Safety circuit auto/manual reset

Hypothetically we have a machine with safety door switch and a E-Stop.
The operator places the work piece into the machine, the door is closed and the operator pushes the START button on the HMI so the machine can start processing the work piece.
In case the E-Stop is pressed or the door opens during operation the power is removed from the actuator and the machine is stopped immediately (Stop
category 0).

Question 1.
Can the E-Stop and the safety limit switch be wired in series into the safety relay?

When the processing of the work piece has completed successfully, the machine is stopped and the operator opens the door to remove the finished work piece, place a new one and closes the door.
In order to restart the process he has to reset the safety relay manually using hardwired RESET push button.

Question 2.
Is it allowed to reset the safety relay automatically from the HMI when the operator presses the START button without using the hardwired RESET push button?

Regarding your first question, this would have to be determined by a risk assessment. Having potential free contacts in series can create fault masking which reduces diagnostic coverage. This may or may not be acceptable. It depends on your risk assessment.

I’m refraining from commenting on the second one because I’m not 100% sure and I’m not in a position to look it up to verify at the moment.

It seems that is has already been discussed in the past couple of times...with no definitive
conclusion.
http://www.plctalk.net/qanda/showthread.php?t=65761
http://www.plctalk.net/qanda/showthread.php?t=82037

I think it’s going to come down to whether or not the reset is part of your safety function. But that’s what I wanted to dig into more later. If you want to research yourself, I would start with that.

' Question 2.
Is it allowed to reset the safety relay automatically from the HMI when the operator presses the START button without using the hardwired RESET push button? '

You may want to consider installing a light screen and a safety mat instead of the door if your safety assessment will allow that. The light screens and safety mat will reset automatically without the added step of using a safety reset button on the HMI. Operator would just push a "cycle start" button or palm buttons. Hope this helps.

What if both safety contacts in the door were closed and machine thought door was closed, even though the door was actually open, and someone accidently hits cycle start on HMI with the door open? What's the risk involved with this? Especially if you do not have a light curtain or safety mat, then you should allow them to continue manually resetting it instead of relying on cycle start button.

Also, you mention a limit switch for the door lock? Is the door actually locked or is it advanced into a seemingly "closed" position? You may need a safety rated door locking switch, but this depends on what your machine is doing, which means looking at the risk assessment as jhenson29 mentioned

I am going to say no, it is not allowed.
NFPA 79 states that a safety relay must be reset with the use of a mechanical pushbutton.
Even if it were allowed, I WOULD NOT, here's why.
I have had the touchscreen membrane FAIL in the on position on several hmi terminals. if they were to do that on your system, what would be the result?
you need to look at your local codes, do a safety assessment and get everyone involved.
james

Well it seems that ABB strongly advises against using an HMI to reset the machine.

https://new.abb.com/low-voltage/products/safety-products/using-an-hmi-for-reset-and-start

@alive15
I will look into the interlock switch but I'm not sure it's needed. The the machine is a small and very slow old hydraulic forming press. Recently we installed guards so there is no access from 3 sides and a sliding door in front on which I want to install a safety position switch.

That a good video about safety switches.
https://www.youtube.com/watch?v=dRxRo4QBoTg

@James Mcquade
I think the reset input of the safety relay must see a falling edge in order to reset it so if the HMI fails in ON position it shouldn't reset the safety relay.

kallileo,

NFPA 79 states that a mechanical pushbutton must be used to reset a safety relay, no electronics.
if the rules have changed, someone please correct me.
james

James Mcquade said:

NFPA 79 states that a mechanical pushbutton must be used to reset a safety relay, no electronics.
if the rules have changed, someone please correct me.

Click to expand...

I don't ever recall reading that as a requirement of NFPA 79. Do you have chapter and verse (and version) for that?

I know it says that you can't use graphical representations for an e-stop device. I don't know of a similar prohibition for a reset device (at least not since 2012).

In section 10.2, it talks about "Pushbutton actuators and color graphic interface devices" and seems to treat them as the same. Section 10.2.2.7 does not point to another section on actuators, that the section on emergency stop does.

Timbert said:

I don't ever recall reading that as a requirement of NFPA 79. Do you have chapter and verse (and version) for that?

I know it says that you can't use graphical representations for an e-stop device. I don't know of a similar prohibition for a reset device (at least not since 2012).

In section 10.2, it talks about "Pushbutton actuators and color graphic interface devices" and seems to treat them as the same. Section 10.2.2.7 does not point to another section on actuators, that the section on emergency stop does.

Click to expand...

Nothing specific about hardware, but it does state the reset signal can only reset the safety relay, not start the machinery.

NFPA 79 9.2.5.4 (2) The reset of the command shall not restart the machinery but only permit restarting.

Not saying yay or nay on the auto reset of the safety relay, that is dependant on the risk assesment. I do know most safety relays have a way to wire them to "auto-reset" once the string of safety devices are all closed.

On question 1 - Safety Relays aren't failure proof, so if you combine the two functions and the one and only safety relay fails, you loose both functions.

On question 2 - I would flip that on its head. Rather than use the HMI screen to reset the Safety Relay, I would use the Reset Button to also Start the process. One less press for the operator, but still working on the safe side.

Rson said:

NFPA 79 9.2.5.4 (2) The reset of the command shall not restart the machinery but only permit restarting.

Click to expand...

BryanG said:

On question 2 - I would flip that on its head. Rather than use the HMI screen to reset the Safety Relay, I would use the Reset Button to also Start the process. One less press for the operator, but still working on the safe side.

Click to expand...

These two statements seem contradictory to me. Please advise if I am misunderstanding either of you.

Rson said:

Nothing specific about hardware, but it does state the reset signal can only reset the safety relay, not start the machinery.

NFPA 79 9.2.5.4 (2) The reset of the command shall not restart the machinery but only permit restarting.

Click to expand...

BryanG said:

On question 2 - I would flip that on its head. Rather than use the HMI screen to reset the Safety Relay, I would use the Reset Button to also Start the process. One less press for the operator, but still working on the safe side.

Click to expand...

kwade said:

These two statements seem contradictory to me. Please advise if I am misunderstanding either of you.

Click to expand...

No, you understand correctly. What BryanG is suggesting would definitely be a violation of NFPA 79. This is a US standard, and the rules are different in the UK, but I would be very surprised to find this doesn't violate there rules there as well.