Where I work, you have to take into account devices that are 'upstream' when locking things out.
So I have to stand on a conveyor to fix something. I lock out the conveyor, and then I have to make sure that anything that feeds onto that conveyor (and me) is also locked out, so I have to go and lock out the screen deck above me as well.
If a machine has the ability to reach me where I'm working, zones or otherwise, it gets locked out.
Now a lot of things could have happened in this case, but the most likely are :
- Nobody realised that this could occur (poor risk assessment or task assessment)
- It was understood that this could occur and procedures were in place but they didn't work for some reason (poor risk management and failure of the hierarchy of controls)
- Procedures were in place (eg. lock out all robots within reach of you on the assembly line, or when working on robot 130,lock out 129 and 131 as well), but this wasn't performed for some reason by the employee. Safety culture at work, production pressures, lack of training in lockout procedure, failure to notice a hazard (again, lack of training), getting casual about lockouts because "nothing's happened the last 50 times I've done it" - lots of reasons to be had there.
One thing is certain though - one "simple" thing could have stopped an accident like this from happening and it only takes that one thing. You have a bunch of defenses at your disposal (eg guarding, lockouts, laser barriers, procedures, training, threats of sacking if you don't follow the procedures) - only one of those had to do it's job and they'd still be alive today.
My 25 years working in the mining industry - where accidents like this still happen regularly - has proven this to me time and time again. It is usually immediately obvious after the investigation what that one simple thing was. I guess we'll find out in due course.