Panel buss short circuit

[RumRunner]

The pictures show the arcing was primarily at the sharpest points or corners on the busbar, and also on the corners of the attached peg for mounting a breaker. Such corners would be where the electric field gradient across open air would be the highest (in other words, where the electric field is changing the most quickly vs. distance). The arcing evidently occurred across an air gap, which is different from arc tracking across an insulator caused by surface contamination or moisture. I could be wrong, but I don't think 480V L-L would normally be sufficient to induce corona and initiate arcing across significant air gaps like that causing the damage in the photos. However, the 480V could likely sustain the arcing once started. For the OP, do you know the approximate distance between the points where the arcing occurred?

I wonder if there could be voltage spikes occurring from the switching of PF correction capacitors, a defective transformer, or something else. Perhaps a surge arrester could be installed on the panel, preferably to clamp significant voltage surges, but also to confirm that a large surge had occurred if the arrester itself was also damaged.

I agree.

480v is not high enough to manifest Corona. The first incident could have developed slowly that no one noticed or simply ignored until the meltdown.

Most "guesses" on how this happened could have merit--from snakes to rats to dust.

However, the decision to replace the burnt busbar (prior to this second incident) and not the entire panel was a hardknocks remedy. Band aid in other words.
The melted busbar evidence is telling that high temperature caused this meltdown.

The arcing that occured inside the panel had raised the temperature so high, the ionized air formed a plasma that the surrounding material turned to carbon--the by-product of combustion.
Carbon is conductive.
The insulating property of composite material even ceramic had developed a bridge where carbon created a path to ground. This carbon is visible on that photo.

The sustained arcing that turned the enclosed ionized air into high temperature plasma could have compromised other insulation like thermoplastic covering for conductors.

I agree with Sahib that meggering the conductors to check the insulating properties of the conductors even after replacing the entire MCC.

 

[Sahib]

The paper at the link below describes arcing in panels and switchboards:

https://www.google.com/url?sa=t&rct=..._GC14WrsDysNCs

It mentions the following about surges in LV networks that could induce arcing, and suggests the installation of surge arresters:




They also mention how an arc that starts closer to the power input side of the busbars will move toward the other end of the busbars due to magnetic forces.
This could be a reason why in the OP's situation the arc was probably sustained at the bottom of the busbars and therefore caused melting there, even though the arc could've started further up.

That the damage to the panel occurred due to surge is not tenable because to reach the panel to damage it, the surge must first pass through cables to the panel. But the dielectric strength of LV cable is 1.5 KV only. So any surge of value greater than 1.5KV could cause breakdown of the LV cable and then would not pass downstream to the panel as the LV cable would become shorted to ground then.

 

[sndbodkin]

Hmm. If surge were the cause of damage inside the panel, it would not spare the connecting cables in the first place because the distance between the conductors much more less. It would be more prudent to meggar the panel than to install surge arrester.

One of the first things we did was meg the panel. There was resistant between each phase and also phase to ground. I believe the copper particles that were implanted into the insulated bracket that held the bus phases in place, along with carbon build up on that bracket may have been where the arc initiated. This bracket held the buses in place at the bottom of the bus location. That is real close to where the voltage arced between phases.

 

[sndbodkin]

But the OP reuses the same panel.

That is correct. The previous electrician reused the panel. The only thing that was replaced was the phase buses.

 

[ptonsparky]

That is correct. The previous electrician reused the panel. The only thing that was replaced was the phase buses.

Well, you have answered your own questions, with the last two posts.

We don't know what caused the first, but leaving the carbon tracked bracket in certainly aided the second flash over.

The advantage of cost cutting procedures.

 

[sndbodkin]

Well, you have answered your own questions, with the last two posts.

We don't know what caused the first, but leaving the carbon tracked bracket in certainly aided the second flash over.

The advantage of cost cutting procedures.

I agree, thank you all for your input. It has been a great help. Happy Thanksgiving!