How is the safety relay for the light curtain set up ?
If it is self-resetting, then you have a very dangerous situation.
An operator can enter the machine for maintenance, when it is powered down. Because of the light curtain he thinks that he is safe.
But if he is so far into the machine during his work that he doesnt interrupt the light curtain, then the machine can automatically start if the power is reestablished.
Maybe the above example is far-fetched, but you have to consider everything.
I think that there should be some kind of operator acknowledgement before the machine can restart.
You can write in the manual, and put on warning signs on the machine, that the main switch should be turned OFF and padlocked. But still, some operators dont read the manual, or cannot read at all, or are simply too careless.