Safety resets in a Guard Logix

phuz · Dec 26, 2017

I'm working on a project where the engineer at the plant is telling me you can use standard inputs for resets, for example resetting an E-stop block.

I've only used the PointIO safety a handful of times and with every application, all inputs related to safety were brought into the IB8S modules, including the resets. I attempted to use a controller tag from a non-safety input for the reset tag, but as I figured, the program does not allow it.

I don't disagree that a reset should be allowed to be controlled from a non-safety IO point, but from my experience, only safety IO is allowed in those routines.

mobsoft · Dec 26, 2017

Under the Logic menu you can map standard to safety tags.

Be sure to use the falling signal for your resets.

lpc921 · Dec 26, 2017

I never used GuardLogix but it is surprising that the program doesn't allow standard tags in safety routines. I wonder if you are aware of the "Safety Tag Mapping" function?

Manual reset function does not need to be from a safety input if risk assessment agrees. However there are a few additional requirements for the reset function depending on which standards you are trying to comply with.

mk42 · Dec 26, 2017

I've seen standard bits used for resets in Siemens safety programs often.

If my little AB experience serves, I don't think standard tags can be read in safety at all. I think there's a way to set up mapping though.

If you have safe inputs to spare, having the reset there might be easiest, which is probably why you've always seen that before.

phuz · Dec 27, 2017

Thanks, huge help guys!

(slightly embarrassed that I didn't know that existed)

Keystone · Dec 28, 2017

mobsoft is right on with the mapping.

To clarify with your reset button make sure it is using a One-Shot falling (OSF). This way the reset can not be maintained - only when the button is released. Prevents screw drivers that accidentally get stuck in the button to reset the machine.

Mispeld · Dec 28, 2017

Keystone said:
To clarify with your reset button make sure it is using a One-Shot falling (OSF). This way the reset can not be maintained - only when the button is released. Prevents screw drivers that accidentally get stuck in the button to reset the machine.

I get the distinction on using an edge signal for safety reset versus maintained. But is there a safety/functional difference if it is the rising edge versus the falling edge of a safety controller input?

mk42 · Dec 28, 2017

Mispeld said:
I get the distinction on using an edge signal for safety reset versus maintained. But is there a safety/functional difference if it is the rising edge versus the falling edge of a safety controller input?

I was wondering that as well.

However, I have (in Siemens systems) seen rising edge used for resetting HW faults (like a short circuit in the card) then falling edge used for software resets (estop or light curtain).

jstolaruk · Dec 28, 2017

Keystone said:
mobsoft is right on with the mapping.

To clarify with your reset button make sure it is using a One-Shot falling (OSF). This way the reset can not be maintained - only when the button is released. Prevents screw drivers that accidentally get stuck in the button to reset the machine.

I declare "stuck" push buttons as faults. It may actually point an electrician to a real problem.

Keystone · Dec 28, 2017

I have not read the full spec, I am sure there are more knowledge people who's posts I would be interested in. I was give the following.:

ISO 13849-1 stipulates instruction reset functions must occur on falling edge signals.

Keystone · Dec 28, 2017

BTW - I am not declaring myself an expert in the safety PLC area. I have done a few systems with them and I like the functionality. Just passing on some thinks I was taught.

jacoffey85 · Dec 28, 2017

mk42 said:
I've seen standard bits used for resets in Siemens safety programs often.

Fun thing I learned a little while back with Tia Portal V13, don't use HMI reset buttons inside of the safety program directly. Make sure to map it to another bit inside of a standard function first, then use that bit inside of the safety logic.

Not having the HMI input synchronized with the scan cycle would sometimes give us a major PLC fault for the safety telegram error.

_Dock_ · Jan 2, 2018

Not sure on guard logix but if using a 440C-CR30 programmable relay you can use a standard input from an expansion module for resets. However, in the software if a reset is held high for to long it will fault the relay. The same logic might be built into the guardlogix, i dont know.

phuz · Jan 2, 2018

_Dock_ said:
Not sure on guard logix but if using a 440C-CR30 programmable relay you can use a standard input from an expansion module for resets. However, in the software if a reset is held high for to long it will fault the relay. The same logic might be built into the guardlogix, i dont know.

Yes, I've done this quite a few times on a 440C. Those are nice cheap little safety units!

_Dock_ · Jan 2, 2018

phuz said:
Yes, I've done this quite a few times on a 440C. Those are nice cheap little safety units!

We use them alot as well, was just reviewing the banner version. They are slightly cheaper and one of the versions is expandable so we may be switching.

If you have any slight chance of needing to zone or expand after manufacturing then it is a no brainer.

Safety resets in a Guard Logix

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