When working on your vehicle is is very important to follow some basic rules. I've read to many articles where someone has there vehicle jacked up with the tires off it falls on them where they are either killed or severely injured.
Always wear safety glasses. I personally wear glasses so I always have my safety glasses on. So many times in my lifetime I've had something bounce of my lenses and though " I'm sure glad I had safety glasses on or I would have lost and eye"
When working on electrical where you need to disconnect the power be sure to disconnect the negative power cable. If you disconnect the ground your positive will be dead as well. Disconnecting the ground is better than the positive because the positive can short with lots of sparks if you ground the terminal as your taking it off.
If you must jack up the vehicle to work under it, be sure to put something under the vehicle in case it falls. I always us as many safety devices as possible. Quite often I will use two jack stands and the jack. Also, if I have the tire off I will throw them under the vehicle as well. In the past 6 months I have read at least two articles where someone took the tires off there car, jacked it up, got under it and it fell on them. Why didn't they at least put the tires under the vehicle. I'm not going to use the derogatory terms like Idiot, moron, stupid, you can insert them yourself.
The bottom line is THINK SAFETY , think "what could happen and how can I prevent it from hurting me"