Using STO for LOTO

Location
Illinois
Occupation
Controls Engineer
OSHA is very clear on not allowing control signals to be used for LOTO, but when thinking about what the STO signals are and the fact that they are "safety" rated makes we wonder if they could be used for LOTO. Since the STO signals are redundant and ensure that no power could go to the motor (via disabling the IGBT's in the drive) couldn't you then run those signals through a disconnect and use that for LOTO ?
 

BillyMac59

Member
Location
Wasaga Beach, Ontario
Occupation
Industrial Electrician
I have an application where the STO (Safe Torque Off) is used as part of the safety circuit. The signal runs thru safety rated contactor and is monitored with a safety relay. To use this signal without incorporating a safety monitoring device seems unconventional. I'll wait for others to comment ...
 
Location
Illinois
Occupation
Controls Engineer
I have an application where the STO (Safe Torque Off) is used as part of the safety circuit. The signal runs thru safety rated contactor and is monitored with a safety relay. To use this signal without incorporating a safety monitoring device seems unconventional. I'll wait for others to comment ...
Typically I think most people use these signals through a safety relay. I usually run them through the timed contacts so I can preform a cat 1 safety stop.
What Im wondering is if I can run these through a trapped key system, or other safety device that can accept a lock for LOTO.
 

EC Dan

Senior Member
Location
Florida
Occupation
E&C Manager
OSHA is very clear on not allowing control signals to be used for LOTO

Hardwired emergency stops do not circumvent the basic principle that electrical LOTO requires physical isolation of the the circuit.
 

SceneryDriver

Senior Member
Location
NJ
Occupation
Electrical and Automation Designer
Even with STO active, there may still be a small about of electrical leakage through the semiconductors that control power to the load. Not enough to cause the motor to spin, and probably not even measurable with the motor connected to the drive's output terminals. However, if you open-circuit the connection to the drive by disconnecting the motor, you run the risk of receiving a nasty shock at/near the DC bus voltage. There wouldn't be much current (any) behind it, but 800V is still nasty.

Additionally, STO wouldn't remove power to the drive in the control cabinet and so the drive (and everything else) would still remain powered. This means you're not supposed to work in the cabinet without proper PPE.

Emergency stops and STO are for stopping... in an emergency. LOTO is for performing work in a machine, and requires physical disconnection of power.


SceneryDriver
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
You can use it as a means of preventing motion without having to physically open power to a VFD. What you cannot do is use it as a means of electrically locking out a motor because it does not remove electrical power. It only prevents motion.

Try measuring the voltage between two output terminals sometime when the safe torque off is activated and no motor is connected.
 

Barney B

Senior Member
Location
Hurst, TX
Occupation
Electrical Instructor/Trainer
You can use it as a means of preventing motion without having to physically open power to a VFD. What you cannot do is use it as a means of electrically locking out a motor because it does not remove electrical power. It only prevents motion.

Try measuring the voltage between two output terminals sometime when the safe torque off is activated and no motor is connected.
Control Devices. Locks/tags shall be installed only on circuit disconnecting means.”

NFPA 70E (2021)120.2(E)
 

drcampbell

Senior Member
Location
The Motor City, Michigan USA
Occupation
Engineer
Even with STO active, there may still be a small about of electrical leakage through the semiconductors that control power to the load. ...
And that's when all's well. If there's an unusual amount of leakage in a semiconductor that's on the way toward failing, it could deliver a significant shock. Likewise if there's contamination in the VFD.
 
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