Safety Circuit Relays, Not Force Guided, Force Guided, Force Guided With EDM

theColonel26

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So I am wondering say you have the following circuits. If you have a safty controller that outputs to a single DPDT Standard relay that controls to E-Stop inputs into another device. Isn't that safer than using a single DPDT Force Guided Relay since if the force guided gets welded on one contact they all are stuck closed?


Now if you 2 force guided relays and use the seconds set of contacts in a NC configuration to the EDM of the safety controller this would obviously be safer correct?



See attached Drawings.

1 Force Guided Relay no EDM.png 2 Force Guided Relays With EDM.png
 
You should always have a pair of safety relays - Force guided - especially with robots.

Basically for the exact reason you mention.

However, in your 2nd circuit - I would use 1 NO contact from EACH relay in your A1/A2 and B1/B2 circuit.

I attached a small example drawing.
 
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That doesn't answer the question of which version of the 1st is safer. I am try to make a point to a colleage that simply using force guided relays in a safety circuit does not make it safer than Standard relays. It is how you use the force guided in the saftey circuit that makes it safter. further if you use force guideds in a saftey circuit incorrectly it can actually make it less safe than just using standard relays.


Also to your comment on the 2nd circuit. Yes I'm sure it would make it safer but I am already using EDM. Those relays don't have enough contacts to do what you susgest and I would have to get different ones. Second that would be over kill since the robot already has 2 Inputs. I have to balance Cost Vs Risk.
 
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You are making the assumption that if one contact on a 'normal' relay welds closed that other contacts will be still be open, that is a huge assumption and from experience incorrect. So my answer is that neither should be considered safe and that as Rson said you must use two force guided relays to give a good level of safety.
 
If you are doing a safety evaluation of a system that requires using force guided relays with the safety monitor, you will not be in compliance with the requirements of the safety circuit using standard relays, so really it’s a moot point.
 
Well, what makes a force-guided relay safer than a standard relay is the fact that you CAN detect a welded contact. You would be correct in saying that if you don't use that feature, you might as well use a regular relay - but it doesn't make it safe.

Your circuit would be dependent on what the risk is for your application. Your first example would only qualify for a category 1 at best (low risk). Your second would qualify for a category 2 (Mid risk)

Personally, I wouldn't put my name on either of those in a robot application unless the robot is really, really small or really weak.
 
Well in this example it's a tiny Denso SCARA that is in an enclosure anyways.



My Senior controls engineer told me to just use ONE force guided relay and use one of NOs on A1/A2 and one NO on B1/B2...... Yeah... That is what prompted this thread. He supposed to know more than I do...



As for safety requirements, this particular customers only requirements per their standard machine spec is that relays in safety circuit be "Safety Rated" which they interpret to mean "Force Guided". They do not require EDM. They expect us to just design the machine safety circuit as we see fit.


On a Side Note: does anyone know of any machine safety design training classes offered in south Michigan or north Indiana?
 
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Well, Jraef is correct you will not be in compliance.

If you read up on safety standards, it states during a transition from Categories or PL levels that "we'll tried components" need to be used. This is the first requirement depending on your assessment, that comes into play even before you determine your circuit design that you speak about. (Dual channel/EDM/etc..)
A standard relay in my opinion does not meet the well tried safety component requirement.
 
Most of the classes I've heard of are based on the European safety standards, not US standards like NFPA 79. I got the impression that they seemed to focus on the calculation of the safety rating (complicated math) more than the system design itself.
 
Most of the classes I've heard of are based on the European safety standards, not US standards like NFPA 79. I got the impression that they seemed to focus on the calculation of the safety rating (complicated math) more than the system design itself.

The calculation proves the validity of the system design.
 
Well in this example it's a tiny Denso SCARA that is in an enclosure anyways.



My Senior controls engineer told me to just use ONE force guided relay and use one of NOs on A1/A2 and one NO on B1/B2...... Yeah... That is what prompted this thread. He supposed to know more than I do...



As for safety requirements, this particular customers only requirements per their standard machine spec is that relays in safety circuit be "Safety Rated" which they interpret to mean "Force Guided". They do not require EDM. They expect us to just design the machine safety circuit as we see fit.


On a Side Note: does anyone know of any machine safety design training classes offered in south Michigan or north Indiana?

It sounds like your application has physical barriers, and therefore may not require a high amount of electrical safety. If the risk assessment is low, a circuit as your senior designer recommends is certainly plausible. Safety circuits aren't the same across the board per application - they typically depend on the physical environment of the machine.

Even with a simple circuit - you want the force guided relays so that if one fails - they all fail.

As for training, I'm in Michigan and I have attended safety classes offered by McNaughton & McKay and Banner Engineering in the past. I found the Banner one the most useful. I believe both were free at the time (several years ago). Most vendors (like Banner) will offer the classes to help sell their products.
 
It sounds like your application has physical barriers, and therefore may not require a high amount of electrical safety. If the risk assessment is low, a circuit as your senior designer recommends is certainly plausible. Safety circuits aren't the same across the board per application - they typically depend on the physical environment of the machine.




Even with a simple circuit - you want the force guided relays so that if one fails - they all fail.


What would be the point of a a single force guided relay controlling 2 NO contacts? If one welds closed them both will be closed. It at least needs 2 one for each change into the Controller.
I guess my next question is; Is there any reason to use force guided relays unless you use EDM or have 2 relays, with 2 contacts one on each channel like you did in your example?


Him telling me to use a single force guided relay seems pointless. Just save money and use a regular one right?



As for training, I'm in Michigan and I have attended safety classes offered by McNaughton & McKay and Banner Engineering in the past. I found the Banner one the most useful. I believe both were free at the time (several years ago). Most vendors (like Banner) will offer the classes to help sell their products.


Hmm I will have to talk to our main vendor. We buy of lot of banner products from them. Including the safety controller SC-26 that I am going to use.
 
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No, you can absolutely use a regular relay if you only need a category B safety circuit.

I see this often with small machines. The MCR acts more like a ON/OFF button than safety.

I don't believe you can technically buy a force-guided relay without at least 1 NO and 1 NC contact - so in your circuit even if you only use the NO contacts, you can still detect a failure with the NC. Now - you might not have it wired or used in any shape or form, but mainenance can test it or a company you sell the machine to could use it as an interlock to see when the control circuit is down.
 
No, you can absolutely use a regular relay if you only need a category B safety circuit.

I see this often with small machines. The MCR acts more like a ON/OFF button than safety.

I don't believe you can technically buy a force-guided relay without at least 1 NO and 1 NC contact - so in your circuit even if you only use the NO contacts, you can still detect a failure with the NC. Now - you might not have it wired or used in any shape or form, but mainenance can test it or a company you sell the machine to could use it as an interlock to see when the control circuit is down.
These have 2 change over contacts. They are DPDT. So you can use them as 2 NO, 2 NC, or 1 NO and 1 NC. So if I used it in a 2 NO configuration I could not uses the NCs since I am using them as dry contacts for the Robot Controller. If the robot controller just had 2 V+ inputs then I could use the NCs as they would be supplied with Main 24VDC power via the 11 and 21 terminals. but the robot requires completing an internal circuit.





I'm not familiar with categories of safety circuit. Have any recommended reading?
 
I'm not familiar with categories of safety circuit. Have any recommended reading?
i would recommend AB's Safebook. There is a lot of information in there. It will tell you about categories, Performance Level (PL), and Safety Integrity Levels (SIL).

Safety is one of those things were a little information is dangerous. I've seen a lot of designs where the "I used safety rated parts so it must be safe" mentality give a completely false sense of security. Yes, you need to use safety-rated components but they need to be used as intended.

Your's in another example of this. It sounds like the correct pieces are there, they just aren't assembled correctly.
 

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