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junkyardgary

Safety question. Driving non STO drive with safety PLC

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 I have a new assembly machine to audit. The builder tells me the motor drives (electric cylinders) are safety rated. The AB Kenetix 5100 driving a servo motor with STO tied into the Gaurdlogix outputs, is fine. The other motor is a stepper motor driven by a Tolomatic non-STO drive, being commanded from the Guardlogix by tag commands. The safety circuit does not kill power to the stepper drive or break the motor leads to the stepper motor. The stepper does stop fine when the light curtain is broken, or E-stop is pressed. I'm not sure what is preventing the stepper drive from moving without STO, other than a stop command from the PLC. 

 My spider senses tell me the weakest link in the safety-device-chain sets the safety level, which would be zilch for the Tolomatic stepper drive. 

What are your thoughts? 

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All the usual disclaimers apply, of course...but...my spidey-senses are tingling too, based on your description. Your risk assessment needs to identify the hazards and how dangerous they are to the operator, based on severity of potential injury and ability to avoid the danger. Once identified and classified, all hazards have to be mitigated to the level required by the risk assessment. The mitigation can be via a safety rated (if necessary...) circuit or through fixed/tooled guarding that only maintenance can remove following proper lockout/tagout procedures.

If the stepper motor is a dangerous enough hazard (as determined by risk assessment) that the operator can access by opening an interlocked guard or reaching through a light curtain, then I'd agree that something needs to be changed. Either adding some fixed/tooled guarding or updating the circuit.

I've never used that stepper motor, but the AMCI units I've used have dual power supply points: one for comms/controls and the other for motor power. The main motor power can be interrupted without interfering with the comms/control side. It's possible that the units on your machine have a similar feature that would make mitigation much simpler.

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I wish it was that easy. Opening the power to the drive, or the motor lead trips it out, and requires a homing/reset to continue. I suggested the builder either put in an STO type drive, or build a physical brake/hold device to lock it in place during a safety circuit event (light curtain or E-stop). I can be over-ridden buy  management, but at least I voiced my opinion, and suggested reasonable fixes. When the inevitable accident happens, I'm usually able to pull out my emails, audits and avoid any disciplinary issues. 

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Oh, my, definitely document it big time. If the hazard is "significant", I'd escalate it too. I'm fortunate to work for a company that takes this seriously and won't approve a machine if it doesn't meet our safety standards. I know it can be frustrating when there's something that's "not the right thing" that you don't have the authority to fix. I would think there would be a machine acceptance process that would identify these shortcomings and give your company some leverage to make the builder fix it. If not...your machine acceptance process needs some work.

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This is a new machine built for the U.S. market?  I would raise hell if the builder doesn't address the hazard.

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agreed. such kind of shenanigans are not acceptable. equipment must pass safety audit.

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