HOA buttons through PLC

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dmejia

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Good afternoon,

I would like to know if wiring an HOA selector switch through a PLC is acceptable as disconnecting means for a motor (NEC section 430.102). The logic is the following: To connect the "Hand" and "Auto" signals to the PLC terminal as an inputs. Whenever the selector switch is in the Hand position, the PLC would send an output (DO) to turn on the motor. Whenever it is in Auto, the PLC would automatically turn On and Off the motor depending on the logic. When it is in the Off position, the PLC would turn off the motor. Technically is the same thing but things are not hardwired.

Thank you for your advise.

Regards,

Daniel
 
i would say no for the fact that you can " FORCE ON" the output via a computer. also how would you lock out the motor for service. it would have to have a lockable motor starter.
 
I don't see why not all functions of an HOA become just a 3 DI's as any thing else, the PLC doesn't know what it is ,it is just a 2 wire DI, you just have to program the need you want for the DO.

The MCC bucket would still be locked out and the PLC could receive a key pad instruction as a backup lockout.

dick
 
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No it is not, an HOA hardwired in a control circuit would not be an acceptable disconnecting means either. The disconnecting means must isolate the supplied power from the controller not just interrupt the control power. See 430.103
 
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dmejia said:
a PLC is acceptable as disconnecting means for a motor (NEC section 430.102).
Click to expand...
No... It is under the control of the PLC - not a human, and not able to lock open, and may not actually physically seperate conductors (Triac/SCR).

Disconneting means are just one of the 'other' places where design is mandated by code for worker/user safety. (We were talking about this in another thread the other day.)
 
e57 said:
And where would my lock and tag go on a PLC?
Click to expand...

If you go back and check my post you will see, I said at the MCC (Motor Control Center) for the lock out and the use of a Key Pad ( soft screen touch @ the PLC) to reprogram the PLC temporarily to disable the control logic.

dick
 
dmejia said:
Good afternoon,

I would like to know if wiring an HOA selector switch through a PLC is acceptable as disconnecting means for a motor (NEC section 430.102). The logic is the following: To connect the "Hand" and "Auto" signals to the PLC terminal as an inputs. Whenever the selector switch is in the Hand position, the PLC would send an output (DO) to turn on the motor. Whenever it is in Auto, the PLC would automatically turn On and Off the motor depending on the logic. When it is in the Off position, the PLC would turn off the motor. Technically is the same thing but things are not hardwired.

Thank you for your advise.

Regards,

Daniel
Click to expand...

This is not an unheard of scheme for motor control but even hardwired it is not an acceptable disconnecting means.
 
eric9822 said:
No it is not, an HOA hardwired in a control circuit would not be an acceptable disconnecting means either. The disconnecting means must isolate the supplied power from the controller not just interrupt the control power. See 430.103
Click to expand...

As a disconnect this is correct. Both power and control must be disconnected. The HOA is not normally sufficient whether PLC or HW.

dicklaxt said:
If you go back and check my post you will see, I said at the MCC (Motor Control Center) for the lock out and the use of a Key Pad ( soft screen touch @ the PLC) to reprogram the PLC temporarily to disable the control logic.

dick
Click to expand...

Most PLCs are not safety rated. Therefore isolation through the PLC is not permitted. You must purchase a special PLC to use it for interlocks.
 
dmejia said:
Good afternoon,

I would like to know if wiring an HOA selector switch through a PLC is acceptable as disconnecting means for a motor (NEC section 430.102). The logic is the following: To connect the "Hand" and "Auto" signals to the PLC terminal as an inputs. Whenever the selector switch is in the Hand position, the PLC would send an output (DO) to turn on the motor. Whenever it is in Auto, the PLC would automatically turn On and Off the motor depending on the logic. When it is in the Off position, the PLC would turn off the motor. Technically is the same thing but things are not hardwired.

Thank you for your advise.

Regards,

Daniel
Click to expand...

Like petersonra said, wiring a HOA switch like this is not unheard of, but this doesn't count as a disconnecting means.

Your PLC does not count as a "controller" per 430.102. So you don't need a disconnect at the PLC location (if thats what you are thinking). Refer to the definition of "controller" at the begining of 430.

Steve
 
Most PLCs are not safety rated. Therefore isolation through the PLC is not permitted. You must purchase a special PLC to use it for interlocks.[/QUOTE]

pfalcon, I agree whole heartedly and when doing so as a single safeguard,it certainly is not permitted nor accepted in the safty stds, however,there is not anything that keeps one from building in a backup safeguard by doing a quick control logic disable command in the event that any run permissive or start commands were interlocked to the motor in question,you could very easily get an auto start command from the PLC programming.......................bearing in mind I did say that it has to be locked out at the MCC.

dick
 
steve66 said:
Like petersonra said, wiring a HOA switch like this is not unheard of, but this doesn't count as a disconnecting means.

Your PLC does not count as a "controller" per 430.102. So you don't need a disconnect at the PLC location (if thats what you are thinking). Refer to the definition of "controller" at the begining of 430.

Steve
Click to expand...

My PLCs frequently fire the starter for the motor, so by 430, yes they are the controller; but this portion would only be the control power not the motive power. Both must be disconnected. As to killing power to the PLC, that is not necessary. It is necessary to kill power to the control circuit powering the starter. The processor may stay live.

dicklaxt said:
Most PLCs are not safety rated. Therefore isolation through the PLC is not permitted. You must purchase a special PLC to use it for interlocks.

pfalcon, I agree whole heartedly and when doing so as a single safeguard,it certainly is not permitted nor accepted in the safty stds, however,there is not anything that keeps one from building in a backup safeguard by doing a quick control logic disable command in the event that any run permissive or start commands were interlocked to the motor in question,you could very easily get an auto start command from the PLC programming.......................bearing in mind I did say that it has to be locked out at the MCC.

dick
Click to expand...

Correct, but I don't permit that to occur. We have had too many incidents where equipment removed for service was not tied back in after return. The PLC backup makes it difficult for most people to verify the Estop functions.

1 Remove defective equipment
2 Jumper out the Estop circuit so they can build manually
3 After repairs, re-install the defective equipment
4 Test the Estops

Oops, we forgot to re-wire the Estop!
Why did it stop then?
Cause the PLC stopped it.
Isn't that good enough?
Yep, until the PLC contact freezes or some yahoo changes the program. Then you get to hear the screams.

IMHO, as a personal preference in machine building, sometimes the backup solution creates worse situations then letting it fail. But that's just me.
 
dmejia said:
I would like to know if wiring an HOA selector switch through a PLC is acceptable as disconnecting means for a motor (NEC section 430.102).
Click to expand...


No, the disconnecting means is required to open the supply conductors, not the PLC input circuit or a control circuit.
 
hurk27 said:
Originally Posted by e57
And where would my lock and tag go on a PLC?
Back in you tool bag/box?;)
Click to expand...

The requirement is also for the motor starter also, and locking out the controls would not remove power from the motor starter if there was one?
Click to expand...
My comment was sarcasm... ;) The MCC and the motor need a disco, unless they are one in the same....

Anyway.... Not to get too far off track, but 'my computer can print documents' :grin: - it does not, :mad: my printer in my basement prints documents.

Since this PLC could feasibly be operated from deep space, or my multiple users in other countries if so equipped, I don't see a PLC as a "disconnecting means" of supply conductors. For that matter a program glitch could have someones arm wrapped around a shaft.

Now the other question here: Are dmejia & dicklaxt the same person - I'm confused?
 
Recently instaled PLC control for a small industrial. At the same time the plant was implementing LOTO. We were continually warning the operators about locking things out and that the PLC was not to be relied on for safety. Point was driven home when a large noisy piece of equipment started as I downloaded a change. PLC control means nothing...lock it off with a disconnect.
 
No e57 we are not the same person just so you know and you can now become unconfused.;)

Here is a schematic of one of 3 motors that work together supporting each other thru PLC logic and remote control voltage,,,,,,,,,,,,,,,if you study this you can see if the PLC is not disabled you can get into trouble with the remote control voltage,possible but improbable PLC card failures amd out of sequence LOTO procedures..also keep in mind that my original comment included that the MCC must be locked out and in this case so must the HOA and a PLC system disable would just put the frosting on the cake as a further safeguard.

I thought when we joined a "Discussion Forum" that we all agreed to disagree and the correct solution/solutions would be sorted out in conversation.

dick
 
dmejia said:
Good afternoon,

I would like to know if wiring an HOA selector switch through a PLC is acceptable as disconnecting means for a motor (NEC section 430.102). The logic is the following: To connect the "Hand" and "Auto" signals to the PLC terminal as an inputs. Whenever the selector switch is in the Hand position, the PLC would send an output (DO) to turn on the motor. Whenever it is in Auto, the PLC would automatically turn On and Off the motor depending on the logic. When it is in the Off position, the PLC would turn off the motor. Technically is the same thing but things are not hardwired.

Thank you for your advise.

Regards,

Daniel
Click to expand...

Control circuit under no circumstances can be considered as a means of disconnect, where the 'means of disconnect' is assigned to devices for compliance with the OSHA LOTO energy isolation.
 
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