Cable Deflection During In-Rush

[paynejh]

I'm trying to find some information about Cable Deflection During In-Rush condition like starting a 250 HP motor. LRC is around 1000 Amps, I remember seeing a video about cable deflection during a short circuit and seem to remember the unsecured cables move about violently.

Thanks,

 

[BILLY101]

Current flowing through a conductor creates a magnetic field.
That is one of the reasons in the NEC for the very specific requirements for supports, raceways, cable tray tie downs, etc.

BILLY

 

[gar]

110208-1944 EST

The force between two long parallel wires D meters apart with I amps flowing in both is

F/meter length = 2 * 10^-7 * I^2 / D

6" = 0.1525 meters
1 newton = 0.2248 pound

1000 A in each wire and a wire spacing of 6" produces a force of

F/meter length = 2 * 10^7 * 10^6 / 0.1525 = 0.2 / 0.1525 = 1.31 newtons / meter

Change the center to center distance to 2 " and the result is 3.93 newtons / meter, or 0.88 # / meter length. Not much. Change to 10,000 A and the force is 88 # / meter. A meter is 39.34 inches.

Somebody check my math.

.

 

[paynejh]

Excellent! thank you very much. Anybody seen any videos? I know I've seen one, I think from Westinghouse, but I cant find it on the NET.

 

[topgone]

The electromagnetic force exerted between two current-carrying conductors depend on: the current in the conductors, the shape and the arrangement of the conductors, natural frequencies of the complete assembly, mounting of structure, insulators, and conductors. (Std. Handbook for EE, CH17:17-27)

Formula:

F = M x (5.4 x I^2)/(S x 10^7);

where F = lbs of force per foot of conductor
M = a factor depending on the circuit type
I = short-circuit current
S = spacing, center-center of cond., in.​

For 3-phase, maximum peak, choosing M = 0.8866 and a spacing of 6 inches, we have:

F = (0.866) x (5.4 x 1000^2)/(6 x 10^7) = 0.07794 lbs/foot of conductor
= 1.13 N per meter of conductor
(1 newton = 0.2248089 lbf and 1 meter = 3.28 feet)

P.S If it were DC current where the Table 17-7 say M = 1, @gar's computation result of 1.31 N/ meter of conductor is spot on!.

 

[Open Neutral]

Excellent! thank you very much. Anybody seen any videos? I know I've seen one, I think from Westinghouse, but I cant find it on the NET.

The one I saw was in power engineering class. The cables had been laid in a trench in the plant floor then backfilled with concrete. But before it fully set, someone powered up the cable...and the load was shorted. The cables FLEW out of the trench, spraying drying concrete everywhere and almost killing several workers before the OCP tripped.

This was the same day we saw pictures of what the 5' dia, 10 ft long generator does when someone closes the paralleling switch not when the three lights are out, but when they were on. The generator was in the parking lot, with a hole in the building wall and 2 flattened cars in view.

 

[dbuckley]

This was the same day we saw pictures of what the 5' dia, 10 ft long generator does when someone closes the paralleling switch not when the three lights are out, but when they were on. The generator was in the parking lot, with a hole in the building wall and 2 flattened cars in view.

[topic swerve]
A little while ago on this very forum I was challenged to provide some photographic evidence of bad things happening when gensets get put on line out of phase, but could not find anything. You don't happen to have copies of those photos perchance?

[Back on topic]
I heard a story from a co-worker some years ago, concerning his brother, who was studying power engineering at a UK uni, and they had a "cables jumping around under force of current moment" in their power lab; I cant recall the details but I recall it was quite a scary outcome.

 

[Open Neutral]

[topic swerve]
A little while ago on this very forum I was challenged to provide some photographic evidence of bad things happening when gensets get put on line out of phase, but could not find anything. You don't happen to have copies of those photos perchance?

I wish I could help. Alas it HAS been a few deca^H^H years since then. I happened to call that prof up last year to say hello and he's sorta losing it, sigh.

I can tell you they made a real impression on the class. It was a sobering reminder that mistakes can kill you and others, and not just from touching the hot wire.

 

[zbang]

This was the same day we saw pictures of what the 5' dia, 10 ft long generator does when someone closes the paralleling switch not when the three lights are out, but when they were on.

Do not try this at home.