Rising main vibration

Status
Not open for further replies.
S

sshields

Member
Hi there, I work in a hospital and we have a split rising main bus bar system which for some reason is vibrating like a transformer. This is occuring on the essensial supply and the non essential supply. The same noise and vibration is occuring on the second set bus bars, can you help
 
Since I can assume that this vibration was not there before, can you think of any changes between the time it did not vibrate and now. Is everything secure?

Does it occur under light loads and heavy loads. You want to find out if it is electrically induced or is vibration from another source like an unbalanced motor transmitting the vibration to the bus bars, in which case it would not matter what electrical load ther was on the system.
 
ss,
noise from a bus duct can only come from movement. since there are no designed moving parts of a bus duct other than expansion joints i would recommend you contact a infrared thermographer and have the area thermographed. i would also rope or lock off the area where the noise is coming from. we have all heard humming noises come from feeders that are subjected to a short circuit in it's system. this could be the problem and if it is something has changed from it's original installation and the bus might be overloaded. the other possibilty might be a failure in the bus duct's insulating supports inside the bus. contact the bus duct's local manufacturer's representative and have him/her listen to the problem and get the manufacturer's feedback.................. i have seen this one time only and it turned out that two of the three aluminum buses were damaged causing all the current to flow over one plate -- this was easily found with the infrared camera as a large hot spot. this bus was feeding 1/3 of a 33 story hotel. we had them move their customers to the lower floors and reduced the load as much as possible until two replacement ducts could be manufactured. schedualled a midnight shutdown and changed out the damaged bus. remember, bus duct is one item that when it fails it does not usually contain the explosion within it's enclosure. if you have any combustible materials in this area i suggest you move them................... my $.02.........
 
The failure of bus ducts can be caused by a couple of things, one is the cracking of bus bar insulation (bus sleeving) combined with the accumulation of moisture or debris. Cracked insulation results from high ambient temperatures and contamination from bus bar joint compound. Cracked insulation in the presence of moisture or debris provides phase-to-phase or phase-to-ground electrical tracking paths, which will result in catastrophic failure of the buses. A second reason is when bus ducts are exposed to appreciable ohmic heating during operation on heavily loaded circuits, they may experience loosening of bolted connections, specifcally splice plate bolts, because of the repeated cycling of connected loads. As stated above, take safety precautions, and start infrared scanning. Then do a safe shutdown to provide a physical inspection of bus ducts bolted connections at sample sections of the buses in the bus ducts and check for proper torque, or check bolt joints for low resistance. This activity will include visual inspection of interior portions of bus ducts to identify aging
degradation of insulating and metallic components and water/debris intrusion. The external portions of bus ducts and structural supports should be also be inspected. A less catastrophic possibility could be the connections at the transformer. if isolators were not used at this connection point, the transformer could be the cause. Still a problem, because prolonged use under this condition could lead to loose connections.
 
Verify loading of the riser is not close to maximum capacity.
You need to check for current harmonics, and if present, make sure they are not effecting the voltage on the riser. Risers are very difficult to get a good IR scan on but, it wouldn't hurt.
If the buzzing goes up and down in magnatude, look at the elevator power supply. There may be a filter bank failed; that might explain why it is heard on both normal and essential risers.

You need to find the cause ASAP for obvious reasons.
 
rising main bus bar noise

rising main bus bar noise

Thank you for the reply, I have a little more information for you.
Temp of bars 32C
Current on nuetral 14A
Temp on two seperate transformers, hot enough to cook an egg
Load current Tx1-800 to 900A max out at 1000A
Load current Tx2- 400 to 600A max out at 1000A
Supply SWA to rising main has sweat and silver speckle on the outer sheath

I think there may be more of a problem than I first thought.

Your ideas would be greatfully received

thanks

scott
 
Could it be that the hum you initially heard is being telegraphed from the tramsformers? They do make a bit of noise....

Either way is sounds as though (as it is for a hospital) that some immediate attention to be taken for both the transormers and the busswork at reducing the load if they are over-loaded*, and getting a systems up-grade. If this type of work is not your normal scope of work, you should probhably contact an electrical engineer and very well equiped contractor for immediate and extensive evaluation. System failure at a Hospital is no laughing matter.

*That neutral amperage is one really balanced system for mixed line-neutral loading, or un-characteristicly low for the other amperages? What are the phase legs at each? And what is the rating/data of the transformer?

"hot enough to cook an egg" Is not good.........
 
Last edited:
Scott,
Temp on two seperate transformers, hot enough to cook an egg
Click to expand...
If these are dry transformers, remember that most are designed for a temperature rise of at least 80?C, and some with a design rise of 150?C, and this is over a 40?C ambient.
Don
 
Does not seem all that hot to me either. I have stood next to large transformers that are heavily loaded that are so warm it is unpleasant to just stand next to them, but that did not mean they were likely to explode or anything. Many of them have a fairly loud hum as well.

Some investigation is in order, but I would not get all that worked up about it until I ruled out some simple things. Like a loose fitting somewhere. They can make some very weird noises that travel along the conduits and other rigid objects.
 
consider how bus duct is made--- a typical bus would have a single bus bar for each thousand amps it is rated for. a three thousand amp rated bus would have three insulated bus bars alongside each other for each phase. sometimes as a connection, or joint as it's called, fails maybe only one bar looses it's connection --- the load automatically shifts to the other two bars. and remember, this may only happen at certain load conditions. i have had bus ground out due to water -- blow maybe a three thousand amp fuse -- and blow itself free from the grounding surface. the maintainance staff didn't find the damaged section -- replaced the fuse and went back "on line". we found it during their two year infrared scan. it is very easy to scan bus ducts --- but you need the right camera and electrically inclined thermographer. when your looking at a 100 amp disconnect switch it is not uncommon to find five to ten degree temp. rise in the blade area when the equipment it servse it under full load. when it comes to bus duct, say 4000 amps, a one degree rise in a joint area needs attention. the newer cameras are not as sensitive as the original ---ya, there nice and light and computerized. i got used to the sensitivity of the old liquid nitrogen cooled units and didn't feel i was doing my customers proper service with the new units. pain to drag it up and down stairwells and ladders, but it produced a better service. a hospital is supposed to be scanned on some type of schedual --- bus duct should be scanned every two years and if problems are found, maybe yearly. it should not be that expensive. but if the operator doesn't know what to look for, or uses the wrong camera -- it's useless....................

ps: it doesn't take much water or even moisture to cause a bus duct explosion. we had a 2000 amp emergency bus blow up because an electrician who was using the office building's electrical closet as his office, installed a a/c duct into the electrical room from the corridor duct. took two years, but the cold air condensed enough moisture to cause the joint below the duct to fail. many building engineers and managers don't realize how critical it is to protect bus duct from moisture. in office buildings there should be a one to two inch collar built around all the bus duct floor penatrations to protect the bus if there is a flood from a broken pipe of sprinkler head. i always tell the engineers "you can pizz on this bus duct and blow it up"......................
 
Last edited:
My comment about "Hot enought to cook an egg", is more about the cooking of eggs. ;) It's not good to cook eggs on transformers. Either way, it apparently requires 275+F to fry, 212F (100C) to boil depending on altitude. And a little too high for most transformers.

Anyway, IMO for a hospital, I don't think that questionable tempatures and sounds be over-looked or ignored. Think about it.... The guy saying if something did happen, "The guys on the internet forum said it was OK."
 
Status
Not open for further replies.
Top