yes this is exactly what Apps is talking about. I made this mistake of "zeroing" the tlo values in the table of tools I wasn't using to try and make them safe, this was a mistake.
The way to make unused tool locations "safe" is to put a positive value larger than the z travel of the machine, so I always put a 20. in my vf2 for unused tools to make that location z "safe". A really big value will usually generate a z overtravel alarm and no motion which is even safer.
A zero tlo value will move the spindle DOWN to your G54, 55.... etc. z offset value which is always negative if you use a probing system to set coordinate offset Z values. Having a large value in the tlo table for tool locations that haven't been touched off, will usually prevent an accident.
joev |