I'm not sure if this is intentional. Machine casings are completely immune to explosions(almost, explained below)
Was doing more testing with the AWESOME new Industrial Grinder. I wanted to see what happened when a grinder was fed more than 128eu/t. As expected it exploded, but none of the Machine Casings were broken. I thought maybe my setup was strange, so I rebuilt the whole thing from scratch and overloaded all 4 attached grinders. Yay big explosion, lots of terrain damage, but the all the Machine Casings were still intact.
Test#2: TNT
I built a 4x4x4 out of each Machine Casing type; Standard, Advanced Reinforced.
6 tnt + 2 restone torches inside each cube. Place the torches and fill in the top hole. Run a way and... nothing. I could see the animation and hear the sound but no casings were destroyed or dropped and all the surrounding terrain was intact.
Test#3 NUKE
Same setup with each type of casing, but only 1 nuke.
The Standard and Advanced casings did have 1 missing block on one of the sides that the nuke was touching. A small amount of the explosion got out but it did about as much damage to the terrain as a regular TNT explosion.
The Reinforced casing had no damage at all. I tested this multiple times and with multiple nukes. I even stood right next to the 4x4x4 in survival mode. All I got was the hunger and poison effects.