Can someone please explain what is happening in this short video?

[mikeadams2]

I saw this clip on one of the Mike Holt DVDs and I don't remember exactly what it was a demonstration of. Can someone please explain? Thank you in advance.

http://www.youtube.com/watch?v=AYQEeuv6OK0

 

[augie47]

appears to be showing the deflection or movement of conductors subjected to a short circuit. You can experience a similar albeit far less dramatic phenomenon with conductors in conduit when subject to a large inrush of current.... they can "rattle"

 

[GoldDigger]

If you are interested in the underlying physics, each conductor creates a magnetic field surrounding itself which is proportional to the magnitude of the current.
The moving electrons in the other conductor are moving at right angles to that magnetic field and so are subject to a force which is proportional to both the field and the current and is directed away from the other conductor.

The result is a force that is trying to separate the two wires and is proportional to the square of the current.
So, what could be a barely noticeable force under normal current can become an incredibly violent force under fault currents.
The calculation of the force with DC is straightforward, and since the direction of the force (repulsive) is independent of the direction of the current as long as it is opposite between the two conductors, the force is also repulsive for AC.

Tapatalk!

 

[ActionDave]

It looks to me like it is showing the power of an arc blast, the conductors are just part of mechanism to make it happen.

 

[GoldDigger]

But the arc blast is not what blows the conductors apart.
The same thing will happen to them regardless of how much energy is part of the arc flash or whether it even happens in the same room.
This same magnetic repulsion is also what makes the arc rise in a Jacob's Ladder.

Tapatalk!

 

[Eddy Current]

appears to be showing the deflection or movement of conductors subjected to a short circuit. You can experience a similar albeit far less dramatic phenomenon with conductors in conduit when subject to a large inrush of current.... they can "rattle"

So thats the rattle you hear in a conduit when a breaker trips, except this isn't in a conduit?

 

[Fulthrotl]

appears to be showing the deflection or movement of conductors subjected to a short circuit. You can experience a similar albeit far less dramatic phenomenon with conductors in conduit when subject to a large inrush of current.... they can "rattle"

i've heard that once... guy reversed a bus tap on a 480 volt section of bus that
was fed by a 4" emt with 500 mcm's in it.

so that B phase and ground were reversed, a bolted fault.
it sounded like someone beating on the pipe with a chipping hammer.

 

[augie47]

Good way to see if your couplings and support structure are secure

 

[Jraef]

It's the reason why we are supposed to brace conductors by strapping or lacing, and why bus bars must be braced for the available fault current level.

Here is the same concept demonstrated with HV solid bus bar and in slow motion so you can see the effects.

http://www.youtube.com/watch?v=rmdzIFFN9OE&feature=youtube_gdata_player

The good stuff starts at about 16 seconds. You'll be able to see the "waves" of force created by the AC flowing that makes the magnetic forces expand and contract.

 

[mgookin]

It's the reason why we are supposed to brace conductors by strapping or lacing, and why bus bars must be braced for the available fault current level.

Here is the same concept demonstrated with HV solid bus bar and in slow motion so you can see the effects.

http://www.youtube.com/watch?v=rmdzIFFN9OE&feature=youtube_gdata_player

The good stuff starts at about 16 seconds. You'll be able to see the "waves" of force created by the AC flowing that makes the magnetic forces expand and contract.

Thanks. Great video and your explanation makes it all come together. Lesson learned for the day. It's now beer:30!

 

[Ragin Cajun]

I witnessed a fault test on aluminum bus duct. Three ~ 1/2 x 6" bars with ~1" separation. After the test they were bent such they touched each other. Huge forces involved in faults, even when things dont "fail".

Wal in a metal fab shop where they had large auto welders. You could hear the conductors rattle in the cable tray as the welders were in operation.

RC

 

[just the cowboy]

I worked on a machine once the demonstrated this great, it was a non-destructive test machine that would pass thousands of amps at low voltage thru a piece of metal to check it molecular properties. The cables connecting it (12 x 000 fine strand cables) would jump as much as 8 feet. hmy:

 

[junkhound]

re: This same magnetic repulsion is also what makes the arc rise in a Jacob's Ladder.

Just to clarify for any young guys: an arc will move away from the source on 2 bus bars with high current (even will move down for high current) but magnetic forces are not the cause of the rising arc seen in a classroom type Jacob's ladder.

A Jacob's ladder typically is limited to mA and the dominant force BY FAR is the buoyant force of the air due to hot arc. If you don't believe this, hold a Jacob's ladder upside down or even sideways just a few degrees down - the arc stays right at the starting point- smallest gap, does NOT go toward the tip if upside down - hot air rises (and pushes arc with it).

 

[Jraef]

I got a call once on a 350HP soft starter I had installed on a gang (arbor) saw at a lumber mill, it has a long shaft with 52 circular saw blades set up to cut a 4" thick slab of wood into 4" fence boards. To get the motor to start at all with all the friction from the blade guides, it had to get 450% current for almost 30 seconds. The users had disabled the soft starter because of the banging sounds it made. When I got there, they had moved it and ASSumed that my lacing was just "old fashioned BS" that was unnecessary. The banging was the cables whipping around inside the soft starter enclosure after the little ty-raps they had used snapped off. If they started is across the line, which took 700% FLC for 1 second, the cables still banged, but for only a second and nobody noticed as much. The soft starter, in reducing the current, extended out that amount of time where the current has really high. Nobody had ever taught their electricians why lacing (or proper bracing) was necessary.