Even with breaker off you're not 100% safe
 

https://cimg8.ibsrv.net/gimg/www.gre...c73e1ad395.jpg
https://cimg3.ibsrv.net/gimg/www.gre...9266e7ea99.jpgI think the vent on this one blew out.
https://cimg5.ibsrv.net/gimg/www.gre...1733d49b10.jpgIf you look at the rail in the center of the photo you can see where it was arcing just beforw the middle bend. I have the rachet on the bolt to the right and my hand slipped causing the short.
The dust is from the fire extinguisher.
so in short, to be safer, temporarily cover the rail in the center of the above photo.