Vibration in electrical feeder

[Troyla]

I was hired to look at the following issue. This one is new to me.

Background: A 12 storey building has a 400A, 3 phase riser that feeds 120/208V power from the 2nd to the 12th floor. The riser consists of an electrical panel on each floor with feed through lugs to feed the next floor. The majority of the load is from office equipment. No apparent mechanical equipment fed from this riser. This riser consists of 4 single conductor teck cables and is fed from a 400A circuit breaker in the main switchboard. The switchboard is fed from an adjacent utility vault.

Problem: From the load side of the circuit breaker to about the 7th floor, there is excessive vibration in the feeder that also causes the panels to vibrate and hum on all floors. I measured the current on the feeder and it is slightly overloaded at 350A. I did not measure any other parameters such as harmonics.

Question: What could possibly cause this? My initial thought was some vibrating mechanical equipment that is fed from one of the panels. I could not confirm that this was the case. Has anybody heard of harmonics causing such a situation? Or any other theories?

Any help would be greatly appreciated..
Thanks

 

[Skogsgurra]

" 4 single conductor " is a bell that rings. And, if you have lots of office equipment, there is also a lot of odd harmonics - mainly third.

That makes peak current a lot higher than the 500 A expected (350*sqrt(2)). Electro-magnetic forces conductor-conductor and probably also conductor-panel seems to be the reason for the vibrations.

Are the conductors running hot? Is the neutral hot?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

Magnetic forces are the likely cause. Usually they are "triplexed" before pulling or at least before terminating, meaning the conductors are twisted around each other to minimized the effect of the magnetic forces around each conductor trying to repel the other 2 (the neutral is not affected by this).

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