Electrical Questions? Ask an Electrician Online.
Hi, I'm Mike G and I'll be assisting you. Installing a breaker with the breaker in the on position will produce an arc when it comes in contact with the buss. Another possibility is a buss failure insulation causing the bus's to bend back and touch the enclosure. If that happened there would be an arc flash mark on the box. A picture of the open panel at the breaker location would aid in making a comment.