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Posted By: ds247 Rattling conduit - 08/03/04 03:47 AM
When there is an overload, often just before the breaker trips the conduit will often start to rattle and shudder. Any one have an idea why? Just curious. Thanks
Posted By: Scott35 Re: Rattling conduit - 08/03/04 04:30 AM
That's the sound of the Conductors literally "Bouncing Around" inside the Conduit, from the high level of Fault Current + related magnetic forces.

Scott35
Posted By: Bjarney Re: Rattling conduit - 08/03/04 06:53 PM
Likely the conductors are trying to take the shape of a set of sine waves in the process. It can be a very memorable sound.
Posted By: frenchelectrican Re: Rattling conduit - 08/04/04 06:45 AM
also try use the jumper cable to start up large diesel engine also you can see the jumper cable do move even smaller jumper cable do move the same way with med/large gas engine too due the current and mangatic force on the wires to push each other out ward it happend to me many time espcally when the starter have direct short inside the starting motor [Linked Image]

merci, marc

ps it also do show up with 3 phase service too when someone is stupid egough to bunch each phase in one conduct and it will viberate and build up pretty good amout of heat too [Linked Image]


[This message has been edited by frenchelectrican (edited 08-04-2004).]
Posted By: golf junkie Re: Rattling conduit - 08/04/04 11:22 PM
"It can be a very memorable sound."

It scares the H**l out of me!
Posted By: Bjarney Re: Rattling conduit - 08/05/04 03:31 AM
Used to do work in a plant that had a pair of 500hp trailer-mounted screw compressors, served by an overhead 480V span. Motor starting would cause the three individual conductors to “repel“ each other.

Running, the three conductors hung in a vertical line, but during starting they would deform into an odd triangle. It looked quite weird.

If the overload occurs very often, periodic meggering to catch cable-insulation deterioration may be wise, partly depending on the cost of a related production outage.
Posted By: Scott35 Re: Rattling conduit - 08/07/04 06:18 AM
Glad Marc posted this:

Quote

also try use the jumper cable to start up large diesel engine also you can see the jumper cable do move

I once saw the very same thing occur - with of all things, the jump starting of one Diesel Electric Locomotive from another Diesel Electric Locomotive (this was at a Branch Line's Station where Locos are kept, fueled and light maintenence performed for equipment used as local switch duties).
The unit being "jumped" (an SW1500) was hard starting, which nearly drained the Batteries; so a running, warmed up unit (GP9) was used to jump start the switcher unit (the SW1500).

The cables bounced quite hard during locked rotor through the first few rotations of the Crankshaft, then they bounced accordingly as the "tighter" pistons / cylinders were moving up on compression.

Being that these Prime Movers are 2-stroke Diesels with Blowers (Superchargers) and Turbochargers, the cranking compression is lower, and compression increases when things move faster; - i.e. the scavanging devices kick in; but still has a large volume of air to squeeze into a small space during starting.

Wierdest stuff to see (and of course- no camera!). Jump Starting a Locomotive was, in lieu of a better word; "Odd Enough", but the reactions of the cables added more "!@#$%" and [Linked Image] to that Day's event!

p.s. since Marc has/does Engineer Locomotives (operates them), I suspect the scenario above is where His reply is targeted towards.

Also, as Scott ("Bjarney") replied, hard starting / large Motor starting does unique things to stationary Conductors - and their reactions are heard with a very "Distinctful Sound" when the Conductors are in Conduit.

Hydro-Pneumatic Plunger type Elevators (actually their Pump Motors) normally set things in motion when they are thrown across the line (started / turned on / begin doing something Kinetic, rather than occupying space and their Rotor has 100% Static Inertia situation...).
Conduits, Boxes, Strapping, Conductors - along with Panelboards, bussing and OCPD belch out a harsh "Buzz/Shake" for about 100-250 milli seconds.

Scott 35
Posted By: Scott35 Re: Rattling conduit - 08/17/04 07:26 AM
ds247;

Quote

When there is an overload, often just before the breaker trips the conduit will often start to rattle and shudder. Any one have an idea why? Just curious. Thanks

Just wanted to know if the replies so far have been sufficient to describe the "wazzup???" and the "whaddaheck???" involved, per your original question
(Did the replies answer the questions OK, or just make more questions)
[Linked Image]

Scott35
Posted By: Frank Martino Re: Rattling conduit - 08/23/04 09:37 AM
The last time I heard a conduit rattling was because there was a fault current passing on the conduit.
Posted By: mxslick Re: Rattling conduit - 12/08/04 05:51 AM
Wanted to bump this up and add that I've also heard some motor contactors/overload relays emit a high-pitched squeal during motor starting....

Many years ago I also saw pictures of destructive testing of industrial switchgear..copper buss as big as one's leg were distorted into odd shapes, insulating supports totally destroyed, and the test engineers were reportedly knocked out of their chairs in the bunker by the blast!!

The fault current in the test was over 2 MILLION amps!! Yikes!! (I think the voltage was 4Kv.)
Posted By: DougW Re: Rattling conduit - 01/30/05 11:47 PM
The AAAAHNKMPH of an overloading panel is quite a memorable sound...
Posted By: Joey D Re: Rattling conduit - 01/31/05 12:05 AM
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.
Posted By: sparkync Re: Rattling conduit - 03/17/05 01:04 PM
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
Posted By: jraef Re: Rattling conduit - 04/21/05 10:34 PM
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.
Posted By: tshea Re: Rattling conduit - 04/26/05 08:22 PM
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.
Posted By: e57 Re: Rattling conduit - 05/02/05 01:16 AM
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).]
Posted By: poorboy Re: Rattling conduit - 06/23/05 01:19 AM
Interesting. Have heard it in one building I often work in when the elevator starts.
Posted By: mxslick Re: Rattling conduit - 11/18/06 02:35 AM
Bump..

Any one got some new war stories for this one? [Linked Image]
Posted By: SteveFehr Re: Rattling conduit - 11/18/06 03:31 AM
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).]
Posted By: jraef Re: Rattling conduit - 11/18/06 08:56 AM
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.
Posted By: Albert Re: Rattling conduit - 11/27/06 06:10 PM
I live a few blocks from a neighborhood distribution substation with step-down transformers, circuit breakers and voltage regulators feeding three overhead circuits. On occasions when there's a fault (usually due to falling tree limbs in a storm) I'll sometimes hear a loud, distinctive 60Hz "groan" from the sub, simultaneous with my lights dimming or going out.
Posted By: mxslick Re: Rattling conduit - 03/26/07 07:08 PM
Bump again...

Keep the stories coming!!

Albert, a POCO friend was in a substation during an earthquake once...he said the combined noises from the transformers going into overload from the faults, the breakers reclosing repeatedly and worst of all, watching the solid tubular bus work deflect was almost scary enough to make him retire!!

He was quite uncomfortable going into substations for a while after that incident.
Posted By: Kenbo Re: Rattling conduit - 03/27/07 09:04 PM
Never been in a substation during an earthquake (don’t get them in Scotland thanks)

But I have been in a hydro station when one of the sets was brought on line a fraction out of phase. Needles to say when a set stalls just for that fraction of a second the whole building shook, that combined with the bang was something I will never forget. Nearly needed to change my shorts

Tongland power station
Posted By: Mountain Electrician Re: Rattling conduit - 04/16/07 09:19 PM
We were servicing a 4160 to 480 1500KVA pad mount xformer, and when we turned the power back on an operator had left the control switch for a 200 HP motor in the hand position (no PB, just a HOA) and when my helper slammed in the second cut out, the motor went to single phasing and the noise the 4160 overhead started making was intense. It seemed to travel down the overhead distribution lines untill the breaker tripped. Never before or after have I heard a sound quite like that.
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