Category 3 Safety Circuit with a VFD

I believe the Safe OFF has two separate inputs and that does the job but lets wait for more input.

Keith Cress
kcress -

The answer may depend upon which part of the world you reside in, which standards are applied there, how they are typically interpreted, and what you are doing with the drive/motor system. But in MY part of the world (Canada) for general industrial machinery for which the risk assessment does not demand a higher structure category or PL than what the drive's STO functions are declared compliant to ... The drive's built-in STO functions on their own are accepted without need of any external contactors unless the drive manufacturer calls for external contactors as part of the recommended wiring - in which case (actually, in either case), the installation has to be as prescribed by the drive's manufacturer. I haven't run across any recent PowerFlex drives that needed supplementary contactors.

So ... Probably ... Dual channel safety relay output directly to dual channel drive STO inputs, 3-phase power to drive with no external contactors, 3-phase drive to motor with no external contactors ... unless the instructions for your particular drive recommends otherwise.

Of course, all of this discussion is under the presumption that the hazard that you are mitigating is "powered motion". You still of course need a lockable disconnect switch, circuit breakers or fuses, etc for lock-out and circuit-protection and so forth - but not additional motor-starter contactors.

If risk asseessment requires PL e then the drive's PL d certification isn't good enough - but that's often in a "hazard elimination or hazard substitution" situation to reduce the risk by some other means.

edit just to ensure awareness: Structure categories, performance levels (PL), and risk evaluation is as per ISO 13849-1.

Thank you Brian. I live in the U.S. I originally designed the motor circuit with one safety contactor between motor and drive and the safety relay contacts drive control inputs and safe off inputs. I will reach out to the drive manufacturer and see what they recommend.

Without the contactors, you can only attain Cat 3 using a VFD that has STO as an option IF that VFD has been evaluated and approved as such. The basic PowerFlex 40 has not, so to attain it you would need to add the contactors. The PowerFlex 40P (with the STO option card) version has that capability, but you must also make sure it has it IN CONJUNCTION with your specific safety relay. The PF40P is now an old product and may not have been evaluated with some of the newer Safety Relays.

But if you are going to change drives, just change to the PF525, it has the STO built-in and is qualified to be able to attain Cat 3 PLd without needing to add the contactors.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

jraef thanks for the information. I contacted Rockwell and they informed me that the drive with safe-off option is Catergory 3 compliant. The design I'm implementing will use an isolating safety contactor between the drive and motor also with a disconnect switch. There are few parameters in the drive that I will have to check also (Stop Mode, Relay Out Select). Thank you for all your help. Here's a link to the drive manual.