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MCC hot bus bars
2

MCC hot bus bars

MCC hot bus bars

(OP)
We discovered two out of three riser bus bars (600 Amp) had lose bolts to the main bus (1200 Amp).  The joint evidently was hot enough that the MCC skin was too hot to touch.  The result is discolored copper in the small area where two bolts join each riser to the main bus.  The only load through this junction was one 250 HP motor - 480 V.

Does anyone have a suggestion how to determine if the copper has been degraded or it's useful properties were reduced?  We will look at the bolt hole size for erosion as one criteria to see if the copper bars need replaced.

RE: MCC hot bus bars

You could try a resistance test using ductor after repair. I'm not sure what's reasonable acceptance criteria, but you can compare to similar joints.

My suspicion is there is no problem with electrical properties of the copper (resistance), or in overall strength of the copper, but possibly some small change in the surface smoothness or flatness may make it a little more challenging to form a good bolted joint.

RE: MCC hot bus bars

Is it possible to do simple energized millivolt-drop tests on the busbar joints?  All that is needed is a power-rated multimeter (e.g., Fluke 27) set for AC millivolts that will not be harmed if unintentionally applied phase-to-phase.  Of course the load currents must be reasonably balanced for the readings to have meaning.
 

RE: MCC hot bus bars

Check with full load -- if voltage drop is acceptable and
doesn't overheat, it is OK.  (what else can a bus bar do ? )

<nbucska@pcperipherals.com>

RE: MCC hot bus bars

2
Clean the joint. If you can obtain a bright copper finish without severe pitting then the copper has not been affected. Copper that has been "fried" has a rough, brittle, grey look that does not smooth out or become bright no matter how much you clean it...the piece just gets smaller. I believe this is the result of the alloys separating from the base copper, but I am not a metallurgist and don't want to be so don't hold me to it.  

Anyway, keep in mind that most copper joints are silver-plated. If the silver is gone after cleaning then you should re-plate the joint before assembling. Finally, any fasteners (bolts, spring washers, etc..) used in the joint should be replaced as the heat has probably annealed the steel such that it is weakened.

As suggested by the above posts, you can test the joint once repaired with: (under load) a thermographic camera, a infrared thermometer, a millivolt drop test, or (when deenergized) a DLRO or ductor.

RE: MCC hot bus bars

Suggestions:
1. It should also be checked for any cracks
2. Support/bracings could be checked for damages/cracks due to thermal expansion of the copper bars
3. The copper conductance approximately stays the same if there was not any meltdown anywhere reducing cross-sectional areas
4. Any bolt connection could be checked and cleaned

RE: MCC hot bus bars

(OP)
Thanks to all who responded.  We have replaced the bolts & insulator material inorder to allow immediate requirements to run one or two motors. Local arcing has taken a toll on the square bolt holes and surrounding areas.  At least one section of copper will likely be replaced prior to final commissioning of the MCC.

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