Safety Relay to PLC

R

Thread Starter

RoboKD

Hello,

I am a little new to designing systems but have worked on them for many years. I have to make a system that control two drives over devicenet, and safeties needed are E-stops(3) and a door switch. I plan on using a safety relay, and have a question about connecting it to a PLC/Drive.

Do I just take the safety relay output and connect it to a PLC input for enabling of the drives over DeviceNet, or do I need to take the safety relay output(s) to the individual enables of the drives over standard wiring (no DeviceNet)?

Thank you
 
Hello,

The later one is the correct. The output of the safety relay should be hard-wired to enable the drives, which provide a fail safe operation.

& the PLC outputs (logic requirements)to connected to the input of the Safety plc.

>Do I just take the safety relay output and connect it to a
>PLC input for enabling of the drives over DeviceNet, or do I
>need to take the safety relay output(s) to the individual
>enables of the drives over standard wiring (no DeviceNet)?
 
Short answer: it depends, but probably not.

Have you done a proper risk assessment to determine the required category/performance level for your application?

If not, that's the first step. Once you determine the required category, you can THEN design your safety control system.

I would STRONGLY recommend getting some expert assistance with your project. Safety is not something to play with.

One thought:
If your application needs a category high enough to need a safety relay, then it will probably not be acceptable to do the safety control from a non-safety rated PLC or via DeviceNet. It most likely has to be redundantly hard wired. It may not be enough to just drop the hard-wired enable input through the safety relay, either. Some newer drives have built-in safe-torque-off or safe-speed circuits. If yours doesn't, you may have to investigate other methods to achieve the required redundancy.
 
G

Gerald Beaudoin

Depending on the application/risks/consequences you may even need to consider using drives that have a "safe torque off" feature that is safety certified, or in some cases, you must even confirm "zero speed" to be safe before even allowing doors to be opened. All of this requires a proper risk assessment. You cannot rely on the input signals to the drives to definitively do what it is supposed to do. Electronics can fail, IGBT's can fail. Since you are new to this domain, you should seriously consider hiring an experienced professional to evaluate the project and guide your decisions. This approach will give you a ton of information for any future projects and also allow you to sleep at night.

Cheers
Gerald Beaudoin
 
Thanks Guys,

the machine is pretty basic. A door switch and E-stops are all that is needed. The drive does have the safe torque off option and will be using it.

Can you recommend some good reading about "determining risk assessment" and the level that should apply?
 
Current practice is that the door switch and the e-stops are hard-wired into the motor control circuit which is in the MCC. This is done in the form of a series "AND" loop. A safety relay is not required. It is also not necessary to go through a PLC.

>the machine is pretty basic. A door switch and E-stops are
>all that is needed. The drive does have the safe torque off
>option and will be using it.
>
>Can you recommend some good reading about "determining risk
>assessment" and the level that should apply?
 
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