Load Balancing after 30 years

[VanElectric]

Our facility recently had a Thermography test done and this disconnect was flagged for unbalanced loads, which is causing heating problems.

I need to figure out how to redistribute the loads on a 3 phase 277/480V main that is feeding one distribution panel that also feeds 2 other panels through 2 transformers.

I have current imbalances of Phase A 47 Amps, Phase B 38 Amps and Phase C 61 Amps.

 

[don_resqcapt19]

As long as the current is below the equipment rating, that amount of imbalance should not be causing a heating problem.
Don

 

[Jljohnson]

I completely agree with Don. I suggest looking for a loose connection somewhere in the distribution system. Probably at the point where the IR scan showed the highest level of heat. That amount of imbalance should (I repeat SHOULD) not cause overheating as long as the equipment is rated for the proper amperage.

 

[VanElectric]

Thank you for the info. I will get an opportunity to check this circuit this weekend.

 

[Wes G]

Balancing act.

Balancing act.

Thermographic will detect the heavier load whether or not it is above equipment rating. the reason for calling it out is the possibility ofit being caused by improper loading of phases.
Check the farthest transformer from where the differance was noted and keep working back until you find where the heavier load enters the system. Next go to the panel fed by that transformer and start checking the loads. At that point a person can move a load or two to ballance the load at that panel if indicated. Or it might be that some loads just weren't operating at the time of the survey.
On a three phase system there is always the possibility that not only is one phase loaded heavier but also if someone was not paying attention to that aspect of ballancing they might also have hooked up a 277V multi-wire circuit wrong to where you have more than one circuit of the same phase on the same neutral.
I have run across this several times, so always check my current on the grounded conductors of the panel where the inballance occurs to assure that this hasn't happened.
___________________
Wes Gerrans
Instructor
Northwest Kansas technical College
Goodland, KS

 

[charlie tuna]

what was the size of the disconnect switch --- 60 amps?? if it was 100 amps there is no problem here.. it is easy to balance loads provided you have enough single phase loads without common neutrals and remember, if you transfer 5 amps from one phase to another the result will be 10 amps total. i am assuming this is a 60 amp switch and you are attempting to reduce the load on "C" phase and shift that load to "B" phase. shifting ten amps would bring things into near balanced loading. what was the temp. rise on this problem. the job of the thermographer is to look at load to justify temp. increases. this ability can only be taught by expirience and it takes years to be good at it. you could have a bent blade causing "some" of the heat problem on "C" phase and the balance from load condition. but infrared scanning is a very good tool provided you have a electrically knowledgable thermographer and a facility that follows the recommended repair job.. remember, once a termination is properly made and not overloaded, it is good for ever. my first building in the infrared business was a 55 story office building. we scanned it every two years. the first scan found about 140 problems. the building would repair the simple problems and hire us to do the more complicated problems. after about six or seven scans we could only find maybe six or seven problems!!! their insurance premium reduction for having their building infrareded more than payed for the scan.....

 

[don_resqcapt19]

Wes,

the reason for calling it out is the possibility ofit being caused by improper loading of phases.

I still don't see a problem as long as the loads are within the rating of the equipment. How does this small imbalance cause heating problems if the load is within the equipment rating?
Don

 

[CRKTRAC]

What does the IR image look like?

What does the IR image look like?

What does the Infrared image look like? Where are the temperature readings on the image. Is the switch cover hot to the touch?

Infrared images can be deceptive. Just because you see what appears to be a very hot spot on the image does not mean it is hot. It could be hot. Howerver many times it is just seeing the extra load.

Good readings can be measured in plastics, insulators, cavities, etc.

You will not record properly temperature readings from switchblades, exposed conductors or fuse ends.

 

[AE-29]

load balancing

load balancing

Question, is the disconnect feeding this 3 phase 277/480 panel or is it feeding a motor or service equipment?. First I would check for a loose connection at the disconnect... If this is the feed to these panels it should be rated over 100 amps. Now what I would do is check the 3 panels that it feeds and using a clamp on check each load on the panels looking for excess loading on "C" phase. Also since you have the covers down, check for tightness at each breaker both at the buss and the load side of the ckt bkrs.Add up your find of each phase on each panel. Obviously, every load won't be on at the same time so you will have some imbalance. you have about a 30 % imbalance between "B" and "C" and greater on A-C.Try changing some loads to "A" phase once you have identified where your larger draw are. Thermography is a useful tool that has aided me in locating defects before they become a problem. Good luck

 

[VanElectric]

To answer everyone's questions this is an explanation of the report.

From the photograph it is a disonnect on our main distribution panel. I didn't check the disconnect rating but there are 125A fuses in the mains. They are of the blade type. In the image the area just below the copper ferrule is where the heating stands out. I just noticed though, that 1 of the fuses is a different manufacturer and it displays cooler than the other 2.

As before the amperage on the circuit is Phase A 47A, Phase B 38A and Phase C 61A. The temeratures on the report show Phase A 38 degC, Phase B 35 degC and Phase C 41 degC.

This circuit feeds a 225A 277/480V 3Phase 4W panel and 2 other subs come out of it that are transformed down using 5KVA xfmrs to 100A panels.

By the way, I was unable to check this circuit during the outage.