safety relay needed????

[clarke]

We have a large machine with two discs rotating at high speed, to shut down the machine if it becomes unbalanced a vibration switch is to be fitted. I have to send a signal to an s7 plc via an et200 rio panel to signal that the vibration switch has been triggered. Is it o.k to let the plc shutdown the machine or do i need to use a safety relay to open a contact and drop out the contactor supply and set the signal to the plc as I only have one shutdown contact on the vibration switch.
I'd appreciate any advice ty.

 

[buzzp]

Does the PLC perform a controlled shut-down (brakes applied or decrease speed)? If you just open a contactor then the 'mass' will continue to turn (and vibrate) until it coasts to a stop. If you have a braking system that is handled by the PLC, it would be best to let the PLC handle the shutdown (I assume this PLC controls the contactor). In your case (based on what I know) there is no 'standards' that may apply out of the ordinary stuff.

 

[JesperMP]

A safety relay is only required if personal safety is at risk.
As you are protecting machinery rather than people with the vibration switch you do not need a safety relay.

It is possible that you need a safety relay and have to worry about bringing the discs to stop quickly in order to provide personal safety, but thats another discussion.

Also, a safety relay allways works on two signals (or two contacts in the same sensor), so you cannot connect that vibration switch to a safety relay anyway.

I would let the PLC evaluate the signal from the vibration switch. Its is much easier to interlock it with your control and to program it to ignore the switch during startup etc.

 

[EEAutomEng]

I agree with Jesper, however, it is possible to use a safety relay to shut down the machine. Some safety relays have the ability to be driven by semiconductor outputs from a PLC. You have to remember that the purpose of a safety relay is to provide redundancy. The ideal scheme would be to drive one safety relay by two plc outputs and have that safety relay switch the contactor. Use one normally closed contact from the contactor for the feedback loop of the safety relay so if the contactor welds for some reason, the safety relay will not reset and the machine will not restart. There you have redundany on the inputs of the safety relay and redundancy on the outputs with back-checking. Hope that helps.

 

[snoogie]

How does the existing e-stop on the system work ? I would have thought you want the vibration switch in the e-stop circuit one way or another to stop the rest of the machine asap ? It sounds like you are adding this switch after you have already had a problem with the machine and i have to beleive that things go really wrong if this thing goes off balance when running. If the machine is driven by an AC or DC drive with braking (resistor or regen) ability i would do two things. Forget about remote i/o for a start (if thats what a et200 is) Hardwire the vibration switch back to the panel (i know its a pain but you will sleep at night) and put it in series with a relay with one contact going to the PLC and a second driving a timmed e-stop contact via a adjustable pneumatic timmer (not solid state) then if the switch activates, the contact thats connected to the PLC tells the drive to do a controlled stop (or put a brake on)and the other contact in the timmer circuit starts to count down to e-stop such that if the contact to the PLC fails then the drive will be disconnected (via the e-stop circuit i hope you have a drive contactor somewhere to pull out) after "x" seconds and coast to a stop. This is safe and utilises the drives faster stopping ability to slow the motion as much as possible before positivly disconnecting the drive. I am sure you could get a safety relay to do the turning off. (the e-stop should be safety type if in europe any way)
Snoogie

 

[EEAutomEng]

Snoogie has a good idea. Instead of a pneumatic timer I would use a safety relay that has category 1 stop contacts (that means off-delay that is fixed or adjustable). That way, there is some redundancy (2 time delay contacts inside the safety relay) instead of just 1 contact from an electromechanical relay that can fail closed.