Manual vs. Automatic Reset

I have a question concerning automatic reset vs. manual reset of a safety relay.

I'm wondering if configuring the relay for automatic reset lowers the SIL level, even if the switch wiring is the same as if it were configured for manual reset. I'm using the following diagram as a reference.



I'm thinking that automatic vs. manual should make a difference, but I have no info to back it up.

The picture didn't come through as expected. Here's a link to the diagram.

https://www.dropbox.com/s/jbz29kedobfogxb/Safety%20Relay.JPG?dl=0

That looks like the ESD relay from Automation Direct, correct?
I just installed one on a panel. I went for the manual reset. I don't know anything about SIL but to me it just makes sense to use a manual reset. If a Emergency Shut Down has been pressed it will require that someone make a effort to make sure everything is off and ready before resetting. That way the process doesn't just start back up if the operator pulls the button back and the hazard still exist.

Just my opinion.

Unless the safety relay is being used for a light curtain, I always opt for the manual reset, be it a push button but usually a PLC output. With the light curtain, it depends on the machine's safety assessment and even then I still might use a PLC output and reset the SR when the operator presses the Cycle Start.

"reset the SR when the operator presses the Cycle Start"

I hadn't considered using the existing "Start" button as the reset. That would eliminate the need to install another button and satisfy the manual reset aspect of it. Thanks for the idea.

As far as I know, When a safety relay is tripped, automatic resetting of that relay is not allowed. You must consider the state of the machine operation, who is working on the system, damage to the system if an automatic restart is initiated.

I do not have my code book with me, maybe someone will post the code section.

regards,
james

Be careful with words.

Resetting a safety relay and resetting an E-Stop circuit are not always the same thing. Safety relays can be used for different purposes.

I can a have a safety relay that monitors if my safety gates are in the closed "safe to run" position/condition. That relay may be wired in series with some e-stop buttons as part of my emergency stop circuit. I may choose to have the gate saftey relay automatically reset when the gates are closed. My E-stop safety relay would most definitley have a manual reset. No equipment will ever run unless the E-Stop relay(circuit) is reset. That relay drops out if a button is hit or gate is opened.

jstolaruk said:

reset the SR when the operator presses the Cycle Start.

Click to expand...

This satisfies the requirement for operator action to reset an E-Stop/Guard safety circuit and it was my recommended option to OEMs when I was an integrator. Unfortunately, there are a lot of people out there not thinking logically about the safety standards, so I usually ended up with something extremely overkill (while not actually being safer) or arguing about something that was legitimately unsafe and how I couldn't do it for them.

My current employer uses a separate HMI button and won't even display buttons that require the safety circuit until the safety circuit is reset. This isn't actually safer, it just makes one button push into two. If you want to be crazy overkill safe, you would require a two hand reset.

A big no no is the machine doing stuff as soon as it turns on (homing) and the biggest issue for machines. You need to have operator input (even a splash screen explaining things are going to home when they press OK). I usually also tied this to the start button but would provide a manual home button as well.

EDIT: I really hate buttons that say things like "Setup" that cause movement. If a button will make something move, it should use a word/words that clearly imply movement, like "Move to Setup".

jrexrode and CapinWinky,

"reset the SR when the operator presses the Cycle Start"

I disagree, the cycle start pushbutton cannot be used as the reset pb. The reset pb is a dedicated pushbutton.

regards,
james

James Mcquade said:

jrexrode and CapinWinky,

"reset the SR when the operator presses the Cycle Start"

I disagree, the cycle start pushbutton cannot be used as the reset pb. The reset pb is a dedicated pushbutton.

regards,
james

Click to expand...

To reset an E-stop, I agree. As something to reset a safety relay monitoring a light curtain, its possible.

Just to be clear, there is absolutely nothing against using a cycle start button to reset an E-Stop circuit AND start the cycle in one push in any major standard and you could easily get your UL/CE certification while still doing this (as I have on several machines over the years). I would argue that this practice is in no way less safe than a separate reset button as an operator would clearly want the machine to start if they were pressing the Start button and had pulled all the E-Stops back out already.

If you are referring to the wording in NFPA 79, CSA Z432, or ISO 14118 that resetting an E-Stop is not allowed to start motion, they are referring to the button itself (as in twisting it so it pops back out), not the safety/E-Stop relay/circuit/PLC.

My preference is to use a "Faults clear" pushbutton to reset the safety circuits as well. Only after all the faults are cleared and the safeties reset, a cycle can be initiated.

James Mcquade said:

jrexrode and CapinWinky,

"reset the SR when the operator presses the Cycle Start"

I disagree, the cycle start pushbutton cannot be used as the reset pb. The reset pb is a dedicated pushbutton.

regards,
james

Click to expand...

We've some Engel molding machines that use the cycle start button to reset the safety scanner (light curtain) circuit. From dealings with them, I know they take their machine safety extremely seriously. I'm not saying that they would use a cycle start button to reset the e-stop, but they seem to take no issue in using it to reset the scanner circuit.

For clarification of the discussion, my application is basically a guard door circuit, not a machine e-stop. There is a separate e-stop circuit for the machine.

I've noticed that a few engineers think that the Reset input on a safety relay overrides the the safety function. Recently, it was a "CE Guru" contractor brought in by my customer to sign off on CE compliance. He was insisting that the reset circuit be performed with a dual contact "safe" push button.

Your points about the light curtain are valid and I have done this in the past, resetting the light curtain circuit after loading parts into the machine using the cycle start pb.

My confusion is if you are using the cycle start pb to reset the E-stop circuit after its tripped. If you are, then that's a big problem for me. When debugging a machine you did NOT
build, you have no idea of what is going to happen. Even if you did write the program, unexpected things can happen.

This happened to my co-workers 3 years ago. Our sister company built a machine and shipped it to us. During the runoff, the machine stopped mid cycle and an e-stop had to be pushed. We followed the manual to reset the E-stop, pressed the button, and the machine homed itself !! The highly volitile material was dropped 4 ft. The customer representative was not a happy camper. The programmer was asked why he wrote it that way, that's how we always do it, I didn't think the machine would be loaded.

I know some of you are tired of reading about my experiences, but it proves my point. Safety first, everyone is safe and goes home to the family.

regards,
james