Possible arc flash ???

[Jraef]

View attachment 11795 View attachment 11796 View attachment 11797 Can someone enlighten me on what could have caused this arc on the door to this switchgear. This was found at a fire scene this morning. The machine that this switchgear goes to was not suspect to the fire cause but the machine was damaged in the fire. The fuses are not blown. If you zoom in the metal bar over the left fuse is damaged. The switch gear was still on but the main supplying it was off when I arrived. When I tried to move the lever to the off postion it was stuck.

I think it does look like an arc was present, likely for a while. But if you are sure this was not part of the cause of the fire yet the machine it feeds was IN the fire, then another possibility is this (again, a SWAT):

Smoke and ionized gasses came up the conduit from the machine fire, decreasing the dielectric of the air in the switch enclosure or at the very least, coating everything inside with conductive carbon. That allowed the line side of the switch to jump the air gap either phase to phase or phase to ground, striking an arc. The arc then became self-sustaining as long as the current flowed and there was enough ionized gas and/or subsequent vaporized metal to maintain it. Because the arc was on the line side of the switch, the fuses never saw the current, so they never cleared.

 

[billk554]

just a thought but could this be a bottom fed switch?

 

[hlfireinspector]

just a thought but could this be a bottom fed switch?

Lines going out of the top of the box ran to the machine.

 

[hlfireinspector]

What indications of heat do you have in the areas near the 120V subpanel? I've never seen completely unloaded breakers trip because of a hot environment, but thermal-magnetic breakers like what's pictured do have to be derated if the ambient temperate gets above 40?C or they will trip prematurely.

120 Volt sub panel was on the other end and opposite side of the building. Heat in this area melted the exit sign off the door right next to the 120 Volt sub panel.

 

[hlfireinspector]

I agree with what everyone has said. The visual indications are pointing to an arc flash event across the line side connections of the disconnect switch. There is also a very good indicator of the event in photo 3. Take a close up view on the bottom right side of the enclosure. There is a great deal of debries in the bottom of the switch box, and if you look you can see that there is an open KO that shows that the flash also hit the box to the right through the open KO.

Did not even notice the flash on the disco next to it.