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.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Aside from STO being inappropriate to supplant LO/TO procedures, I think it might be important to understand what STO is INTENDED for.

In any VFD, it is not a good idea to constantly remove and apply power to the VFD, because all VFDs must have what's called a "pre-charge circuit" that prevents the DC bus capacitors from damaging themselves and/or the rectifier when they are first energized. Capacitors try to charge themselves up instantly, and do so at the available fault current level. That's what can damage things in the drive. So a pre-charge circuit places a resistance in series with the caps for the first second or so, then bypasses that resistor once the caps are fully charged. Excessive cycling of power stresses the resistor and bypass circuit and if one of them fails, the drive is toast shortly thereafter. So what does that have to do with STO?
STO is intended to replace a line side contactor that would have been used to ensure a machine cannot move while an operator is doing a routine task, like changing a bit or blade, adjusting a belt, removing or inserting a product, etc. ROUTINE tasks done within the confines of other safety systems such as guards and key locks, not repairs of replacements of the machine itself. It does not remove all potential sources of energy as LO/TO would do, it merely allows a routine task to take place without stressing the VFD components.
 
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