I disagree back. If you look at the less-close picture, the 12th screw seems to be the most blackened. However, I looked even closer in a graphic program. The most oxidation seems to be concentrated around the bracket that holds the bus (and conducts the total neutral current).
Look again at the two studs to the left of the bus. The larger is holding the neutral-conductor lug, and the smaller is unused, but appears to be the same piece of metal that is the bracket itself. It's rather dark. Remember, the bracket is what conducts the neutral current.
If you look at a piece of bus, there are two rows of holes: the upper row is where the conductors land, and the lower row is where the self-tapping screws that hold the bus to the bracket are screwed. I believe this bracket is more discolored than the bus itself.
Additionally, the bottom-most wires are at least as charred as the upper ones, if not more so. Look how far around the first bend they are blackened. These wires are too evenly burned for one of them to be the source, in my opinion.
If indeed triplen harmonics are contributing to the heat, it would be concentrated in the neutral bus, and not the individual circuits, unless each and every wire is subject to such currents. Remember that third-order harmonics add in-phase, and this damage looks like an overloaded bus.
Plus, this much damage spread out this evenly is indicative of long-term overload, and not a short-term event. It takes a long time for this much oxidation to occur, along with a steady supply of heat. A localized heat source capable of this much energy would do a great deal of damage in a small area.
My conclusion is that the metal parts, the bus itself along with the bracket, have been subject to current beyond their design, and for a long time. I may be incorrect about the harmonics, but I believe I am correct that the bus and its bracket are the source of the heating.
augie47 said:
I would be curious as to the connection between the neutral bar and the main neutral lug. The bar containing the #12s obviously has been hot...the heat may have generated from the bar connection to the neutral also.
supergeek said:
I was thinking of that too. There are just 2 screws holding the long neutral bar to an 'L' shaped piece of copper where the main lug bolts on. I wondered if that connection was bad and if that could be the reason all the #12's burned up. When it gets changed out, I'm going to take a good look at that connection and see how tight everything is.
I believe these posts are heading in the right direction. The OP mentioned that the building is under renovation, and the measured neutral load is now under 20a. This is obviously not what was going on when the damage occured. We need to know what the loads were when the building was in full use.
A large non-linear loading, such as computers, fluorescent lighting, etc., is more likely to have created the third-order harmonics current than anything else, except for plain old resistance in the bus's mounting bracket.
If looseness of the individual conductor screws was the problem, we would expect to see greater conductor damage for the amount of insulation damage. Was there ever complaint of voltage level, power quality, or EMI interference issues? Arcing would likely have melted the individual wires.
My brain is screaming at me to end this now, so I'll see your responses tomorrow. Good night.
)... and the conductors both above and below seem to gradually get less charred. This is a sign of a loose connection that has done quite a bit of arcing. Being the bus bar is the largeest of the conductors about the connection, the heating effect has spread vertically. Note how the charring has not made it to the first bend in the top (close up pic) and bottom-one-in-the-group (away pic) branch neutrals.
