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Old June 23rd, 2022, 04:50 AM   #1
rQx
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Safety relay - diffrence between power on and emergency stop

Hi,

We have a machine that we wan't to restart after power on if the emergency stop is OK. But if the emergency stop is tripped with the button we wan't it to be resetted with the E-STOP. Is this something what i achievable? We don't have personal on site so after power failure we wan't it to restart.

/Tim
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Old June 23rd, 2022, 05:31 AM   #2
parky
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It depends on the Relay, most have an input for a reset button, it will depend but if this is linked out it turns it into auto reset, some have another input again linked to auto reset, I suggest you download the manual for the unit & it should show the auto reset.
Here is one for a Pilz unit.
Note: Auto restart can be dangerous, even if no personnel are in the location there are certain measures that need to be taken for example a sign stating machinery will start automatically, also other precautions in the event of a fault.
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Old June 23rd, 2022, 05:34 AM   #3
rQx
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Quote:
Originally Posted by parky View Post
It depends on the Relay, most have an input for a reset button, it will depend but if this is linked out it turns it into auto reset, some have another input again linked to auto reset, I suggest you download the manual for the unit & it should show the auto reset.
Here is one for a Pilz unit.
Note: Auto restart can be dangerous, even if no personnel are in the location there are certain measures that need to be taken for example a sign stating machinery will start automatically, also other precautions in the event of a fault.
I know there is auto restart function, but I wan't "both".
Manual restart when emergency stop is pushed
Auto reset if power is applied (and no emergency stop pushed)
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Old June 23rd, 2022, 05:38 AM   #4
TurpoUrpo
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First, why?

Think about scenario - someone presses e-stop, resets the e-stop button (not the relay) and someone powers off the system and the powers on.

Do you want it to automatically reset the safety on that situation?

Second, are you sure that is even allowed. What I remember working with safety it has always been - power on, check everything is clear and safe - reset safety and just then power on the machine.
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Old June 23rd, 2022, 05:41 AM   #5
parky
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The only thing I can suggest is you put in a N/C contact in series with the auto reset but it has to be a mechanical latched e-stop, so only when it is released it will start. or use a separate e-stop relay & safety relay slave one off the other.
Not ideal but will work, seen this on batching vessels, the e-stop circuit is separate from a lid safety relay although in this case the e-stop stops everything but the lid safety only disables the agitator.
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Old June 23rd, 2022, 05:48 AM   #6
parky
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Doh, so simple use a lock off e-stop button so if pressed then it requires an operator to reset it, simple.
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Old June 23rd, 2022, 05:50 AM   #7
BryanG
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Not commenting on the 'should you' bit, the 'can you' bit can be done with an on-pulse timer. It closes for a short time at power up, you parallel the 'normally open' contact with the reset button. Though it depends on what checking the Emergency Stop relay does at power up.
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Old June 23rd, 2022, 09:09 AM   #8
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Not much info to go on.

I doubt you can achieve this programmatically with a basic safety relay as it would look for an edge transition, but you certainly can with a safety PLC.

If it's just for short term power outages then I would just use a UPS for anything of any length of time I would probably use a Auto / Manual restart key switch for when the site is unmanned and manned.

Risk assessments and visible signage are going to be a must, along with training around auto restart.

For example, not much wrong with auto restarting a remote pumping station or HVAC, but you wouldn't make that argument with a mechanical press for instance.
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Old June 23rd, 2022, 10:57 AM   #9
ValeoBill
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If you want the manual control to bypass the safety circuit, what's the point of having it? As others have noted, an autoreset can be done with most safety relays. Not sure where you're from but making such changes here would require risk assessment and engineering approval. Be careful...
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Old June 23rd, 2022, 12:41 PM   #10
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Do you have relays only in this panel or are you using a PLC?

If you are using a PLC, just wire a control power reset relay into the safety circuit. Read the E-stop as an input, control power as an input, the safety reset pusbutton (if applicable) as an input, and write your logic for your conditions.

The above is not using the PLC as a safety relay - it is only using it as a means of resetting the safety relay.
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Old June 23rd, 2022, 01:21 PM   #11
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I think the OP means that on a power loss, the safety relay should automatically reset as this is an un-manned station.
There is only a need to use a e-stop with latching (preferably key release) and make the circuit auto reset, if the e-stop is pressed, then it latches in and cannot reset until the e-stop is released, in the event of a power down the relay will auto reset. perhaps the OP could clarify this.
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Old June 23rd, 2022, 01:27 PM   #12
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Didn't think of that. I guess a bit more robust than a key on the front of the panel.

(Until it's in someone's pocket and they are in the pub!)
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Old June 23rd, 2022, 05:47 PM   #13
QvixoteRoux
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By safety standars any machine at power ON must have all safety devices manually reset (when they are configured for manual reset)


If this is not going to be considered, make it with the PLC logic, at first scan check if the E-stop(s) are correct then start the machine, if not wait until reset


I should have not said that last part since I once had a valid CSME (valid 2016-2020)
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Old June 23rd, 2022, 06:48 PM   #14
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I had to implement this once. We had a process where the consequences of not automatically restarting after a power failure were significantly more severe than the automatic restart. There were zero exposed moving parts, and significant chemical hazards involved if the fans weren't running.

What we did was to leave the safety relay in monitored manual reset, but to wire an output relay from the PLC to permit the PLC to reset the safety circuit, in parallel with the physical reset pushbutton.

Inside the PLC logic, we had a latched bit which monitored the state of the safety relay. At power up, we checked the bit and the logic essentially said "if, and only if, the safety relay was healthy when the PLC went down, try exactly once to reset it 5 seconds after power on".

This means that if the safeties were tripped when the power went off, the safeties stayed tripped when it came back on. If someone presses an e/stop while the power is off, safeties remain tripped when the power comes back on*. But if the safeties were healthy when the power went out, and no e/stop was pressed when the power came back on, the safety circuit would get reset and the system would auto restart.

(*provided the e/stop wasn't both pressed and reset again while the power remained off).


Of course, you absolutely must do an extensive risk assessment and document, document, document. If you had any guarded areas, I personally would never sign off on this. If you had any exposed moving parts, same deal, even if they were protected by light curtains. My application was closer to a laboratory environment than an industrial environment, so the level of physical/mechanical hazards present permitted the idea to be entertained.
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Old June 23rd, 2022, 07:24 PM   #15
James Mcquade
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a risk assessment must be done and approved before trying to do this.

i have reread your post and the risk assessment still needs to be done.
i think some sort of warning buzzer needs to be in place to alert someone that the machine is going to start.
james
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