter can often reduce the starting current enough to avoid the strong magnetic forces that are making the cables repel each other.

Had a couple of those ultra-high efficient air conditioner condensors cause the same distinctive sound. Ended up replacing the conduit and wire with romex. Will remember the "twist the wire together" trick. Sounds (no pun intended)like it might work.

You can order triplexed conductors from most larger supply houses.

Or these guys: http://www.customwireandcable.com/

There is also a direction to twist, and ratio, so doing it yourself with a large feeder is un-wise. But it is the same direction, and amount of twist as a same sized romex or MC cable.

An article on the subject: http://www.findarticles.com/p/articles/mi_qa3726/is_200304/ai_n9218102


[This message has been edited by e57 (edited 05-01-2005).]

[This message has been edited by e57 (edited 05-01-2005).]

Interesting. Have heard it in one building I often work in when the elevator starts.

Bump..

Any one got some new war stories for this one?

The force between two parallel wires is defined by F/L = mu0 * I1 * I2 / 2Pid.

Two wires that are touching (insulation to insulation) and carrying 1000A between them will push apart with a force of about 1lb/ft. If they're hit with 20KAIC durring a bolted fault, that jumps to about 275lbs/ft! This is why you have to tie down cables in some applications.

If, instead of running parallel conductors through one conduit, you run the 3 phases together (or better yet, triplex them) and split the parallel conductors up in different conduits like that, the magnetic fields will cancel and pull the cables together as opposed to tearing them apart. This will cause less inductive losses in the conduit, too.

[This message has been edited by SteveFehr (edited 11-17-2006).]

I once got called out on a problem with a 500HP soft starter on an "Edger", a horizontal circular gang saw with 54 blades on a shaft cutting slabs of wood into fence boards. Whenever they first started the machine in the morning, there was an awful banging sound coming from the soft starter, but the rest of the day it was fine. Everyone there, including the plant electricians, were affraid of the soft starter because they didn't understand it, but they did know that it didn't have any moving parts so there shouldn't be anything banging.

What it turned out to be was that in the morning, the saw blades would shrink in circumfrence, but actually widen by just a tiny fraction of an inch. They had blade guides on them, Delrin blocks with slots in them that actually touched the blades to keep them from warping. In the morning when the blades were fatter, the small amount of additional friction, multiplied by 54 blades, translated into so much extra load that the soft starter would stay in current limit until the ramp time would expire, then go across-the-line. When it did, because the blades weren't moving yet, the current jumped to 3500A, and the cables inside the soft starter cabinet broke out of their straps and started banging on the enclosure walls.