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.

 

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?

 

I would keep the actual safety function out of the standard PLC path.


There’s nothing wrong with sending a status from the safety relay into the PLC so the program knows why the drives are inhibited, but I wouldn’t make that PLC input and a normal network command the thing that actually provides the safe stop.


Since the drives in this case have STO, that is the part I’d be looking at for the hardwired safety path, with the exact arrangement based on the required performance level from the risk assessment. The PLC can still monitor the relay separately for diagnostics and operator messages.


I found this while looking through some safety-relay wiring material and thought it might be useful to anyone who comes across this discussion:
https://iqelectro.com/blogs/news/safety-light-curtain-wiring-diagram


The example is based around an E-stop and a light curtain rather than a door switch and VFD STO, so it isn’t the exact same circuit. What I found useful is the separation between the safety outputs and the ordinary PLC status signal — the PLC gets information about the safety state, but it isn’t relied on to perform the safety function itself.


I’d still start with the machine risk assessment and then wire the STO according to the drive and safety-relay manuals rather than treating a normal DeviceNet enable as equivalent to a safety input.