We were called in to give an opinion on the panel failure below. The panel was rated at 400a, 480vac. Upon arrival the panel had already been removed and a replacement unit was on rush order. The panel had suffered significant damage. The line side feeder bus to the disconnect switches had been burned off on several branches and there was significant internal arcing. This panel was made up of fused switches stacked sideways. The fuses were held in place by a small clip that was held across the fuse blade with 2 screws.

It was my opinion that the cause of the failure was the result of a bent clip allowing the fuse blade to burn and fill the panel with ionized gases that subsequently reduced the dielectric strength of the air, causing internal arcing and failure. The upstream protection was a bolted pressure switch with a ground trip and current limiting fuses. The ground trip was set at maximum value and maximum time delay.

Right — small fuse retainer clip — uses screw pressure for clamping with no spring tension. While performing a facility audit at another location, I found another panel from the same manufacturer with a similar condition in the early stages.