How to properly implement Safety Relay for application

[mioduz]

I want to improve a semi custom product I am delivering to a customer with implementation of safety relays. The system is fairly simple with just a couple of small motors and a couple solenoid valves. All of these are to be controlled through the PLC. All of which are designed to go to a fail condition in an E-STOP state. One Local E-Stop pushbutton and one set of contacts in for a remote input.

Traditionally I would just use a NON safety rated relay. The Local and remote Estops in Series to the Coil. The NO contacts would be in series with the Digital Output Common terminal of PLC.

I can of course do the same with a safety relay but there has to "more" to this to improve the product. Can anyone elaborate how to improve this?

 

[mk42]

I want to improve a semi custom product I am delivering to a customer with implementation of safety relays. The system is fairly simple with just a couple of small motors and a couple solenoid valves. All of these are to be controlled through the PLC. All of which are designed to go to a fail condition in an E-STOP state. One Local E-Stop pushbutton and one set of contacts in for a remote input.

Traditionally I would just use a NON safety rated relay. The Local and remote Estops in Series to the Coil. The NO contacts would be in series with the Digital Output Common terminal of PLC.

I can of course do the same with a safety relay but there has to "more" to this to improve the product. Can anyone elaborate how to improve this?

I don't understand your question. The "improvement" to the product with a dual channel safety relay over a single channel non-safety relay is that it is significantly less likely that nothing changes when you push the estop button. Therefore less likely people get hurt and you get sued into oblivion.

 

[rQx]

It seems like you describe that you have a regular contactor and in series with the coil you have your estop. And that you thinking of switching it to a "safety rated" contactor.

My experience with Allen Bradley is that the safety rated contactor and a regular contactor has the same B10d values and are designed the same.

The improvment in your case would be to implement a safety monitoring relay like a 440R-N23132.

In the end it all comes down to your required performance level that your risk assesment provide. From it you can descide if your solution meet the requirement, wich it can, or if you need to improve.

 

[kamenges]

Be careful thinking that simply replacing a common component with a "safety rated" component will gain you any advantage. This was a common misconception among designers in the US when safety rated components started filtering into the design space about 25 years ago or so.

Generally speaking, safety rated components give you two things. First, they have testing data that backs up their capability and longevity claims. Second, they are designed in such a way that they fail in a consistent and detectable manner. As rQx said, there really isn't much of a difference between Allen-Bradley's normal IEC contactors and their IEC safety contactors that would guarantee that one would operate better than the other. However, the IEC safety contactors have positive guided (or mirror) contacts, meaning a normally open and normally closed contact in the same contactor cannot be closed at the same time. This allows a user to detect a welded contactor. Combined with the use of redundant contactors you end up with a safer system.

So, it isn't so much the use of a safety component that is so important. It's about how you use it.

Just as importantly, as rQx said, your design decisions should be driven by a risk assessment. That will tell you what level of protection is required. Without that you are just throwing parts at the machine without knowing what your real end goal is.

Keith

 

[James Mcquade]

Welcome to the forum.

Please look at NFPA 79, Electrical Standard For Industrial Machinery. Also look at NEC 70 - electrical codebook, NEC 70E - Arc Flash. A safety relay has been required for machinery for a good while for E-stops. if i am wrong, someone please correct me.
regards,
philip

 

[I_Automation]

The safety relay would be a separate E-stop system NOT controlled by the PLC or even controlling the PKC for safety. The PLC should have an input that the safety relay is set and monitor that to run it;s outputs



BUT


The safety relay when tripped should drop all voltage to the PLC output cards that control things that need to shut off. If it's a standard PLC and something happens internally and an output can not be turned off, then the external safety relay dropping power to the output is the only thing that can save an operator.


This is an example of a SLC I have that when faulted or put in program mode does not turn off any of the outputs that were on at the time: This better have had a separate safety relay controlled power source