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Heat transfer at electrical lug connections.

Heat transfer at electrical lug connections.

Heat transfer at electrical lug connections.

(OP)
Hello.


I am faced with a real problem. I need to perform an analysis on electrical lug connections used in very large circuit breakers.

The lug connections use about 1" thick solid copper bus and terminate to 4 large DLO 313kcmil cables. Each side of bus has two lugs, a total of 4 per end of bus.

Now comes the problem. I need to assume some heat generation. If I am given the current and voltage that this unit (per bus connection) will be seeing, what and how do I assume contact resistance and Q heat generation?

Also, where should I apply this Q? I am assuming I shall apply this Q right at the interface of the two surfaces (Bus/Lug) in the form of heat flux at those surface areas.

If my assumptions are correct, please confirm and if not, please advise.

Thank you and I love this forum.


   - Dan

RE: Heat transfer at electrical lug connections.

It is a very real problem, your initial contact resistance is in milli-ohms range and is a function of the bolt torques used in making the connections. The conductivity of Cu is such that the temperature in the lug itself is quite uniform so the big issue is the resistance at the bolted interface not so much the temperature gradients.

The resistance increases with time due to oxidation and varous environmental corrosion effects, and the connections need to be checked regularly for critical applications.

RE: Heat transfer at electrical lug connections.

I think your question about assumptions belongs in electrical, not heat transfer. Once you figure out your assumptions these guys can help.

I think the answer is: both.  The volume resistivity is well known and quantifiable.  The contact surface resistance exists but will be relatively small under usual conditions of a good connection, and difficult to quantify.   

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RE: Heat transfer at electrical lug connections.

(OP)
Thanks for the responses.

Hacksaw, I am actually interested in the steady state heat generation at the interface. Do you know of some correlation? Should I post this same question in electrical?

pete, the model I am running is trying to simulate what happened to one of our components that failed. We know the lugs get hot, but we want to try and predict lug temps.

RE: Heat transfer at electrical lug connections.

Don't forget than one of the biggest, hidden, resistances is the spreading resistance.  Everywhere that the conductor either changes cross-sectional area, materials, or even direction, incurs an additional resistance factor above the basic resistances of the materials.

So, therefore, you'll need to determine the actual flow of the current through the copper, spreading or turning as it gets to its contact with lug, the spreading or turning within the lug and toward the connection, and then the connection itsef, and everything past that.  It's a fairly non-trivial problem, and you also need to determine how much accuracy you need versus how much modelling you're going to have to apply to achieve that accuracy, and whether it's worth the effort.

TTFN

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RE: Heat transfer at electrical lug connections.

I might point out that thermal imaging of several existing units at different loads would allow you to create a correlation that could help you predict installed lug temperatures.

Regards
StoneCold
 

RE: Heat transfer at electrical lug connections.

(OP)
IRstuff, the current flow prediction is going to be beyond my work scope. And I think we arent going to get that detailed. We just want a rough idea of lug temperatures or even deltas from case to case.

StoneCold that actually is a good idea, but I dont know if we have access to thermal imaging right now.

RE: Heat transfer at electrical lug connections.

Just to clarify, the failure you think occurred based on heat generated between lug and bus... or at the crimped barrel of the lug?

Either way, I don't think you will get far with an analysis that starts with an assumption about the unknown your are trying to find.  In other words, you have to assume some degradation of the interface but how much?  That's what you're trying to figure out.  And again for normal connection that contact resistance at the plane of the contact is small compared to other effects... smaller than the current distribution effect which has been mentioned I would bet.

You could perhaps work backwards and say "I know the temperature go this high (2000F to melt copper for example).  What would the contact resistance have to be to cause that?  Even then it's not particularly valuable because the contact degrades... obviously it was bad just before failure but what made it start is the question.  

Maybe I'm not understanding exactly the situation...

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RE: Heat transfer at electrical lug connections.

Without some detailed knowledge of how the current flows, or could flow, you'll just get a gross approximation, which may tell you absolutely nothing useful.

TTFN

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RE: Heat transfer at electrical lug connections.

bdecker,

we had a dc drive 950 amps, the connections buss bar and very heavy yet they were running about 100-150C within a year or so. When the connections were re-made, the temperature was 40-50C but life-time was 6-12 mos. At issue was sulfide corrosion that persisted even with a/c. Plating did not help, just regular checks, and taking advantage of down time.

The heat generation was from the mating junction. The bars had enough surface area to dissipate most of the heat and only heated up as the connection resistance builtup. Seems that we were getting thermal trips rather than meltdown failures.

Clean connections and properly torqued bolts were required. The contact resistance was less than a milliohm so we could not measure it directly; We could measure the junction voltage, but you had to be careful crawling in the switch gear. IR temp scanner was a lot safer.

good luck
 

RE: Heat transfer at electrical lug connections.

There are greases available to protect the lugs from corrosion.

RE: Heat transfer at electrical lug connections.

My mistake - Based on that copper-to-copper chart linked there can be quite significant heat generated at the interface.

The vertical axis of the chart is labeled microOhm/mm^2.

A little thought tells that these cannot be correct units since contact resistance is inversely proportional to contact area.  I assume it should be microOhm*mm^2.

Now convert it to an equivalent length of copper (in direction of current flow using R = rho*d/A
d = R*A/rho
Choose R*A = 1000 microOhm*mm^2 from the curve

d = 1000 microOhm*mm^2 / (1.7E-8 * ohm*meter)
d = 0.06 m (effective length of copper which creates same resistance as the contact).

That's a pretty big equivalent lenght of copper... especially considering all the heat is generated right at the contact.

Obviously there are many factors - materials, roughness, contact pressure. The bottom line - I was going from memory and remembered wrong. Sorry.

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RE: Heat transfer at electrical lug connections.

(OP)
Thank you all for the replies, please give me some time to study them.

desertfox, that website is fantastic. It really sheds light on some questions that I had.

hacksaw, if the contact resistance is so low. Than what is the driver of the temperature rise at lug connections? Is it the current density or 'current flow' that people have mentioned?

electricpete, I am not going to be assuming a degredation at the interface. I will assume the interface had just been made and there is no corrosion. I am performing this analysis with the least amount of variables at this point. Later, I will consider oxidation.

I just need to understand what drives temperature rise in lug connections.

And from the looks of it, the overlap/thickness ratio is a huge driver of resistance. As seen on the website provided by desertfox.

I may have more questions to come, as I study this article.

RE: Heat transfer at electrical lug connections.

Hi dbecker

Can you give more details on your failure, have you any pictures that you can post?
When the switchgear was first designed, did it undergo a temperature rise test? if so then you should have records of the temperature rise at the point of interest which should give you some idea of the heat generated in that area.
One possible failure scenario I thought about was, that the joint is bolted up using torque figures, however using torque figures is subject to an error of about +/-25% so you could end up with more or less preload in the bolt, which would reflect in joint pressure either being high or low.
Suppose the bolts had 25% extra preload when the gear is in service, is it possible that when the joint reaches its service temperature, that due to differential thermal expansion, the bolt washer has embedded itself slightly into the copper (ie copper has yielded), now lets say the gear is turned off and the joint cools down,the materials contract back to their nominal size but because the copper has yielded slightly (permanent deformation) then the joint has a reduced contact pressure or even worse no joint pressure at all and any subsequent use would result in joint failure.
Obviously without more detail of your joint and failure I might be way off the mark.

desertfox

RE: Heat transfer at electrical lug connections.

bdecker,

say you've 1 milliohm resistance in the joint and 950 amps dc through the made up connection. Thats about 1 volt across the connection, and just shy of a kw of heat generation. The solid copper wasn't the source of heat, the conductor ohmage was good for only a 20-40C rise, we had a lot of cooling and open grills for an enclosure.

RE: Heat transfer at electrical lug connections.

(OP)
Hello again to all,


  I will try to answer the questions one at at time.

electricpete, correct me if I'm wrong but isn't contact resistance unnaffected by contact area given a constant surface pressure?

desertfox, I was given word that the failure did not occur at the lugs themselves (exceeding temperature limit) but in other places that wont pertain to this problem. Either way, I am still asked to perform a thermal analysis on the lugs.
Also, they are using belleville washers to keep the pressure somewhat constant during cooling/heating.

hacksaw, I was given information that the resistance at the joint was 12 micro-ohms. That is TINY!

According to my calculations, if 1300 amps flows through the joint. The voltage drop is 0.0156V. And the heat generated due to that drop is Q = VA = 0.0156x1300=20.28 Watts. That sounds like a very small amount of heat.

But Im not done, the contact surface area has been calculated to be 1736 mm^2. Which comes out to a heat flux (Q'') = 20.28 / (1736/1000^2) = 11,682 W/m^2

Please advise if this is a logical approach to obtaining heat generation due to contact resistance only.

Thanks again all.

RE: Heat transfer at electrical lug connections.

Hi dbecker

I am looking into your last post but I can't believe that the joint is dissapating 11.68 Kw/m^2.
Well I thought it was your lug joint that failed reading your other posts, but nevermind can you provide some more info on this failure, then you might get better assistance.

desertfox  

RE: Heat transfer at electrical lug connections.

(OP)
Hi Desertfox,

 Yes I initially thought the lugs were exceeding rated temp, but that is not the case. Still, I am tasked to predict lug temps. And I'm looking for the best method to assume heat gen.

Yea I find it hard to believe too, that 11.68kw is being dissipated in the connection.

The actual failure did not occur at the lugs, that was the information given to me initially which turned out wrong.

In any case, I can say for certain that the lug connection is passing 1300 Amps and the terminal connection is 1736 mm^2 with all certainty. And that the terminal resistance was measured to be 12 micro-ohms. This data is to be taken as baseline input data.

Material 1 is silver plated copper, thickness is 20mm.

Material 2 is pure copper with a thickness of 12.8mm.

Overlap distance is 32 mm.

In the article you supplied to me desertfox, there is a correlation for resistance vs overlap/thickness.

Both of my conductors have different thickness, therefore how do I apply this correlation now?

Thanks for your time.

 

RE: Heat transfer at electrical lug connections.

hi dbecker

In that reference site a gave in my first post there is another chapter which deals with heat generated in a conductor see below.
http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec3.htm#Heat Generated by a Conductor
In regard to your overlap the truth is I don't know, if you have different thicknesses however I would just use the thinner section for that calculation.
I think the heat generated at the joint is around the 28 Watts you calculated in your earlier post, although the overlap factor needs to be included.
I am not convinced that the heat generated is that helpful because there might be sufficient cooling aroud the joint by convection, radiation and conduction to dissiapate the 28 Watts, which is why I mentioned whether a temperature rise test had been done for this gear, if it as then you should be able to see what temperature the joint finally achieved under normal conditions.
Another question did this failure occur under normal conditions or fault conditions?

http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec3.htm#Heat Generated by a Conductor

RE: Heat transfer at electrical lug connections.

Quote:

correct me if I'm wrong but isn't contact resistance unnaffected by contact area given a constant surface pressure?
Contact resistance in ohms is inversely proportional to area.  Double the area... halve the resistance in ohms.

As I stated above, it appears the value plotted in the link is supposed to be (R*A) in units of ohm-mm^2.  That would be a number that depends on contact pressure materials, etc, but not area.   To back out the resistance of a given geometry, divide that number by area.

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RE: Heat transfer at electrical lug connections.

bdecker,

The contact resistance you are using might be okay for freshly made connection not for typical industrial application. If you were running 28 W junction heat that results in something on the order of 0.5C/cm temp gradient along the buss.
 

RE: Heat transfer at electrical lug connections.

(OP)
Yea hacksaw the contact wattage output is like 20 watts according to my calculations. This is very low, I think the calculation is wrong perhaps.

I guess my data that points to contact resistance being 12 micro-ohms is way to small. I have to talk to the test engineer about this. That is a very small resistance for a seemingly high temperature lug connection.

I know for a fact that even freshly made lug connections get hot by at least a 20-30C rise. And that is without 6 months of oxidation.

I think I need to get more accurate information.

So far everyones input has helped my greatly.

Also, electricpete; check out desertfox's website. It said

"It has been shown above that the contact resistance is dependent more on the total applied pressure than on the area of contact. If the total applied pressure remains constant and the contact area is varied, as is the case in a switch blade moving between spring loaded contacts, the total contact resistance remains practically constant."


Or am I misunderstanding?

Thanks for your time all.

RE: Heat transfer at electrical lug connections.

Quote:

also, electricpete; check out desertfox's website. It said

"It has been shown above that the contact resistance is dependent more on the total applied pressure than on the area of contact. If the total applied pressure remains constant and the contact area is varied, as is the case in a switch blade moving between spring loaded contacts, the total contact resistance remains practically constant."


Or am I misunderstanding?
imo you are misunderstanding because the copper.org terminology is very poorly presented if not downright wrong.

Case A: Area=1mm^2, Applied Force = 50N, Pressure = 50N/mm^2
Case B: Area=2mm^2, Applied Force = 100N, Pressure = 50N/mm^2

Case B has got to have half the resistance (in ohms) of case A. That is the reality.

I think when your link talks about "resistance", it is not talking about resistance in ohms.  Look right below the paragraph you quoted they give the equation Ri = C / p^n where Ri[sic] is "resistance of contact"[sic] and n is between 0.4 and 1.  But below that is a graph with resistance as vertical axis which is intended to represent the equation... you can tell by the shape (Y~1/X^n). If the vertical axis was resistance in ohms, then we would see ohms on the vertical axis, but we don't.  There is some units of area in there mm^2 in there written as microhm/mm^2 [sic]..  As pointed out previously, the mm^2 belongs in the numerator not the denominator.

The quantity "Ri"[sic] they are trying to describe would be the inverse of conductance per area.  i.e. Ri[sic] = 1/[G/A] where G=1/R.  It can also be written as Ri[sic]=R*A and the units are micro-ohm*mm^2.

Sorry for getting carried away with the sic but it's hard to describe it without using their terminolgoy.

In a knife switch it is not the pressure that is constant, it is the total force that is constant as the contacts slide further closed, not the pressure.  With constant total force and increasing area the the pressure goes down.   Change in pressure cancels out change in area to give constant resistance.    

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RE: Heat transfer at electrical lug connections.

Hi dbecker & electricpete

Indeed figure 16 the graph of "Resistance against Pressure" could be explained better,I looked in an old copy of "Copper for Busbars" which dates back to 1965 and I found that same graph, but underneath it the following was written:-

Quote:

:-"Nevertheless it is convient,when illustrating in graphical form the variation of contact resistance with pressure to reduce both these quantities to a "unit area" basis, bearing in mind that both axes really represent average quantities taken over the whole "apparent" area of contact.

Further on it goes onto say that to obtain the total joint resistance then:-

Quote:

divide the ordinates by the contact area .

So these two small statements can make you look at the graph in a completely new light.

desertfox

RE: Heat transfer at electrical lug connections.

hi dbecker/electricpete

Here is another reference about the resistance of bolted connections, this might make it clearer.
One thing I should mention with respect to joint area is that we calculate the average pressure on the joint not the actual pressure which by the nature of the metallic surfaces is in reality unknown.

http://books.google.co.uk/books?id=EkStW7v8VPkC&pg=PA220&lpg=PA220&dq=resistance+
of+electrical+busbar+joints&source=bl&ots=FpdQ_
jmVti&sig=jJCOJT0PL1SuXCiGQa-eJ0Z7x4c&hl=en&ei=eYGXS8GrNaj40wTd8rjqCw&sa=X&oi=
book_result&ct=result&resnum=3&ved=0CAoQ6AEwAjgy#v=
onepage&q=resistance%20of%20electrical%20busbar%20joints&f=false

It would be helpful if the OP could provide more specific details about the failure, as I have requested several times now, without more information the thread can't get much further.
The way I look at this discussion about resistance is:-
take a one metre length of conductor whatever its its area, now double the area of that conductor keeping the length constant and you will halve the resistance, conversely double the length and keep the area constant and the resistance doubles.
In the bolted joint however my interpretation is that if the actual pressure on the joint is not changed significantly by a slight increase or decrease in area of the conductors, then the resistance will not change, part of the reason I say that is because the force distribution is mainly centred around the fasteners ie washers, nuts and bolt heads, once outside these area's the force tends to drop off significantly.
I could by wrong but these are my thoughts.

desertfox

RE: Heat transfer at electrical lug connections.

A backdoor approach is to use the known data, i.e., the known temperature rise and the known current to estimate the apparent resistance.

Since all of the power is eventually dissipated into the air, we can write:

power = area*htc*known_temperature_rise

let:
htc = 2.5 W/m^2-K (could be as as high as 10 W/m^2-K)
area = 4sides*1inch/side*2.5inchlength+4cables*700mm/cable(diam)*pi*2.5inchlength (assume that total heated length is divided by 2 to get the average length)

Then, power(=680W)/(1300A)^2 = resistance

Obviously, lots of swags here, but, for 60°C rise, the apparent resistance is on the order 0.4 milliohm, which is considerably higher than the "12 micro-ohm" measurement.

Anyway, food for thought...

TTFN

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RE: Heat transfer at electrical lug connections.


Quote (copper.org):

It has been shown above that the contact resistance is dependent more on the total applied pressure than on the area of contact. If the total applied pressure remains constant and the contact area is varied, as is the case in a switch blade moving between spring loaded contacts, the total contact resistance remains practically constant.
Another nitpick on the copper.org terminology above... throughout the entire page they use "pressure" to indicate force per area.  Here they are quite obviously using the term "pressure" to indicate force.   One clue are the words "total applied" in front of pressure.  Another clue is physics (the sentence is only correct if we interpret pressure to mean force).

The copper.org page assumes uniform contact pressure across whatever area we're talking about and I made the same assumption in my discussion.  I agree with  desertfox the actual contact stress distribution in bolted connection may not be uniform.

I'm a little unclear about the big picture of this thread.  It has already been identified sulfide corrrosion is the reason for the gradual resistance increase?  (rather than relaxation of bolt tension for example).   I would think as mentioned grease would exclude the contaminants.   I would also think there has got to be some kind of plating that could be resistant.    Why is it that you need a thermal model?   The role of temperature in this sulfide corrosion would be affecting the reaction rate?  Is there a threshhold temperature of interest that you are looking at?
 

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RE: Heat transfer at electrical lug connections.

(OP)
electricpete, yes we have a threshold temperature that must not be exceeded. That is the point of all of my inquiries, how to better model the contact resistances so I can account for all the heat dissipated and in turn calculated component temperatures.

I am really after finding lug metal temperatures, that is the ultimate goal. I am using a 3D CFD model to compute local HTCs on the lug surface. All I need now are heat generation rate in the form of heat fluxes on my contact areas for my ANSYS model.

Give me a day to read over the last few responses, it's getting pretty hairy!

 Thanks again,



 

RE: Heat transfer at electrical lug connections.

electricpete,

the sulfide issue was specific instance unrelated to original post, but was mentioned to provide context for my own first hand experience relating to the op's query. interesting problem to say the least.

RE: Heat transfer at electrical lug connections.

Aha. I was definitely cornfused on that ponit. Thanks for straightening me out hacksaw.

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RE: Heat transfer at electrical lug connections.

dbecker -

I just saw this thread, so I'm coming to the conversation a little late.

By way of background, I do both thermal imaging and heat transfer analysis using FEA and CFD methods.  I don't use ANSYS, so am not familiar with the details of its setup.  

Here are some comments: (all include "in my opinion")
1. If you are looking at a failure or over temperature event driven by issues at the connection then the resistive heating in the remainder of the conductors due to their resistivity or shape changes will be minor by comparison.  Yes, systems under load heat up, but failing systems whether due to loose connections, dirty connections, or corroded connections heat up a lot more.  You should be able to ignore everything that contributes to normal load heating and look only at the unusual, which would be the increased resistance at the joint / part interface.  

2. Using the heat generation of 20.28 watts on the interface is what I would do.  I have done a sample analysis using that method, which I have used as a basis for papers at SPIE Thermosense and at EPRI's IRUG conference.  I think it gives an excellent estimate of what will be going on. A couple of images from the paper are included in the brochure that is at the link I provide below.

3. You did not specify whether the component is air-filled, oil-filled, or under vacuum.  If air or vacuum, radiation will have some effect, but most of the parts will be low emissivity and will not partake in significant radiative exchange.  However, you need to assess whether nearby parts with higher emissivity are getting sufficiently hot by conduction or convection to be significant participants in radiation to the inside of the enclosure.  If the unit is oil filled, then radiation will only be a factor on the outside of the enclosure to the surroundings.  

4. Your most recent posting indicates that you are using the CFD model to assess heat transfer coefficients.  That makes it sound like you are then doing manual calculations or non-CFD FEA calculations of the heat transfer.  I would not use that route.  Having CFD capability allows you to model the interior flow which will affect the heat removal from the interior parts and will also affect the temperature patterns being developed on the enclosure.  These latter patterns may impact how the overall component dumps its heat to the surroundings.  [I realize as I write this that using HTC for the parts means you are not dealing with an evacuated part.]  I would suggest using the CFD for the full solution, including natural convection, especially on the interior.  You might be able to use a film coefficient (plus radiation if appropriate) between the outside of the enclosure and the surroundings.  If the system is oil filled, you will need density vs temperature properties for the oil.  (BTW, if you have that data, I would appreciate your sharing it.  I ran into an analysis that needed it, but could not find much.)  If you wanted to, you could shorten the analysis time by doing pure heat transfer calcs in FEA without flow.  I would suggest, in that case, that you increase the thermal conductivity of the fluid (air?) in the component by a factor of 2-3 to allow for the convective component and treat the problem as a conduction only exercise.  

5. Since you question the resistance measurement, one approach would be to do a parametric study of T(lug) as the dependent variable vs heat generation rate at the contacts.  This would be an implied function of the resistance of the contact.  The test data of 12 microohms is a resistance, not a resistivity. (I say that because of all the discussion above about units and areas, etc.  (not all of which I followed))  I would start with the 1300 amps you cite above, giving I2R= 20.28 watts as you state.  You can then develop a curve of lug T vs. assumed resistance where the resistance is used to calculate the heat generation at the interface.  20watts may not seem like a big number, but it can do a lot of heating in a small volume.  

I hope this helps.  
Jack


Jack M. Kleinfeld, P.E.  Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering
www.KleinfeldTechnical.com  

RE: Heat transfer at electrical lug connections.

This is a link to a brochure on my website (which does not have an on-web link) that has two images from the FEA analysis I did of heating in an electrical component.  This is strictly as a demonstration of the output obtained using the heat generation on the mating faces.  BTW, this was done without CFD, so the pattern of heat on the component surface is not accurate.  I also used the method of increasing the k of the air in the component to represent convective heat transfer.

http://www.kleinfeldtechnical.com/ktsinfo/FEAforThermographersBrochure.pdf

Jack


Jack M. Kleinfeld, P.E.  Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering
www.KleinfeldTechnical.com  

RE: Heat transfer at electrical lug connections.

(OP)
Thank you JKEngineer!

A couple things, the enclosure is air filled at ambient air pressure. There are vents and fans to help cool the lugs. I have the boundary conditions for the fans and vents and will be using these as my CFD BC's. The CFD is going to compute for my surface HTCs and from there I can apply them in my FEA (ANSYS) model and compute surface temps.

The only reasy I need heat generation or heat flux at the contact interface is for the ANSYS model to converge, I need the heat gen as boundary conditions for the ANSYS model.

Whoever is familiar with FEA, this is going to be a steady state solution with HTCs applied on all surfaces of the lug and no radiation assumed (T^4-T^4) is very low.

I will use 20.28W as I calculated before thanks for confirming that with me JKEngineer.

I will get back to you on this when I finish studying those links.

Thanks for all the help everyone.

RE: Heat transfer at electrical lug connections.

(OP)
Hello everyone. I am back. I am at the site looking at the circuit breaker.

desertfox, I spoke to the field engineer and he said the failure was partly due to a 130C temp rise at the lug. Not entirely due to that (long story). I mentioned earlier that the failure was ultimately caused by something out of the scope of this project (something that had to do with the breaker itself).

BUT! I must still calculate the lug temps because the field engineer is not going to take another chance and wants metal temp predictions.

So, I gathered more information.

The breaker is an AC breaker, 3 phase, 50-60Hz.

Current = 1300 amps
Voltage = 1380 V
8 cables per pole, 4 going in and 4 going out per pole
A total of 24 cables
A total of 6 bus bars with 4 lug connections each

Each busbar connection is made of copper, thickness were stated earlier as were overlap distances.

I am getting information that because this is AC, that the current density is going to be much higher due to skin effect, and that assuming DC is a less conservative (more dangerous) approach because it will underestimate the rise in current density associated with AC.

Please get back to me, I am grateful for all your input.

RE: Heat transfer at electrical lug connections.

Hi dbecker

I have recently calculated torque loadings in busbar joints and calculated stresses for temperature rises a bit less than you quote but the calcs highlight failure possibilities.
Now if you had a 130 degs temp rise it sounds to me that there was a fault on the system and that would certainly produce high stresses at the joint.
To save me trawling back through the posts can you give the section sizes of copper thats bolted together and not just the thickness (ie 50 x 10 and a 32 overlap), also how many  bolts and what size and torque loading, are you using belville washers or just plain?
If you provide this info I can do some calcs for you.

desertfox
 

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