busbar meltdown

[mwatson]

Does anyone know of a calculation or rule of thumb for predicting when a copper busbar will enter a meltdown cycle. The point at which the resistance and temp start to rise until meltdown.
I have a suspect busbar, should be 3000Amp rated but it is only carrying 1200Amp and has a surface temp of approx 50c. Ambient temp is 20C.

 

[jghrist]

I don't think there would be a rule of thumb for thermal runaway. Calculation would be complex and would require precise knowledge of the environment vis a vis convection.

Unless you have a bad connection, it sounds like the spacings are much closer than assumed in determining the 3000A rating, assuming the rating is for a 30°C rise.

 

[rbulsara]

I would think the melting point of copper (for the given composition) will be a fixed temperature. It will melt at that temperature, regardless of how it got there. But beforet that you will have many other problems such as bus getting red hot, possible fire, cable insulation meltdwon if any cables are connected to that bus etc.

Most instances overheating of bus and cable are due to loose connections if they are not carrying rated current. Copper bus are atleast rated for 90 deg. C temperature.

What you need to do is perform thermal scanning (infra red) of your bus and conncetions in question and review the uniformity of temperatures. Involve a professional testing agency to carry out the scan and interprete the results.

 

[rbulsara]

Also see if the bus has adequate ventilation or cooling. I am not suggesting 50deg C is really considered overheating or not. You need to compare that to rest of the system and its enviornment.

 

[rbulsara]

A google search indicates melting point of copper is 1084 deg. C. I beleive it is for pure copper.

 

[rbulsara]

So it is unlikely that the a copper bus will melt because of just overload but it WILL melt and even vaporize if there is an sustained arcing fault or a severe short circuit. A loose connection eventually can lead to such conditions.

 

[itsmoked]

Would it make sense to incorporate IR sensors that could shed load/open breakers if the BB temp exceeds some obvious temperature, thereby eliminating the prospect?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com