I was disconnecting batteries under the seat of a 5 Ton
Tractor (Bob tail is what we called them in my Army Reserve Unit) and the box they were in appeared to be fiberglass...that is until I smacked the wrench into the side of the box and caused sparks to flyyyyy...I had 2 Senior NCO's outside just laughing at me as I evacuated the cab most expediently.
I now make sure to take cables off in the proper order.