Powerflex 525 STO button inside enclosure

Hey everyone, just want to preface my post by saying that I'm not knowledgeable/trained on machine safety in any way, but I'm installing some PF525s in place of some older AB drives on a boiler coal feeder system. Some of the plant guys brought up the idea of utilizing the Safe Torque Off function using an E-stop button just for peace of mind during for the mechanics during maintenance, but they also don't want the E-stop mounted on the outside of the enclosure because an accidental stoppage of a drive would trip the boiler. Would it be kosher to mount the E-stop button inside the enclosure so that when the boiler and coal feeders were down for maintenance, they could open the enclosure and hit the button and know that the drive/motor is in a safe state? I could see there being issues around a couple of things, namely using an E-stop button in a non E-stop manner, and mounting the actuator to a safety input inside a cabinet. Just wanted to run it by people who are more experienced with this side of things, I would rather not put the plant in violation of safety standards which they would then have to stop the operation of the boiler to rectify.


EDIT: There are currently no E-stops mounted on the outside of the drive enclosures.




Thanks!

IMHO Estops should NEVER be used for Lock out / energy isolation. They are for stopping a machine when a bad event occurs. In the past a safety contactor would be used to stop a drive in an emergency. The STO option was developed to replace the contactor, not replace isolation disconnects.

If stopping the drive for maintenance is something that happens on every shift, then a Keyed E-stop and a written procedure, might be acceptable.

Ken Moore said:

IMHO Estops should NEVER be used for Lock out / energy isolation. They are for stopping a machine when a bad event occurs. In the past a safety contactor would be used to stop a drive in an emergency. The STO option was developed to replace the contactor, not replace isolation disconnects.

If stopping the drive for maintenance is something that happens on every shift, then a Keyed E-stop and a written procedure, might be acceptable.

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I figured that something along these lines might be the case. I was skeptical of having it inside, but wanted to see if anyone had any concrete info. I'll look into the keyed E-stops, that's a good idea. I could see the keys being lost though . It may be easier to just have the electricians pull the motor leads off whenever the mechanics are doing maintenance on the belts if they want to go that far. They've been doing maintenance on them with none of these measures in place for 30 years, so if there's no good answer that's probably alright. I would normally say just LOTO the drives, but they need to be able to jog the belt when they're working on them.


Thanks Ken, I'll have a think about what you said.


EDIT: The drives definitely aren't stopped every shift, the plant normally goes weeks/months between going offline.

They need to isolate, lock out, and verify a motor power disconnect before performing maintenance.

This is an element of safety procedure that is completely unrelated to emergency stop.

I would install a small lockable disconnect switch/enclosure on the conveyor next to the motor. Have the guys lock this out when they are performing maintenance. If the disconnect is accidentally opened while the motor is running the drive won't be real happy but it probably won't fail either. Most modern drives a fairly tolerant of that occurrence.

Keith

kamenges said:

I would install a small lockable disconnect switch/enclosure on the conveyor next to the motor. Have the guys lock this out when they are performing maintenance. If the disconnect is accidentally opened while the motor is running the drive won't be real happy but it probably won't fail either. Most modern drives a fairly tolerant of that occurrence.

Keith

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Yes, and you could wire an auxiliary normally closed early break contact from the disconnect back to the VFD STO board. Then the drive will kill its output and help protect itself from the potential damage that could be caused by having the power circuit opened under load.

Daft idea.

I would link out the STO terminals and get a Lock Out Tag Out procedure in place.

janner_10 said:

Daft idea.

I would link out the STO terminals and get a Lock Out Tag Out procedure in place.

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There is a LOTO procedure, but they often need to jog the belt while doing maintenance to inspect it. I’ll mention to the guys that it’s not a great idea and that they may want to look into other methods. Thanks everyone

> they could open the enclosure


> but they need to be able to jog the belt when they're working on them.


What is the nature of the enclosure? Is that a drive enclosure or a larger enclosure that they enter to do the work?


Either way, is it reasonable to have the physical opening of the (keyed?) enclosure, similar to a keyed E-Stop, be the action that disables the drive? And is it reasonable that (leaving and?) closing the enclosure (and replacing and turning the key) would re-enable the drive along with some control that wouold enable the jog?


This seems like a Rube Goldberg/Heath Robinson, but if "plant guys" and mechanics are asking for increased safety I think it is incumbent on the plant to provide an option that is fail-safe when the procedure is followed, and that requires explicit, non-accidental action to get around the procedure.

mylespetro said:

... I would normally say just LOTO the drives,
but they need to be able to jog the belt when they're working on them.

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OK, THAT ^^^ is actually a very valid reason to use the STO functionality.

The PURPOSE of STO is not as an "Emergency Stop", because that would be better done with isolation contactors. The STO is for when you NEED to retain the ability to control the motor when necessary doing routine maintenance or process tasks, but not by powering it down and back up again every time, because that stresses the drive and leads to premature failure (of the pre-charge circuit, which leads to total drive failure).

As to where to put the STO actuator, keep in mind that in most cases, opening an enclosure with power devices inside, in this case the VFD, will require proper PPE, meaning gloves, face shields, etc. even if it is a low energy system. So putting the button inside may create more problems than it solves. I would go for the keyed switch.

On the other hand, NEITHER STO or any E-Stop CONTROL function is safe for working on the motor or machine itself, you MUST have a LO/TO system of isolation of potential energy, aka a "disconnect switch" (even contactors are not considered safe in this regard).

What is the current safety system now? Is there no estop circuit or anything?

drbitboy said:

> they could open the enclosure


> but they need to be able to jog the belt when they're working on them.


What is the nature of the enclosure? Is that a drive enclosure or a larger enclosure that they enter to do the work?

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The enclosures are just standard Hoffman enclosures (roughly 16"W x 24"H x 12"D) mounted on strut stands near each belt. Not any existing door switches at the moment.

Just leaving site now, drives are always de-energized under LOTO if anyone is working on the belt, as it should be. They only ever jog to inspect the belt visually when it is not being worked on. Therefore I think the STO is unnecessary in this case. Thanks again to everyone who chimed in, really appreciate it

I would imagine the drive has some sort of circuit breaker or fuse in the box with it. If you get a decent circuit breaker in front of the drive, they can trip and lock out there.

In either case, if that box has 480 running into it, I would certainly have a disconnect with a door interlock to open the door in the first place.

Rson said:

I would imagine the drive has some sort of circuit breaker or fuse in the box with it. If you get a decent circuit breaker in front of the drive, they can trip and lock out there.

In either case, if that box has 480 running into it, I would certainly have a disconnect with a door interlock to open the door in the first place.

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Disconnect is located on an enclosure immediately adjacent, and as I stated above they do have a strict LOTO procedure. I misunderstood that they would ever have people working on the belt with the drive powered up. Once all parties are free and clear and sign off on the LOTO, it is then powered up and jogged for a visual inspection.