The AB literature on their safe off option says this:
My understanding is that PL e is the highest PL there is. That would seem to suggest it is about as safe as it gets.
Well, sort of. Yes, the Performance Level doesn't go higher than e, but there is also the safety "category" involved as well. Cat. 3 means a SINGLE failure of a component will not cause a loss of safety function, but you can also have Cat 4, in which MULTIPLE failures will still not result in a loss of the safety function. Cat. 3 can have anything from PL-b to PL-e, Cat. 4 is always going to be PL-e.
So in the AB Drives world, you can get Cat. 3 PL-e using the STO option on the PF40 and PF525 drives, but you still need a Safety Relay to engage the STO command. So it cannot be listed higher than Cat 3 because the safety relay monitors the potential loss of safety sensor device ahead of it, but the VFD STO system does not monitor the safety relay functionality itself, so the failure of the switch AND the relay could negate the safe functionality. The STO function basically just eliminates the requirement for an additional safety contactor between the drive and the motor, meaning the STO option in the drive has the same or better reliability in preventing the motor from developing torque as a safety contactor would. That rating necessitates evaluating the MTTF (Mean Time To failure) of the safety contactors and testing to ensure that the MTTF of the STO function in the VFD is the same or better. In all reports I have read, the STO did indeed come out with a significantly higher MTTF number, meaning there was (statistically) MORE time between failures than with a safety contactor. And for those who don't know, there is a difference between a "safety contactor" and an every day contactor, mainly in that the contact movement to the Open condition must be "positively driven". That means no gravity or indirect bell crank movement, there must be a mechanism (spring) of sufficient strength acting directly on the armature cross arm to force all three of the contacts open in the even of being told to open under conditions that might otherwise weld contacts shut.
The highest level of Safety options available on VFD you can get, with the PF755 for example, is Cat. 4 PL-e and with that you do NOT need the Safety Relay any more either. That's because the Cat. 4 Safety Circuit inside of the drive is monitoring the sensor, PLUS it is continuously monitoring itself and has a fail safe mode since it is ALSO the end effector in the system.
One of these days soon, OSHA is going to adopt all of these IEC safety standards as
requirements instead of just "suggesting" them, so if any of you plan on doing industrial work in the future and get a chance to take a class offered by a vendor of safety relays and safety systems, I highly suggest taking the time to do it. That light at the end of the tunnel is a freight train heading your way...
