Could also be EN61508 -there's a few parts to this and a dead boring read at that.

Therac 25 is what can happen when you get it wrong. There were no safety interlocks and dodgy software. As I mentioned in the other thread you started that you cannot rely on the microprocessor alone for safety. I also mentioned that the emergency stop was to cut power directly - not via the microprocessor. So for a 'simple' device like a numerically controlled lathe the requirements could be as simple as:
1/electrical wiring safety
2/emc
3/an emergency stop switch and maybe a safety cover with an interlock switch.
4/ labelling and a user manual pointing out safe operating procedures


In most safety systems the rules are pretty simple:

1/ You're allowed one failure
2/ The failure must be detected
3/ One failure cannot create a dangerous situation

The EN standards don't tell you exactly how to do it - they generally give guidelines and recommendations. Its up to you to prove that you comply. If you're unsure, you'll have to employ the services of a company that specialises in this type of work to advise and certify your equipment. You probably want to do this anyhow - if something does go wrong and you get sued - you have to demonstrate 'due diligence' in your design.

As Steve said - its a 'bag of worms'. How did I learn (not that I would call myself an expert)? Read the standards, look at other similar equipment, speak to suppliers. If you don't want to purchase the standards - go to the local public university library. They will most likely have them.