480V MCC Breaker Hot to the Touch

[sanders11]

I have an issue regarding an Cutler-Hammer 480V Main Molded-Case Circuit Breaker for an MCC. The breaker is an 800A breaker carrying a load of ~720A. The breaker case itself is actually very warm to the touch. I have had the breaker thermographically surveyed with no temperature variances found, other than the temperature of the breaker itself. This is a new breaker (approximately 4 months old). The previous breaker also showed this same temperature rise (it was replaced because of another issue). The ambient temperature of the electrical room is not excessive (60-90 degrees farenheit) although it is not air conditioned. There is actually a 120V fan installed on the MCC door to release the excessive heat. There are multiple busses in this electrical room but this is the only breaker that shows this type of heating characteristic. There is nothing to cause any harmonic distortion like VFD's; the load is pretty much lighting transformers/panels and normal motor loads. Does anyone have any explanation for the heat created by this circuit breaker?

 

[petersonra]

Re: 480V MCC Breaker Hot to the Touch

big breakers that are running close to full load current are often warm to the touch, sometimes even uncomfortably so.

these breakers are thermal-magnetic, meaning there is a magnetic trip part that is nearly instantaneous to deal with short circuits, and a thermal part that just heats up and eventually trips when it gets too hot, sort of like when a thermostat switches or overload heaters on a motor starter trip.

 

[realolman]

Shouldn't it be loaded to less than 80 % instead of 90?

 

[iwire]

Shouldn't it be loaded to less than 80 % instead of 90?

It can be loaded to 100% non continuous (up to 3 hours) and 80% forever.

I would expect an 800 amp breaker running at 720 amps to be warm.

 

[Pierre C Belarge]

Without knowing the exact temperature of the breaker, it is hard to say, but I agree with the others. This may just be a normal case where the breaker is near capacity, and the temperature rise due to the amount of current flowing (resistance) will feel warm, maybe even hot to the touch.

 

[realolman]

Would the manufacturer be able to shed a little light on the question?

 

[don_resqcapt19]

I believe that the UL standard for breakers permits a 50?C rise over a 40?C ambient. That would be very hot to the touch.
Don

 

[templdl]

Often times when one expects a device to be cool to the touch and it is warm it is judged to be hot like transformers and motors are often judged.
However, "hot" and "warm" are ambiguous terms and it is seldom that an actual temperature is often included. Because breakers do have a level of resistance there will be a voltage drop which will equate to heating watts. The greater the current that passes through the breaker the greater the heating watts. I do have a few "heating watt" values in my C-H/(W) breaker data base. I had thought I had all of the breaker frame values but could only find (3) frames in my brief search the largest of which is a 400a frame. I show it to have heating watts of 16.6w per pole at its rating that's almost 50w or about 20w more than a 250a frame. So expect that 800a frame breaker to be possibly in the area of 100w as my best guess.

 

[mike wright]

I read in an underwriters manual once that if you can hold your hand on a motor or breaker and count one thousand one, one thousand two, one thousand three it may be hot but not too hot. I have used this as a rule of thumb, no pun intended, for many years. mike

 

[cpal]

It would not hurt to get a IR gun and shoot the lugs and buss also, it would be interesting to see the skin temp of the feeder conductors at intervals away from the lugs.

It's a safe bet that this equip is not listed for 100% as a unit. but that has been mentioned.

make sure that the 720 in not continuious and if the lug temp is elevated above 75 degC or 167F, I'd consult the manufacture.


Charlie

 

[realolman]

I read in an underwriters manual once that if you can hold your hand on a motor or breaker and count one thousand one, one thousand two, one thousand three it may be hot but not too hot. I have used this as a rule of thumb, no pun intended, for many years. mike

You can lay your hand on something that is 140F , but you ain't gonna keep it there very long. I think the exact time is one thousand one, one thou

( actually it's shorter but I couldn't think how to write it that would make sensible reference to your rule of thumb ( ...like that was sensible) ):smile:

 

[charlie tuna]

when a breaker is running 80% + for long periods of time it's supposed to get "hot" or "warm" whatever you want to call it. now, if your infrareding this breaker and the load on each phase is fairly close -- then the breaker should look evenly "hot" or "warm". if on the other hand one or two of the phases WITHIN the breaker looks like it is the heat source --- your breaker is the cause and should be replaced. if the load on one phase is higher than the others -- you can expect a slight temperature rise in the contact area of the breaker. the amount of temperature rise is a judgement call in the thermographer's part --- he must justify temperature rise with actual load conditions -- the only way to do this is really--by expieriances...... i know of one breaker that is shockingly "hot" in a particular motor control center -- it feeds a escalator and is constantly under 85% operating conditions. been that way for years!!!

 

[hivolt]

The breaker is undersized. Please keep in mind that molded case breaker ratings are rated continuously when installed in open air. This is not practical. Because breakers are installed in MCC which has limited ventilation and in close proximty to other loaded breakers and bus, the molded case breaker must be de-rated. I would not load this breaker more than 80% (640A).