S7-1500F with ET200sp -Safety outputs connection

[grnick50]

Hello all,

I am planning an application where I have a cluster of conveyor belts (~20) where each on has its own local E-stop. I want to use an ET200sp station near that cluster and connect the E-stops to safety input cards.
Then via Profisafe in another ET200sp I want to have safety output cards that control the motor starters of the said conveyors.
My question is about the possible ways of utilizing the safety outputs.
I see in the documentation that in order to achieve the required safety level (SIL3), two Safety outputs are used to drive 2 power contactors in series.

In my case the motors are in the range of 45-55kW so having double contactors in series is not an option as this would increase the space requirement of the MCC, and space is limited.
What I would wish is for the safety outputs to control small relays in series (3RH2131-1BB40) that will interrupt the control voltage that drives the power conductors. Much like the 3TK28 safety relay.

Could another option be to use the control voltage that is switched by the safety cards as common supply voltage for conventional DOs.
Could another option be to use F-PM and have DO cards at the left of the PM?

Regards Nik

 

[mk42]

In my case the motors are in the range of 45-55kW so having double contactors in series is not an option as this would increase the space requirement of the MCC, and space is limited.
What I would wish is for the safety outputs to control small relays in series (3RH2131-1BB40) that will interrupt the control voltage that drives the power conductors. Much like the 3TK28 safety relay.

What happens if the main contactor welds closed? You'd have no way to turn off the motor. That's why safety requires redundancy.

 

[grnick50]

I totally understand the point of the two contactors in series.

As it is at the moment there is no PLC in the installation and the E-stops are wired to to the conventional automation interrupting the control voltage. The instalation is old and there are issues with that cabling.

A PLC will be installed to control the process and the cabling will be fixed-replaced across the plant. With regard to the safety I want to replicate the previous functionality without the conventional cabling but using Profisafe.
There is no study indicating the SIL level.
All that is required is that the control voltages in the MCC that is far from the local E-stops to be interrupted in a secure manner.

 

[steffen_sohl]

1 output will do, the outputs are monitored in the system, so they are PLe
but you'll still need 2 power contactors to meet PLe

 

[mk42]

There is no study indicating the SIL level.

If that's the case, then I wouldn't touch it until that study has been done.

At least in the US, there is all kinds of liability when you touch a safety system. Replicating what is already there and may or may not have met standards when it was built long ago is a great way to be on the hook for large sums of money.

I know the liability landscape is a bit different in EU land, but I 'm fairly certain the Machine Directive says you have to do risk assessments.

Your original post said you were asking about a SIL 3 system. If you want to do that, you need more than one contactor. If you don't know what SIL you need, then you can't design a safety circuit.

 

[grnick50]

Sil3 is mentioned in the documentation when using 2 contactors. Maybe my statement was unclear.

 

[boneless]

Could one use a 0-voltage spool to trip the breaker in front of the contactor? You will need the breaker anyway, so no extra space is needed...

 

[Dirt]

+1 to all what mk42 said!