VFD machine safety

I have a mixer driven with AB 753 drives and I'm trying to make it safe for the operators to wipe off the blades when the mixer head is raised. I'm looking at the Safe Torque option with the AB drives. Is that sufficient safety for wiping down the blades? Also looking at a local disconnect to isolate the motors. Any thoughts, advice or opinion's are greatly appreciated.

A local disconnect is a good idea for maintenance folks that are servicing the motor or the mechanism attached to it - something that a lock can be put on.


As far as the the drive, a dual contact switch that monitors that the mixer is in its work position, connected to a safety relay whose output is wired to the drives Safe-Off input would be a start.

Err....

LOTO at the local disconnect is probably the most straight-forward method. However if you don't have LOTO procedures in place for operators then that is a roadblock you must get past.

The safe-torque option allows for an electrical interlock, however to properly design the safety circuit you should do a proper risk-assessment to determine hazard risk allowing you to define the appropriate level of protection required. jstolaruk has provided what will most likely be your solution.

NEC 430.102(B) I believe covers the requirements for this.

I would buy a disconnect with a N.O. aux contact and use the contact with the plc logic / vfd.

added safety.

BEFORE you can do anything, you MUST get with management, engineering, safety, maintenance, and the operators and do a risk assessment.

that will guide you in what also must be done besides the LOTO.

regards,
james

STO can only be used after a proper Safety Assessment has been performed, and yes, it still needs a Safety Relay to implement it. But the design purpose of the STO option is for performing repetitive tasks on a machine that would otherwise risk damaging the equipment if powered up and down every time, something that is not good for VFDs in particular.

Hi there,

Castell interlocks are a good method of protecting the operater when cleaning equipment such as mixer blades.

They can be set up to be as straight forward as removing the castel key shuts down the system, or a more complex system that includes a master key which is used to energise the system, and local keys which can only be released when the master key has been removed.

The idea is that the operator always keeps the unique key on them when cleaning the blades, and the system can't be started until the key has been reinstated.

Of course safety/risk assessments would be required to ensure these technique could be used. Can't beat a good old fashioned padlock through an isolator, following LOTO procedures.

I would also recommend looking at trapped key type interlocks. Castell is one brand. Another is Fortress.

An idea (you still. Wes to do a risk assessment) would be to have a mechanical lock that keeps the mixer down. To open this lock, you need a key that is trapped in the electrical disconnect when the power is on. Removing the key disconnects the power.
When you use it to open the mechanical lock on the mixer the key becomes trapped there so no one can use it to turn the power back on when the mixer is open. When you close and lock the mixer, the key is released so it can be used to put the power back on at the disconnect.