Vibrating Wires?

Status
Not open for further replies.
M

msoe03

Member
Location
Schaumburg, IL
I am new to the forum, but I've glanced through from time to time. I've been working as an engineer for a consulting firm for the past three years, so I am pretty new to the industry. I feel rather stumped right now and I am hoping that some of you might have some insight. I was at a jobsite yesterday and was informed that there is a wire in a conduit within the facility that is constantly vibrating. From what I understand, the feeder is for a fieldhouse distribution panel that was disrupted a few weeks ago when the feeder was cut accidentally during demolition. I have never heard of a feeder vibrating continously, but then again, I am always learning new things. The owner would like to know what is causing this and if it will be a problem in the future. I have heard of wires in a conduit vibrating for a short time when there is a large in-rush of current. I do not believe that this is the case. Any ideas would be greatly appreciated. Thanks.

-Jill
 
What size conductors and what is the load on them when vibrating?

If it happens to be a large feeder contained in parallel raceways I would check to see if they have one of each phase in each conduit.
 
Depending on what you find based on Bob's suggestion, also check to see if the conduit system is rigid the whole way. Some type of equipment nearby could be causing the vibration, by either a physical connection through hangers supports, or structural steel, or by some low frequency.

The conductors could also be overloaded, or one phase anyway, check for balanced load.

You could also have a low level fault in that case check to see if the conduit is actually carrying some current, by use of a ammeter.
 
And remember to get his definition of constantly.

I have gone on many a service call where "half" the house was out only to find a tripped GFCI feeding five or six outlets.
 
A large motor starting will sometimes cause the wires to rattle in a conduit.
I agree that "constantly" needs a better definition. Is the sound constant and does it 24/7 or does it make noise "all the time" where it comes and goes often?
 
I've witnessed wire vibrating in the conduit while under a load near the limit of the conductor. I don't know if it's normal but I've seen it many times and there was nothing 'wrong' with the installation.

Bob on the left coast.
 
I went back out to the jobsite yesterday and I have more of an idea what is going on out there. Panel FHP-S feeds panel FHP-SB in a field house. Panel FHP-SB serves all single phase loads such as the field house lighting (metal halide with magnetic ballasts), temperature controls and receptacles. The feeder is 150 amps with 3#1/0 phase conductors, 1#1/0 nuetral and 1#6 ground. The feeder is no more than 20-30 feet in length. During construction, the original feeder was cut. The contractor repaired the damaged conduit and ran new conductors.

The three phase conductors in panel FHP-S all vibrate. The vibration is not visible, but rather only felt to the touch. The vibration is constant only when the loads in the sub panel are on. The nuetral and ground conductors do not vibrate, nor do any of the other conductors in the panel. Using an amprobe, the current on each phase at panel PHP-S were:
A: 110A
B: 69.2A
C: 114A

The feeder is located under the concrete slab for the majority of the run. The only location the conduit is visible is from panel FHP-S to the floor. The conduit in that location does not vibrate. Unfortunately, concrete has already been poured over the area that the conduit was cut.

In panel FHP-SB only the B phase conductor vibrates. Similar to the other panel, the nuetral and ground conductors do not vibrate. The B phase and C phase conductors are switched in this panel, but I do not believe that would cause the B phase conductor to vibrate. Using an amprobe, the current on each phase at panel PHP-SB were:
A: 110A
B: 113A
C: 69A

With 1/0 conductors, the feeder does not seem overloaded. Could it be possible that a bad magnetic ballast on the B-phase in the sub panel would be causing the vibrating? It seems odd that only the B phase conductor vibrates in panel FHP-SB and in panel FHP-S all phase conductors vibrate. Or maybe this could this be a sign of a low-level leak in the conductors?

Any ideas are appreciated. Thanks for all your help thus far.
 
msoe03-What are the panel sizes and voltages? Is there a transformr that feeds the single phase panel? In other words how does the single phase panel get fed out of the three phase panel?
 
There is no transformer. Both panels are 120/208V 3 phase 4 wire. The loads on the subpanel (FHP-SB) are single phase (i.e. using phases A, B or C). Panel FHP-S is 400A MLO. Panel FHP-SB is 225A MLO with a 150A feeder and 150A breaker in panel FHP-S.
 
This is new to me since I have never heard of vibrating wires. I've also never heard of or seen a three phase panel where you are able to switch on/off the individual phases. It's either all or nothing. I'm curious you took an amp probe at both locations and came up with identicle loads for phase A (110A) this leads me to believe that on the 400A panel there are no breakers attached to phase A and that this is really the subpanel load. Is that correct? Phase B also has me stumped. Typically whenever you probe an amp the main panel should consist of the total load for both panels and the subpanel should be less. Your total amps for the subpanel based on your numbers as you know (292A) are just slightly less than the main panel. This leads me to belive two things that if there are switched breakers the wiring may be switched around so that when you think you are turning off one phase you are actually trasfering that load to another phase and you may want to reread both panels just to be sure the loads are correct.

Just a guess. Let me know what you think.
 
To see vibrating wires in action, visit a local sawmill and climb up on the cyclone while it's in action.

To return to the subject, all mechanical objects have a natural frequency of vibration, f sub n. That frequency is defined as the square root of the stiffness coefficient / mass (I don't know how to write an equation here,) but
fn=sqrt(k/m)

It is inevitable that the B wire, lug, support, etc., has a natural frequency, as in a tuning fork. The wires next to it would have a different frequency, different lengths and not being supported exactly the same - chances are practically zero. Most physical objects have very low frequency responses, as in highrises during earthquakes. But piano wire is wire, isn't it? If this is not a 60Hz (or multiple) hum, it's probably just the wire/support, which is being excited somehow and noticable through the fingers.
 
necnotevenclose - I don't know where you are getting the idea that in order to serve single phase loads from a three phase panel you need to switch individual phases on and off. The loads are single phase...not the panel. Both panels are three phase. The subpanel is 120/208V 3 phase 4 wire and is able to serve 120V single phase loads (1 pole breaker, 1 hot 1 nuetral). The amprobe readings are of each phase conductor (A, B and C) at each panel. Perhaps my wording was confusing at first, but this should clarify the issue.

peteo - Do you think that the B phase conductor from the sub panel could be causing the other conductors to vibrate so by the time the conductors reach the main panel, all three are vibrating?
 
msoe03 said:
..The amprobe readings are of each phase conductor (A, B and C) at each panel. Perhaps my wording was confusing at first, but this should clarify the issue.
Click to expand...
You did transpose B with C phase when listing load values between panels. That confused me too, but I thought it was a typo, not a real cross up between panel phases.
panel PHP-S were:
A: 110A
B: 69.2A
C: 114A

panel PHP-SB were:
A: 110A
B: 113A
C: 69A
Click to expand...
See how it looks like B & C phases could be crossed between panels?
 
In a previous post I explained that the B phase and C phase were switched in the sub panel. Therefore the amp readings reflect this. But I don't think this would cause vibration?
 
msoe03 said:
B phase and C phase were switched in the sub panel. ..But I don't think this would cause vibration?
Click to expand...
What!!!!!! Any future 3-phase motors & power factors we be destroyed by such a panel, vibration may be the least of your problems if 3-phase motors burned up.

For any future 3-phase load, instead of firing power peaks in progressive angles 120 degrees apart, you've got the firing order 240 degrees apart.

That crazy looking waveform may work for all single phase loads, but who knows how those angles balance between single phases, sharing the same conduit, or cancel emf forces, or (+,-) sequence phase harmonics?

I've never seen or heard of such a thing, but even if disconnecting that subpanel to switch B & C phase feeds to the proper lugs does nothing for vibration, at least it may prevent a disaster for the unsuspecting fool who later tries to add a 3-phase load.

--'I require three things from a man. He must be handson, ruthless, and stupid.'
-- Dorothy Parker
 
Yes, I definitely agree with you. I was going to inform the architect so the electrician will be aware that he will need to fix his mistake before the job is done (since he pulled new conductors after the conduit was hit). Luckily for the owner there are no three phase loads on the panel...but this could change someday.

I am still stumped as to what is causing the vibration. But at least one possible problem was avoided!
 
ramsy said:
For any future 3-phase load, instead of firing power peaks in progressive angles 120 degrees apart, you've got the firing order 240 degrees apart.
Click to expand...
The only effect I can think of from swapping lines is the running direction of a 3-phase motor.
 
ramsy said:
For any future 3-phase load, instead of firing power peaks in progressive angles 120 degrees apart, you've got the firing order 240 degrees apart.
Click to expand...
Its confusing, but I thought the way to make a motor spin backwards was crossing A & C phases, and damage would depend on what happens to 3-phase equipment when it spins backwards.

But, honestly Iduno what happens when crossing B & C on a 3-phase motor load (A,C,B)?

If A & B phase sequences occur 240 degrees apart on the motor, maybe the motor's inductive-kick-back could generate an emf differential or equipment vibration, kind of like what might be expected if an xfmr fired two phase coils at once.
 
Status
Not open for further replies.
Top