What do you know about control reliable circuits?

[parky]

I also meant drop the control supply to the motor contactors unless this is already catered for i.e. if these are controlled from PLC outputs then put the common feed to the output card through the safety relay, this way you increase the safety.

 

[Timbert]

I would assume so because the same current that welded it on one would weld the other...

Well, we all know what happens when you assume....

Then I would be very surprised that this happen simultaneously on 2 série contactors.
What's melt a contact is heat generated. Amps alone or voltage alone aren't making any energy, you need a contact resistance to held voltage + flowing current to heat up the contact at the melting point. If this happen on 1 of the contactor, it will significantly makes the voltage drop on the other reducing the chances to melt the other one...

Same when opening under load, the arc generated is halved on the 2 contactors when both are suppose to be good enough to extinguish it alone.

Contractors weld shut when either the coil/spring fails. Or a contact pad is worn and it arcs on closing and opening.

The probability of them both welding shut at an identical time is insignificant.

This is also covered with the diagnostic time of the circuit, how often they are actuated and tested.

Is it possible that both weld at the same time? Sure. Is it likely? Absolutely not. Modern safety design isn't trying to make something 100% safe (pro tip: You can't) but rather is the probability low enough to be tolerated.

Assuming the contactor is rated to handle the load (see above re: assuming), the contactor shouldn't fail. The reason it might weld is because the contact resistance is too high or the spring force too low. Just the normal current shouldn't do it

Let's say the contactor is 99.9% reliable. To have both contactors fail at the exact same time would be a one in a million chance. I sure hope your contactors have an expected lifetime much greater than 1000 cycles.

But this touches on the issue of diversity for high levels of safe design. By using two different methods (such as one large safety contactor and six smaller contactors for example) you lessen the likelihood of some common cause failure happening.